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Title: Testing Generator Voltage
Post by: Mike Karseboom on May 19, 2014, 09:34:49 pm
The outdoor festival I worked over this last weekend supplied a whisperwatt 70 (56kW) generator for audio and stage power.  I have gotten power from this type of generator before but not paid much attention to how these things are hooked up before.  However I have a CEP spider box and a length of 6/4 with hubbel California standard CS6365 twist lock plug and was asked to bring that as a distro.


After reading on these forums how important it is to always check the generator outputs prior to plugging in any equipment, I tried to do just that.  I read the generator manual and checked all the switch settings.  Good thing too - it was set to 3 phase 480V and I needed single phase 240V.  That switch is alluded to in several sections of the manual and on labels on the generator. Strangely I could not find the location of this critical switch from the manual.  It is pretty well hidden inside one of the compartments.


Checking the voltage at the on board GFCI edison type outlets, it all looked good.  117 volts AC on the hot leg of the outlet referenced to the neutral or ground. 


Moving on to check the  voltages on the CS6369 outlet for the 240V 50A twist lock, I could not see any voltage between  any of the output legs and ground or between any two legs for that matter.  After a while I went ahead and connected the spider box and indicators looked normal at the spider box.  Voltages at the edison outlets on the spiderbox were as expected.


So my question is whether there is some trick to  measuring voltages on the CS6369 on a generator like this?  It is possible that the metal tips on the test leads of my DVM were too short to make contact with the outlet connectors.  But they were at least 5/8" long and inserted and moved around a bit.



Title: Re: Testing Generator Voltage
Post by: Ray Aberle on May 19, 2014, 10:20:23 pm
The outdoor festival I worked over this last weekend supplied a whisperwatt 70 (56kW) generator for audio and stage power.  I have gotten power from this type of generator before but not paid much attention to how these things are hooked up before.  However I have a CEP spider box and a length of 6/4 with hubbel California standard CS6365 twist lock plug and was asked to bring that as a distro.


After reading on these forums how important it is to always check the generator outputs prior to plugging in any equipment, I tried to do just that.  I read the generator manual and checked all the switch settings.  Good thing too - it was set to 3 phase 480V and I needed single phase 240V.  That switch is alluded to in several sections of the manual and on labels on the generator. Strangely I could not find the location of this critical switch from the manual.  It is pretty well hidden inside one of the compartments.


Checking the voltage at the on board GFCI edison type outlets, it all looked good.  117 volts AC on the hot leg of the outlet referenced to the neutral or ground. 


Moving on to check the  voltages on the CS6369 outlet for the 240V 50A twist lock, I could not see any voltage between  any of the output legs and ground or between any two legs for that matter.  After a while I went ahead and connected the spider box and indicators looked normal at the spider box.  Voltages at the edison outlets on the spiderbox were as expected.


So my question is whether there is some trick to  measuring voltages on the CS6369 on a generator like this?  It is possible that the metal tips on the test leads of my DVM were too short to make contact with the outlet connectors.  But they were at least 5/8" long and inserted and moved around a bit.

Yeah, those outlets on the gennie can be a pain in the butt to hit right. If you look really close, you can see the metal contacts in the connector. You may be good to also check at least two of the outlets, one on each hot leg, just to make sure you have a solid feed.  Then, you can double check that by measuring hot of one leg to the other (and you should read 220-240v there). I usually paranoid myself by double checking neutral and ground to each other from one leg to the other. And maybe those to generator body.

Of course, since I own a 45kW, a lot of time when I am using a generator, it's *mine* which means I have double checked the hell out of it before it goes out anyways. :)

-Ray
Title: Re: Testing Generator Voltage
Post by: Mike Karseboom on May 20, 2014, 12:26:59 am
Yeah, those outlets on the gennie can be a pain in the butt to hit right.

-Ray


Just so I am clear - you are saying I should have seen the voltage on the CS6369?  I just did not get the probes in proper contact.  It is not some trick where there has to be a load on the line or something before you see the voltage?
Title: Re: Testing Generator Voltage
Post by: Tim McCulloch on May 20, 2014, 12:51:51 am

Just so I am clear - you are saying I should have seen the voltage on the CS6369?  I just did not get the probes in proper contact.  It is not some trick where there has to be a load on the line or something before you see the voltage?

You need longer probes or a better light to shine into the contact slots to find the metal.  Or both.

My generator story from this weekend:  preferred vendor, talked to the sales guy (new to me).  I reject the first genset because the terminal cover door is missing.  Oops.  So we have it moved, and Mr. Salesguy starts the other genset, I verify the right voltage and he shuts it down.  Half hour later I'm on the phone with him "uh... is there something special needed to start this thing?"  "No, just turn on the battery switch and press the start button, it should fire up in less than 30 seconds."  No go.  No go again.  He comes over and can't start it, either.  We check all the various shunt trip switches on doors and they're all closed and tight.  He calls out a tech.  Mr. Tech takes about 20 minutes tracing 12vDC around and determines a shunt trip switch is defective and wires around it.  It's the cover to the voltage/phase selector switch (which is inside another compartment to start with).  I'm good with that, and we're off and running.
Title: Re: Testing Generator Voltage
Post by: Ray Aberle on May 20, 2014, 11:55:42 am

Just so I am clear - you are saying I should have seen the voltage on the CS6369?  I just did not get the probes in proper contact.  It is not some trick where there has to be a load on the line or something before you see the voltage?
With....
- A properly functioning generator
- Main voltage selector switch in the "1/240V" position
- Main breaker engaged
- branch circuit breaker engaged
- Properly functioning 6/4 feeder cable
- A properly functioning multimeter

You should read 220-240v between hot and hot, 110-120v between each hot and neutral/ground, and 0v between neutral and ground.

With...
- Properly functioning spider box
- branch circuits on spider box engaged and not GFCI-tripped

You should read 110-120v between any outlet's hot and neutral/ground, and 220-240v between the hot of one leg and the other. (Pick any two outlets; generally spider boxes are wired every other outlet is on the other leg.)

There is not a trick where there has to be a load on the line or anything else before you see voltage. As long as everything's working and turned out, there's going to be voltage.

-Ray
Title: Re: Testing Generator Voltage
Post by: BobWitte on May 20, 2014, 12:26:21 pm
Just to be clear and state the obvious, the pin-out looks like this:


(http://www.hubbellcatalog.com/wiring/images/3pl4wr_50a_125250r.GIF)


The first time I encountered this connector (many years ago) when I didn't know the pin-out I tried to measure to the center "pin/stud". Of course that yielded not much of anything.
Title: Re: Testing Generator Voltage
Post by: Mike Karseboom on May 20, 2014, 03:06:37 pm
Thank you all - excellent info.  In my case I think the probes were just too short.


I did not know this little bit  about measuring 240V between hots on the spider box outlets:


With....
- Properly functioning spider box
...
You should read 110-120v between any outlet's hot and neutral/ground, and 220-240v between the hot of one leg and the other. (Pick any two outlets; generally spider boxes are wired every other outlet is on the other leg.)

-Ray


The other confusing part of this experience has to do with generator grounding.  After reading various posts about this I thought it was a good idea to have a ground rod even though there is no building involved and all power is from plugs mounted on the generator.  When I asked the generator rental company, which is a fairly large supplier with locations in 3 different cities in Oregon, about grounding the generator they said it has never come up before.  They don't even have ground rods.


So when there is no building tie in, do the OSHA rules prevail?  Those seem to be pretty straight forward in saying that if we are just using  cord and plug based loads, the ground/neutral bonded to the generator/trailer frame is sufficient and no ground rod is needed.


If that is true then I won't bother next time with all the hassle of a ground rod.
Title: Re: Testing Generator Voltage
Post by: Ray Aberle on May 20, 2014, 03:43:10 pm
The other confusing part of this experience has to do with generator grounding.  After reading various posts about this I thought it was a good idea to have a ground rod even though there is no building involved and all power is from plugs mounted on the generator.  When I asked the generator rental company, which is a fairly large supplier with locations in 3 different cities in Oregon, about grounding the generator they said it has never come up before.  They don't even have ground rods.

So when there is no building tie in, do the OSHA rules prevail?  Those seem to be pretty straight forward in saying that if we are just using  cord and plug based loads, the ground/neutral bonded to the generator/trailer frame is sufficient and no ground rod is needed.

If that is true then I won't bother next time with all the hassle of a ground rod.

What OSHA might say could be different then what your AHJ might say, and what they say may or may not be different then what is "actually smart to do."

I've got a 45kW WhisperWatt, and if I bring it up to Seattle, the city requires grounding and an inspection by city L&I. I always ground it in one way or another when working in Seattle; most times I pull the permit but sometimes the client does. (Or, at least... they are telling me that they are... but I never see paperwork around, or have an inspector show up... soooo...)

If you're using a big diesel, and you don't ground, and someone shows up (that AHJ)- they may have the authority to shut down your show until the problem is corrected. Personally, I would rather do it to the most extreme point possible that will ensure that someone doesn't have any standing to drop by and demand that it be shut off!

Same deal about a "large supplier" - I worked with one several times in the Seattle area, and I never saw them grounded. It is possible that, as a part of their contract, they're *just* delivering it to the site, and it is the responsibility of the client to ensure that all legal and safety things are taken care of. But without seeing the rental paperwork... hard to know for sure.

The "big guys" charge a LOT of money for generators, but that's also... permitted... delivered... setup... grounded... inspected... It's a full service package and you are assured (as much as is possible) that there won't be any problems.

-Ray
Title: Re: Testing Generator Voltage
Post by: Mike Sokol on May 20, 2014, 08:23:37 pm

You should read 220-240v between hot and hot, 110-120v between each hot and neutral/ground, and 0v between neutral and ground.

Remember that you'll only read 0v between neutral and ground with NO load. Once you put a load on the end of the line, you'll typically see a voltage between neutral and ground that's 1/2 of the total line drop with a single-phase load. So expect to see 2 or 3 volts between neutral and ground when significant power is being drawn, especially from lighting.

This also assumes that your generator is properly G-N bonded. Portable generators under 5KW are mostly floated neutrals and have a GFCI protected output. But anything near the size you would use for a show should already be G-N bonded.

Also, I always earth-ground outside generators for stage power since it's way too easy for power cable's hot wire to be pinched and shorted to a grounded stage. That can flip the entire distro upside down with the Hot at earth potential and the Ground/Neutral at 120-volts above earth potential. If that happens, then it's way too easy for a musician holding a properly grounded guitar to get a serious shock if they touch a metal stage or wet earth. That's why I always insist on the stage being bonded to the generator's neutral-ground point.

For instance, I once did a show for Chumbawumba in Wash DC on the roof of a parking deck. There was a big contractor generator delivered behind the building without a grounding rod to power the show. I had camlocks and a proper distro, but when I hooked everything up for a test I measured 90-volts AC between the sound system "ground" and the metal rail around the roof deck. I was really worried that one of the performers (on an elevated metal stage a rail height) could get a shock and tumble off the roof to the pavement 40 ft below. So I drove in a piece of rebar and hooked a heavy wire under the generator grounding terminal, then used a pair of vice-grip pliers to bond this ground wire to the rebar. Then some electrical tape around the vice-grip handle kept everything snug. Once I did that the voltage on the system ground (EGC - Earth Grounding Conductor) went down to within a volt of earth potential as well as the metal rail around the roof deck.  Problem solved, and we all got the hear them play Tub-Thumbing for their debut trip to American.   

That ground terminal on the generator is there for a reason, even if it's not required for local inspection. I've taken a few big shocks off of ungrounded generators over the years, and I think it's way too dangerous to run them ungrounded. Our musicians/clients are paying us to keep them safe, and I take that responsibility very seriously.
Title: Re: Testing Generator Voltage
Post by: Ray Aberle on May 20, 2014, 08:54:09 pm
Remember that you'll only read 0v between neutral and ground with NO load. Once you put a load on the end of the line, you'll typically see a voltage between neutral and ground that's 1/2 of the total line drop with a single-phase load. So expect to see 2 or 3 volts between neutral and ground when significant power is being drawn, especially from lighting.

Thanks for the clarification, Mike. It is important to note that the best (really, the first thing you do, as in don't do anything else until you have metered it!) time to meter the generator is *before* anything is connected. So, at that point (before connections are made) you would get that 0v reading. If you don't -- as Mike reminisced, that is something that should be addressed immediately!

This also assumes that your generator is properly G-N bonded. Portable generators under 5KW are mostly floated neutrals and have a GFCI protected output. But anything near the size you would use for a show should already be G-N bonded.

Of course, we are talking about the larger sized diesel generators.

-Ray
Title: Re: Testing Generator Voltage
Post by: Mike Karseboom on May 20, 2014, 11:04:30 pm
I actually could not find a ground lug on the whisperwatt 70.  The manual shows a grounding point on the frame but there was nothing there.  I looked all around for something convenient and did not see anything.  Most of the likely spots are either 3/4" heavy duty frame bolts that looked like they would be hard to loosen or 1/2" bolts where the nut side was hard to access.  I ended up unscrewing a bolt on the fender and scraping the paint down below the washer to get bare metal. It almost seemed like they were trying to make it hard for me to ground the darn thing.
 
There is a ground terminal inside the door where the 5 pin type leads go for 3 phase.  But I know even less about that kind of wiring and did not want to touch those.  In hindsite that ground terminal probably would have been the place to go. 
 
If you do establish an earth ground at the generator and want to also ground a metal stage, does that mean running a separate wire from the stage back to the generator ground?  Or is there a way to ground the stage to something more convenient like the spider box?
Title: Re: Testing Generator Voltage
Post by: Tim McCulloch on May 21, 2014, 02:50:03 am
I actually could not find a ground lug on the whisperwatt 70.  The manual shows a grounding point on the frame but there was nothing there.  I looked all around for something convenient and did not see anything.  Most of the likely spots are either 3/4" heavy duty frame bolts that looked like they would be hard to loosen or 1/2" bolts where the nut side was hard to access.  I ended up unscrewing a bolt on the fender and scraping the paint down below the washer to get bare metal. It almost seemed like they were trying to make it hard for me to ground the darn thing.
 
There is a ground terminal inside the door where the 5 pin type leads go for 3 phase.  But I know even less about that kind of wiring and did not want to touch those.  In hindsite that ground terminal probably would have been the place to go. 
 
If you do establish an earth ground at the generator and want to also ground a metal stage, does that mean running a separate wire from the stage back to the generator ground?  Or is there a way to ground the stage to something more convenient like the spider box?

Yes, using the ground connection in the big lug box would have been correct.  Meter resistance between that lug and the neutral lug, it should be 0 Ohms or nearly so.  If it's open, add a jumper between the neutral and ground to bond them, and run a wire from the ground lug to a ground rod.

The stage structure must be grounded.  IIRC it can be bonded to the Grounding Electrode System, but Mike or someone else will need to confirm/deny.
Title: Re: Testing Generator Voltage
Post by: Stephen Swaffer on May 21, 2014, 08:20:39 am
You could bond to the grounding electrode system (ground rods/wiring/genny frame).  It would also be acceptable to use the main spider box ground-IF there is only one cable from genny to stage.  You want it set up in such a way that if something is unplugged, your hot conductors are disconnected at the same time or preferably before the ground is (that is why on most plugs the ground prong is slightly longer than the power prongs.)

This is a safety consideration, so convenience should be a secondary consideration.  That said, I have learned that people are more likely to be safe when it is convenient to do so-unfortunately!
Title: Re: Testing Generator Voltage
Post by: Mike Sokol on May 21, 2014, 07:52:58 pm
You could bond to the grounding electrode system (ground rods/wiring/genny frame).  It would also be acceptable to use the main spider box ground-IF there is only one cable from genny to stage.  You want it set up in such a way that if something is unplugged, your hot conductors are disconnected at the same time or preferably before the ground is (that is why on most plugs the ground prong is slightly longer than the power prongs.)

This is a safety consideration, so convenience should be a secondary consideration.  That said, I have learned that people are more likely to be safe when it is convenient to do so-unfortunately!

Steve is correct. You should have a late-break connection for the EGC if it's part of a mult-plug power feed to the spider box and the spider box is bonded to the stage. And there must be no way the EGC can be disconnected with the power lines still connected. But a home-run EGC to the stage is best if you can plan for it.

I've seen a few large stages with a special grounding cam-lock attached with a bolt. That seems like overkill, but it sure is convenient to put a T on the genny ground and run a standard piece of cam-lock cable to the stage. And yes, it needs to have a green cover, or at least green e-tape.   
Title: Re: Testing Generator Voltage
Post by: Mike Karseboom on May 22, 2014, 11:02:55 am
Thanks for the great replies.  Seems like we get more "facts" and clear answers on this particular forum than most of the others.  Many of the discussions over on the Lab Lounge involve opinion and conjecture.  Good thing due to the inherent dangers working with electrical systems.


Another voltage question -  having to do with adjusting the voltage control on the generator: 


Let me note first the only other load on the generator was about 500W of incadescent lights for the hospitality tent connected with a 50' extension cord directly into the generator GFCI edison outlets.

The audio (and later stage lighting)  was connected to the spider box.  I had 100' of 6/4 soow between the generator and the spider box.  From there is was another 25'  <EDIT 4x 12/3  parallel cables >   or so to the amp/processor rack and the stage monitors and power.




The basic audio set up was 9 QSC PLX series amps,  6 powered QSC K12 stage monitors , and a furman rack mounted "conditioner" and small UPS feeding the processing  (2x DR260) and FOH (SL24.4.2 + laptop).


With all that equipment on and just running background music, I was measuring about 108V as indicated by the Furman and by the UPS.  This seemed a little low to me and I thought, why not bump it up a bit.  Over a period of several adjustments I gradually brought the voltage up to about 116V as shown on the UPS by adjusting the voltage control on the generator.  That control is supposed to give you +/- 15% voltage change without affecting the frequency.  To get to 116V at the UPS I needed to turn the adjustment almost all the way up.  That voltage stayed pretty stable even after more load was put on the spider box from live sound and lighting.


So finally the question:  Is there any reason why I should not have bumped that voltage up at the generator? 

Title: Re: Testing Generator Voltage
Post by: Tim McCulloch on May 22, 2014, 12:06:50 pm
Thanks for the great replies.  Seems like we get more "facts" and clear answers on this particular forum than most of the others.  Many of the discussions over on the Lab Lounge involve opinion and conjecture.  Good thing due to the inherent dangers working with electrical systems.


Another voltage question -  having to do with adjusting the voltage control on the generator: 


Let me note first the only other load on the generator was about 500W of incadescent lights for the hospitality tent connected with a 50' extension cord directly into the generator GFCI edison outlets.

The audio (and later stage lighting)  was connected to the spider box.  I had 100' of 6/4 soow between the generator and the spider box.  From there is was another 25'  <EDIT 4x 12/3  parallel cables >   or so to the amp/processor rack and the stage monitors and power.




The basic audio set up was 9 QSC PLX series amps,  6 powered QSC K12 stage monitors , and a furman rack mounted "conditioner" and small UPS feeding the processing  (2x DR260) and FOH (SL24.4.2 + laptop).


With all that equipment on and just running background music, I was measuring about 108V as indicated by the Furman and by the UPS.  This seemed a little low to me and I thought, why not bump it up a bit.  Over a period of several adjustments I gradually brought the voltage up to about 116V as shown on the UPS by adjusting the voltage control on the generator.  That control is supposed to give you +/- 15% voltage change without affecting the frequency.  To get to 116V at the UPS I needed to turn the adjustment almost all the way up.  That voltage stayed pretty stable even after more load was put on the spider box from live sound and lighting.


So finally the question:  Is there any reason why I should not have bumped that voltage up at the generator?

Voltage adjustment of +15% means your feeder was waaaaaay under-sized for the load and (especially) the length.  I've said this before, but my policy in designing electrical distribution is to go up at least 1 AWG level than is indicated in NEC 400.4 and 400.5.  Only our shortest (25ft or less) L21-30 cables use #10 AWG, the rest are #8; IIRC we used #6 for the 250ft runs to FOH.

One of the things I look for when I first check a generator is how far up the voltage adjustment has been jacked.  It's very typical in construction and similar work to compensate for #16 extension cords used by workers with portable tools.  I had one genset that was trying very hard to put out 150v... (see earlier story about generator tech that couldn't re-bus the set for 120/208v, the 150v unit was the spare).  At least it didn't require a call to the vendor.  This stuff tells me the vendor doesn't run them when they come back, they might change the oil and replace the fuel filter, but that's about it, and it's why I have a preferred vendor who will tech *my* order before the generators are tagged for delivery.

To your question, Mike, the answer is you didn't do anything spectacularly wrong, but you needed a shorter feeder or fatter wire, or both, even after cranking it up.  Don't forget that the vendors using the 120v convenience outlets on the generator will see the same voltage change, though, and you might create problems for them.
Title: Re: Testing Generator Voltage
Post by: BobWitte on May 23, 2014, 10:25:19 am
What we don't know is the voltage at the generator. I have had a similar issue, but the generator voltage itself was set at 108 with only a handful of lights on (maybe 400 watts). Had to bump it almost to the max to get to about 119 at the generator and I had 117 at FOH about 200 foot run.


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Title: Re: Testing Generator Voltage
Post by: Stephen Swaffer on May 23, 2014, 12:49:21 pm
 

 That voltage stayed pretty stable even after more load was put on the spider box from live sound and lighting.

 

If the lighting load that was added was significant-say roughly equal to the load when voltage was adjusted, then I would say the voltage at the genny was low to begin with.  If you drop 6 to 8 volts with a given load, then double your load, your voltage drop will double.  The fact that it remained stable as load was added indicates a reasonable voltage drop on the feeder.

Ignoring contact resistance-obviously unknown-a 61 amp load would result in a voltage drop of 6 volts on a 100 foot/6 AWG feeder.  Or each additional 10 amps/1200 watts should increase the voltage drop by 1 volt.  The ampacity is listed as 45 amps, so it would result in a 4.5 volt drop or 4% if fully loaded.  Following Tim's advice would get the voltage drop back in line with recommendations, but unless the cable was way overloaded it should not have been responsible dropping the voltage to 108 (unless there was a bad connection).

Not trying to be too technical-but sometimes when you actually do the math and apply Ohms Law the results can be surprising.  Making voltage checks at both ends of the feeder would obviously tell the whole story.
Title: Re: Testing Generator Voltage
Post by: Jonathan Johnson on May 29, 2014, 01:35:52 am
Also, I always earth-ground outside generators for stage power since it's way too easy for power cable's hot wire to be pinched and shorted to a grounded stage. That can flip the entire distro upside down with the Hot at earth potential and the Ground/Neutral at 120-volts above earth potential. If that happens, then it's way too easy for a musician holding a properly grounded guitar to get a serious shock if they touch a metal stage or wet earth. That's why I always insist on the stage being bonded to the generator's neutral-ground point.

This is probably the best argument I've read for grounding generators and bonding to other metal in the vicinity.

To clarify the "flip the entire distro upside down": without grounding, the line and neutral from the genny are effectively floating, meaning that there will be 0V between line and dirt, and 0V between neutral and dirt, since there is no complete circuit via the dirt. (I use the term 'dirt' here to clearly show that I'm not referring to the equipment grounding conductor or the frame of the genny.)

But if the line of a floating output inadvertently gets bonded to dirt, i.e., by pinching and piercing the insulation, you now DO have a complete circuit. Because of this bond, line-to-dirt will be 0V, and neutral-to-dirt will be the line-to-neutral voltage, typically 120V. With the neutral bonded to the EGC at the genny (but not to a ground rod or other grounded system), the EGC is also at 120V to dirt.

So now when Joe Vocalist reaches for his "properly grounded" microphone, he completes the circuit and gets zapped.

On the other hand, if you ground the generator's EGC to dirt, when that cable gets pinched it trips a circuit breaker or GFI instead of lying in wait to electrocute someone.
Title: Re: Testing Generator Voltage
Post by: Mike Sokol on May 30, 2014, 08:45:42 pm
On the other hand, if you ground the generator's EGC to dirt, when that cable gets pinched it trips a circuit breaker or GFI instead of lying in wait to electrocute someone.

Yup, my point entirely. The reason that an under 5KW portable generator can have a floated neutral and no "dirt connection" is that it's typically supplying a single power tool of some sort. But my viewpoint is that as soon as you distribute generator power to multiple locations (Amp racks, FOH console, Backline, etc..) there's way too much possibility that a single hot-to-ground failure on one power feed could bias every chassis on the backline to 120-volts above earth potential. And that's how musicians and singers get shocked and possibly killed.   
Title: Re: Testing Generator Voltage
Post by: Stephen Swaffer on May 30, 2014, 10:13:41 pm
The OP mentioned having trouble testing voltage in a twistlock receptacle-14-50Rs and 14-30Rs can be just as bad-don't rely on a meter test to check for live before disassembly.

That said, a method I have used is a spare male plug that is not connected to a cord.  The Hubbel ones are more or less finger safe-though I would wrap tape around the screws for insurance.  I would also only plug/unplug with the genny off or main disconnect thrown. You can then use the holes intended for wire as test points. Keep in mind this is still "live work" and proper safety precautions observed, but if I were setting up/testing gennies on a regular basis, I think I would keep a "test" plug handy.  IMO safer and more reliable than probing a TL or high capacity receptacle. 
Title: Re: Testing Generator Voltage
Post by: Ray Aberle on June 08, 2014, 02:52:31 pm
So I discovered something new with the "newer" control panels on WhisperWatts- at the Relay for Life I did Friday, they had a locally provided 45kW from a reputable local supplier (that never grounds their gennies, but that's a different story!). On the control panel, there was an Engine Speed switch, high speed and low speed. The tech who delivered it (I wasn't there for the drop) had switched it to low speed. When I metered the genset before use, I found 70v/150v instead of 110/220v! By switching the Engine Speed back to normal (high speed) the voltages increased to normal as well.

When the rental tech was picking it up yesterday morning, I told him about this, and he was surprised that this lowering of speed also lowered the voltage. He said he'll speak to their engineers... but first--

Has anyone encountered that same switch on a diesel generator?
Is it supposed to reduce the voltage along with the engine speed (I feel like it's supposed to be like the "Eco-throttle" on Honda EU generators?), or was that control malfunctioning on this unit?

Does it just seem like a stupid thing to have? Haha.

-Ray
Title: Re: Testing Generator Voltage
Post by: John Moore on June 08, 2014, 03:08:46 pm
Had the same issue on one in Feb this year...freaked me out... !   flipped the switch and all was good.. we could only dial in so much voltage gain on the unit, once flipped, we had MORE than enough power...and yes, we ground all our generators, it is Code here on the left coast.
Title: Re: Testing Generator Voltage
Post by: Ray Aberle on June 08, 2014, 03:12:08 pm
Had the same issue on one in Feb this year...freaked me out... !   flipped the switch and all was good.. we could only dial in so much voltage gain on the unit, once flipped, we had MORE than enough power...and yes, we ground all our generators, it is Code here on the left coast.

Code is, of course, state by state, and dependent on your AHJ -- in Seattle, permits are required to be pulled on anything over 5k, with an inspection, but that's anywhere from $90.50 to $271.50 (business hours or not) - and some of my clients don't bother. I'm not in power full time, so I will provide the gennie and ground it as needed, but I let the clients know they are responsible for obtaining the needed permit and scheduling the inspection. Since the inspector(s) will leave the signed off electrical permit after they've been there... I know damn well who has and who has not done this. :) I ensure, though, that everything I do is by the book and up to code.

But yeah, I had the same thing-- cranked up the voltage gain, little change, but then hit that switch and life was good!

-Ray
Title: Re: Testing Generator Voltage
Post by: Ray Aberle on June 08, 2014, 03:22:24 pm
Something else I just wondered... so rental co brings out diesel, doesn't ground it. Would you insist they come back and do so? In my case, they were long gone when I arrived, and I was just using the 50A twistlocks to spider boxes. Would you carry a rod and sledge and just drive one in? (I am certain what would have pissed off the high school that would now have a copper pole jammed in their field. Haha.)

-Ray
Title: Re: Testing Generator Voltage
Post by: Stephen Swaffer on June 08, 2014, 06:16:07 pm
Who hired the rental company?  If you are just plugging into power hired/provided by the venue, I would think your liability would be minimal.

Is there a local inspection requirement?  Frankly, I view inspectors as my friend-they come in handy as the "bad guy" that insists things are done by the book.

As has been discussed on other threads, ground rods are mainly lightning protection.  I would be more concerned about bonding-equipment/stages/etc myself.  Also, if GFCI protection s used there is a measure of safety should a path accidentally be created back to the genny.

That said, grounding is required by the NEC, so if power is my responsibility and I am going to use this vendor in the future, it wouldn't hurt to establish that "If we are setting up for Ray, make sure you ground the genny, we don't want to make an extra trip." 

Also, ground rods are often surprisingly easy to pull.  (I have hit more than one old foundation 7 to 7 1/2 feet down!)  And in soils around here at least, a little water can be used an you can work the ground rod in easier than using a sledge.  Though this reminds me of another experiment Mike was working on for a temporary grounding electrode-but that is another thread.
Title: Re: Testing Generator Voltage
Post by: jason misterka on June 08, 2014, 09:11:00 pm
So I discovered something new with the "newer" control panels on WhisperWatts- at the Relay for Life I did Friday, they had a locally provided 45kW from a reputable local supplier (that never grounds their gennies, but that's a different story!). On the control panel, there was an Engine Speed switch, high speed and low speed. The tech who delivered it (I wasn't there for the drop) had switched it to low speed. When I metered the genset before use, I found 70v/150v instead of 110/220v! By switching the Engine Speed back to normal (high speed) the voltages increased to normal as well.

When the rental tech was picking it up yesterday morning, I told him about this, and he was surprised that this lowering of speed also lowered the voltage. He said he'll speak to their engineers... but first--

Has anyone encountered that same switch on a diesel generator?
Is it supposed to reduce the voltage along with the engine speed (I feel like it's supposed to be like the "Eco-throttle" on Honda EU generators?), or was that control malfunctioning on this unit?

Does it just seem like a stupid thing to have? Haha.

-Ray

Many many generators have that switch, especially the larger generators.

My understanding is that you start it off in lower speed to warm it up and after a few minutes turn it on to full speed. 

You keep the main breaker off until it is at full speed, you check the voltage at the generator panel. Then you check the voltage at your PD while the main breaker if off of course.

You do the reverse when turning the generator off for the night.

A few years ago I was at a festival where the generator was powered off due to a storm.  The stage manager was a bit over-eager and turned the generator back on, left it in low speed mode and switched on the main breaker.

We (audio) were fine because my monitor tech is smart and left our PD main breaker off until he could check the power again.

Lighting , unfortunately, had no main breaker and blew every fuse in every dimmer rack of a 120k rig.  They had a few spares and limped through the night until some one could make it to radio shack the next day.  It could of been way way worse...

Jason
Title: Re: Testing Generator Voltage
Post by: Mike Sokol on June 08, 2014, 09:23:24 pm
So I discovered something new with the "newer" control panels on WhisperWatts- at the Relay for Life I did Friday, they had a locally provided 45kW from a reputable local supplier (that never grounds their gennies, but that's a different story!). On the control panel, there was an Engine Speed switch, high speed and low speed. The tech who delivered it (I wasn't there for the drop) had switched it to low speed. When I metered the genset before use, I found 70v/150v instead of 110/220v! By switching the Engine Speed back to normal (high speed) the voltages increased to normal as well.

Remember that a standard copper/iron AC generator needs to have a specific RPM from the motor to maintain both frequency and voltage. I'm guessing that the low speed is for warmup (as noted earlier), but not for power production. The Honda and Yamaha Inverter generators actually have an electronic inverter on the output which locks the AC frequency to 60 Hz (or 50 Hz) no matter what engine RPM. That's great for economical fuel usage since during times of low power draw the motor can be running much slower and use much less fuel. But alas, for a standard spinning-rotor generator there no such thing as eco-mode, so it has to spin at full speed to maintain 60 Hz even if it's only powering the smallest appliance or guitar amp. Since a number of digital voltmeters also read frequency, try to measure the Hz next time you find a big genny in "slo-mode" and I'm sure it will be a lot less than 60 Hz, maybe 40 or even 30 Hz. 
Title: Re: Testing Generator Voltage
Post by: Tim Padrick on June 09, 2014, 12:08:30 am
... But my viewpoint is that as soon as you distribute generator power to multiple locations (Amp racks, FOH console, Backline, etc..) there's way too much possibility that a single hot-to-ground failure on one power feed could bias every chassis on the backline to 120-volts above earth potential. And that's how musicians and singers get shocked and possibly killed.

Would that not pop the breaker?
Title: Re: Testing Generator Voltage
Post by: Jonathan Johnson on June 09, 2014, 01:27:43 am
...But my viewpoint is that as soon as you distribute generator power to multiple locations (Amp racks, FOH console, Backline, etc..) there's way too much possibility that a single hot-to-ground failure on one power feed could bias every chassis on the backline to 120-volts above earth potential. And that's how musicians and singers get shocked and possibly killed.   

Would that not pop the breaker?

Not necessarily. The scenario is a hot to earth ground fault (earth ground being the metal frame of the stage), but the generator is not grounded (but the EGC in the distro is bonded to neutral at the genny). Because there is no connection between the earth ground and generator ground, there is not a complete circuit, so the breaker doesn't trip. But once Joe Vocalist grabs the wired microphone with a shield connected to chassis ground (assuming all the gear is somehow insulated from the stage, such as with rubber casters or feet), he completes the circuit. The current flow through the human body will not be sufficient to trip the breaker, but it will be sufficient to stop the heart.
Title: Re: Testing Generator Voltage
Post by: Mike Sokol on June 09, 2014, 06:28:21 am
The scenario is a hot to earth ground fault (earth ground being the metal frame of the stage), but the generator is not grounded (but the EGC in the distro is bonded to neutral at the genny). Because there is no connection between the earth ground and generator ground, there is not a complete circuit, so the breaker doesn't trip. But once Joe Vocalist grabs the wired microphone with a shield connected to chassis ground (assuming all the gear is somehow insulated from the stage, such as with rubber casters or feet), he completes the circuit. The current flow through the human body will not be sufficient to trip the breaker, but it will be sufficient to stop the heart.

Exactly. And that's why generator bonding to the metal stage is just as important (and maybe even more important) than "earthing" the generator chassis.
Title: Re: Testing Generator Voltage
Post by: Stephen Swaffer on June 09, 2014, 08:27:50 am
The NEC in 250.4(A)(5) and in 250.4(B)(4) says "....the earth shall not be considered as an effective fault-current path."  Hence the need for an EGC and bonding.

I did not give a definitive yes or no to Ray's question on calling back the genny provider because one of the variables that would play in to my call on the scene is an unknown-and at that it would be a judgement call demanding taking responsibility.

Consider the following scenarios.

A musician playing on a metal/conductive stage.  Stage must be bonded-been discussed so taken as a given.

A musician playing on a wood stage.  No need to bond, and in fact no way to bond.

Now for a "don't try this at home" scenario.

A musician playing in the middle of a mud puddle.  Really need to have that puddle bonded some how.

A musician playing in the middle of a well drained gravel lot that hasn't seen rain in 6 months.  No need to bond and in fact probably no way to bond.

The reality is that 98% of the time you will be working in between these 2 extremes-but not really knowing where you are at with conductivity.  The old adage comes in to play, "Better safe than sorry"  If you drive a ground rod and the moisture in the ground is sufficient to cause a hazard, you will now be "bonded" to that conductive surface.  IMO that is where a ground rod affects personnel safety, in addition to providing lightning protection.
Title: Re: Testing Generator Voltage
Post by: Rob Spence on June 09, 2014, 04:50:29 pm
I carry a 10' length of #4 green with lugs on the ends. I use a vice grip at each end to bond the stage to the distro (I need to add a post to the distro). The vice grips are also handy to serve as handles to pull a ground rod.


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Title: Re: Testing Generator Voltage
Post by: Mike Sokol on June 09, 2014, 06:49:10 pm
I carry a 10' length of #4 green with lugs on the ends. I use a vice grip at each end to bond the stage to the distro (I need to add a post to the distro). The vice grips are also handy to serve as handles to pull a ground rod.

I've done exactly the same thing, and also added a few wraps of e-tape around the handle of the Vice-Grips so that: A) it's obvious they're supposed to be there, and B) they can't vibrate loose. I suppose if you wanted to be extra correct you could use green e-tape or spray paint your Vice-Grips green.

Not sure if the NEC speaks to the use of this sort of bonding/clamping rig, but it's fast, solid and will bite through any existing paint or corrosion way better than a lug. Any code-heads here who can speak to this usage? I've never been challenged by an inspector about my Vice-Grip grounding bonds in the past, but I don't remember any inspectors showing up there either.   
Title: Re: Testing Generator Voltage
Post by: Jonathan Johnson on June 09, 2014, 07:37:14 pm
Not sure if the NEC speaks to the use of this sort of bonding/clamping rig, but it's fast, solid and will bite through any existing paint or corrosion way better than a lug. Any code-heads here who can speak to this usage? I've never been challenged by an inspector about my Vice-Grip grounding bonds in the past, but I don't remember any inspectors showing up there either.

I'm guessing that if an inspector wanted to be picky about it, you would be required to use either a pipe-style ground clamp around a tubular structural member, or bolt a lug to a flat portion of the staging structure.
Title: Re: Testing Generator Voltage
Post by: Kemper Watson on June 09, 2014, 09:31:47 pm
What is the proper depth needed for a ground rod? Will rebar work?
Title: Re: Testing Generator Voltage
Post by: g'bye, Dick Rees on June 09, 2014, 09:55:26 pm
What is the proper depth needed for a ground rod? Will rebar work?

It depends...

If there is a high moisture content the requisite depth will be less than in dry soil.  A knowledgable friend reports having to go to a depth of 40 feet to get a proper ground for his home.
Title: Re: Testing Generator Voltage
Post by: John Moore on June 09, 2014, 10:14:07 pm
Something else I just wondered... so rental co brings out diesel, doesn't ground it. Would you insist they come back and do so? In my case, they were long gone when I arrived, and I was just using the 50A twistlocks to spider boxes. Would you carry a rod and sledge and just drive one in? (I am certain what would have pissed off the high school that would now have a copper pole jammed in their field. Haha.)

-Ray

The rental companies don't ground anything, nor do they supply the rod, we get one, drive it, clamp it and leave it in place or pound it to hell after we are done...
Title: Re: Testing Generator Voltage
Post by: Mike Sokol on June 09, 2014, 10:26:58 pm
It depends...

If there is a high moisture content the requisite depth will be less than in dry soil.  A knowledgable friend reports having to go to a depth of 40 feet to get a proper ground for his home.

It's really difficult to get a good ground in dry or sandy soil. See below for an article from EC&M about some of the issues.

==================================

Achieving an Acceptable Ground in Poor Soil
Oct 1, 1998 Keith Switzer, ERICO, Inc. | Electrical Construction and Maintenance

To ensure your electrical system functions properly, it's important your buried ground system has low impedance. So how do you achieve this goal while keeping safety in mind? When designing and installing electrical power systems, proper grounding is not just a luxury, but a necessity. All good grounding systems should provide a low-impedance path for fault and lightning-induced currents to enter

To ensure your electrical system functions properly, it's important your buried ground system has low impedance. So how do you achieve this goal while keeping safety in mind?

When designing and installing electrical power systems, proper grounding is not just a luxury, but a necessity. All good grounding systems should provide a low-impedance path for fault and lightning-induced currents to enter the earth, ensuring maximum safety from electrical system faults and lightning. More specifically, a properly installed grounding system not only helps safeguard buildings and equipment from damage caused by unintentional fault currents or lightning surges, but also protects a much more important investment: people.

Achieving an acceptable ground is challenging. Proper installation of grounding systems requires knowledge of national standards, conductor materials, and connections and terminations (Fig. 1, in original article). But that's not all. Don't forget to consider soil conditions where you install the ground rods (or grounding grid).

Impact of soil conditions on grounding. Though the overall effectiveness of a buried grounding system depends on many factors, the resistance of the earth (or earth resistivity) significantly impacts overall impedance of the buried conductor. Soil characteristics, such as moisture content, soil temperature and type, determine the overall resistivity of the earth. When grounding your system, always keep the following in mind:

Moisture content.

    The soil's moisture content is important because it helps chemicals in the soil that surround ground conductors carry the electrical current. In general, the higher the moisture content, the lower the soil's resistivity. When moisture content falls below 10%, resistivity increases significantly.

Soil temperature.

    Temperatures below freezing also increase soil resistivity. As soon as moisture turns to ice, resistivity increases sharply. In areas subject to freezing, driving a ground rod below the frost line is required to maintain a low-resistance ground.

Soil type.

    Black dirt, or soils with high organic content, are usually good conductors because they retain higher moisture levels and have a higher electrolyte level: leading to low soil resistivity. Sandy soils, which drain faster, have a much lower moisture content and electrolyte level. Therefore, they have a higher impedance. Solid rock and volcanic ash, such as that found in Hawaii, contain virtually no moisture or electrolytes. These soils have high levels of resistivity, and effective grounding is difficult to achieve. See Table 1 (in original article) for resistivities of different soils.

Measuring earth resistivity. The effectiveness of grounding rods largely depends on whether the soil surrounding the rods can conduct large electrical currents. To design a buried grounding system correctly, you must measure earth resistivity with a ground resistance testing instrument. This instrument should also have switches to change the resistance range. You can use various test methods to measure earth resistivity, but the three most common are:

    Four-point method, the most accurate.

    Variation in-depth method (three-point method).

    Two-point method.

After determining the soil resistivity, you are in a better position to determine what kind of buried grounding scheme will be most effective. Depending on the soil resistivity and grounding scheme requirements, the particular system can vary from a simple buried ground conductor to an extensive ground rod bed. The latter could include a grid system or a ground ring (Fig. 2, in original article). To decrease the grounding system impedance, you can use ground enhancement material or chemical-type electrodes.

How to achieve an acceptable ground. There are various options to lower soil resistivity. One method is to increase the moisture content of the soil. Topsoil resistivity may be reduced 800 ohm-m by increasing the moisture from 5% to 10%. An additional reduction in resistivity, although much smaller, can be obtained by increasing moisture from 10% to 20%. The problem with adding moisture to the soil is that it's not a practical option in most cases.

Another way to lower earth resistivity is to treat the soil with a salt, such as copper sulfate, magnesium sulfate, or sodium chloride. Combined with moisture, the salts leach into the soil to reduce earth resistivity. However, this inexpensive process can also cause problems. First, as the salts wash away, the soil reverts to its untreated condition. As a result, you must recharge the system periodically. Second, some salts may corrode the grounding conductors. Lastly, the salt may contaminate ground water. Local environmental regulations and the Environmental Protection Agency (EPA) may object to adding salts to the soil.

In many places, ensuring a low-resistance ground system is as simple as driving a ground rod into the subsurface soil layer that has a relatively permanent and conductive moisture content. Remember, the ground rod must extend below the lowest frost depth. You can also use ground enhancement material to achieve acceptable system resistance (Fig. 3, in original article).

What you should know when using ground enhancement material. Under almost all soil conditions, the use of a ground enhancement material will improve grounding effectiveness. Some are permanent and require no maintenance. You can use them in areas of poor conductivity, such as rocky ground, mountaintops and sandy soil, where you can't drive ground rods or where limited space makes adequate grounding difficult with conventional methods.

There are several kinds of ground enhancement material available. But use care when choosing the material. It should be compatible with the ground rod, conductor, and connection material. Some options include bentonite clay, coke powder, and specially engineered substances.

Bentonite is a clay substance used in areas with high soil resistivity. However, conduction in bentonite clay only takes place via the movement of ions. Ionic conduction can only occur in a solution, which means the bentonite clay must be moist to provide the required resistance levels. When bentonite clay loses moisture, its resistivity increases and volume decreases. This shrinkage results in a discontinuity in the contact between the bentonite clay and surrounding soil, which further increases system resistance.

Coke powder is another choice. A predominantly carbon substance, coke powder is highly conductive. However, groundwater can wash it away.

A noncorrosive low-resistance enhancement substance is a conductive cement that you can install wet or dry. Depending on the substance, it will not leach into the soil and meets EPA requirements for landfill. The railroad and utility industries have successfully used this material. When installed dry, it absorbs moisture from surrounding soil and hardens, retaining moisture within its structure. When used dry, no mixing is required, and you achieve maximum efficiency in a matter of days. This is because it absorbs enough water from the surrounding soil. You can also premix it with water to a heavy slurry. You can add this to the trench containing the grounding conductor or use it around a ground rod in an augered hole. The material binds the water into a cement making a permanent, highly conductive mass.

Some products offer a test-proven resistivity of 0.12 ohm-m or lower, compared with 2.5 ohm-m for bentonite clay. Unlike bentonite clay, the cement-like material does not depend on the continuous presence of water; nor does it require periodic charging treatments/replacement.

An ideal ground enhancement material should not require maintenance. When designing or installing a buried grounding system, look for materials that do not dissolve or decompose over time, require periodic charging treatments or replacements, or depend on the continuous presence of water to maintain conductivity.

Installation of ground enhancement materials. After selecting the material, consider the method of installation. Placement of ground enhancement material is quick and easy. For installation around a ground rod (Fig. 4, in original article), auger a 3 in. to 6 in. diameter hole to a depth equal to 6 in. less than the rod length. Drop the rod down the hole with the lower end centered and driven into the earth at least 12 in. Make the connection of the grounding conductor to the ground rod. Then, fill most of the hole using ground enhancement material. Lastly, fill the remainder of the hole with the soil removed during augering.

The installation of a conductor in a trench involves six steps as listed below. Refer to Fig. 5, for more guidance. Should you use a conductive-type cement for ground enhancement, see the estimated amount of linear feet obtainable from a bag of material for use as ground conductor covering in Table 2 (on page 64P, in original article).

    Dig a trench at least 4 in. wide by 30 in. deep, or below the frost line, whichever is deeper.

    Spread out enough ground enhancement material (either dry or in a slurry) to cover the bottom of the trench, about 1 in. deep.

    Place the conductor on top of the ground enhancement material.

    Spread more ground enhancement material on top of the conductor to completely cover the conductor, about l in. deep.

    Carefully cover the ground enhancement material with soil to a depth of about 4 in., making sure not to expose the conductor.

    Tamp the soil down, and fill in the trench.

Chemical-type electrodes are another option for difficult grounding situations. These consist of a copper tube filled with salts installed in an augered hole or trench. The electrode is backfilled with a ground enhancement material. The copper tube has holes in it near the top and bottom, and the top of the electrode remains exposed to the atmosphere. Water slowly dissolves the salts, which enter the tube from the top holes exposed to the atmosphere. Highly conductive salt solution leaches into the soil from the holes near the bottom of the tube.

The backfill material is usually bentonite clay or a combination of bentonite clay at the bottom and the cement slurry described above at the top. Chemical-type electrodes require periodic recharging of the salts. Although more expensive than a cement slurry encased ground rod, several long-term tests indicate a chemical-type electrode provides about the same effectiveness.

Measuring installed grounding systems. After installation, you may be required to measure the ground resistance of the installed system. Be aware that the 1996 NEC, Sec. 250-84, requires a single electrode consisting of rod, pipe or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of the type listed in Section 250-81 or 250-83. Always install multiple electrodes so they are more than 6 ft apart.

Maintenance of the grounding system. You need an effective inspection and periodic maintenance program to ensure continuity exists throughout the grounding system. Be sure to regularly inspect it, using an approved ground-testing instrument to test electrical resistance and continuity.
Title: Re: Testing Generator Voltage
Post by: Stephen Swaffer on June 09, 2014, 11:20:17 pm
Regarding Mike's vice grip bonding clamps, I haven't asked my inspector yet, but from past experience the key question would be "Are they listed for the purpose?"  I wonder if Irwin has ever submitted them to UL for testing?  Of course,common sense says they are certainly an effective bond.

I agree that getting a good ground in dry soil is very difficult.  Code is happy with 2 8' rods regardless of effectiveness-and I think for audio/lighting gennys that should be fine-especially considering my earlier examples.  If 2 8' rods doesn't provide an adequate ground, it is doubtful anyone will be grounded well enough just through the earth to get hurt.  A conductive stage/structure is a different story. 

Title: Re: Testing Generator Voltage
Post by: Mike Sokol on June 09, 2014, 11:37:33 pm
I agree that getting a good ground in dry soil is very difficult.  Code is happy with 2 8' rods regardless of effectiveness-and I think for audio/lighting gennys that should be fine-especially considering my earlier examples.  If 2 8' rods doesn't provide an adequate ground, it is doubtful anyone will be grounded well enough just through the earth to get hurt.  A conductive stage/structure is a different story.

It's been discussed before on this forum, but for those who missed it, here's the Ufer Ground: http://en.wikipedia.org/wiki/Ufer_ground (http://en.wikipedia.org/wiki/Ufer_ground)

It should be noted that besides lightning safety, an earth ground helps to keep your local "ground plane" close to earth potential. With a totally floating ground plane, your stage can float to pretty high voltage from leakage currents or inductive voltage from overhead power lines. Yup, if your metal stage is set up anywhere near high-tension lines and not properly "grounded" then you can easily get up to 10,000 volts above earth potential. It's supposed to be less than 5 mA current and not lethal (according to a PoCo engineer who I talked to who designed the test) but that's equivalent to grabbing a spark plug on a running lawnmower engine. I did that ONCE as a kid and don't think it would make your clients happy to be on the receiving end of such a shock. Of course, any kind of shock on an elevated stage can be life threatening simply from the possibility of a startle reflex causing a fall down the steps or off the stage. 
Title: Re: Testing Generator Voltage
Post by: Jonathan Johnson on June 10, 2014, 12:26:10 am
Now what if your generator is positioned several hundred feet away from the stage on dry, sandy soil and your stage is on a lawn that is watered weekly? You might drive a ground rod near the generator, but because of soil conditions and distance you may have a low impedance/resistance between the stage and the earth near the generator. But your personnel experiences low resistance between equipment and earth. Bonding the stage structure (and any other structure where powered equipment is used) to the generator grounding system is wise. I also wonder if maybe driving a ground rod near the stage would be a good idea?

Remember, there are two "grounding" systems involved:

There should only be one intentional bonding point between the EGC and the GEC: at the power source.
Title: Re: Testing Generator Voltage
Post by: Mike Sokol on June 10, 2014, 12:38:24 am
Now what if your generator is positioned several hundred feet away from the stage on dry, sandy soil and your stage is on a lawn that is watered weekly? You might drive a ground rod near the generator, but because of soil conditions and distance you may have a low impedance/resistance between the stage and the earth near the generator. But your personnel experiences low resistance between equipment and earth. Bonding the stage structure (and any other structure where powered equipment is used) to the generator grounding system is wise. I also wonder if maybe driving a ground rod near the stage would be a good idea?

That's why I believe that bonding the generator ground to the stage metal is more important than the actual ground rod impedance in most cases. And perhaps a secondary ground rod at the stage could be a good idea just for the scenario you mention. For instance, I do a gig nearly every year where the big generator is parked along the access road in the gravel, while the metal stage is 100 feet away sitting on the wet grass. I certainly want a ground rod at the generator, but see the advantage of adding one to the stage as well. More to consider.
Title: Re: Testing Generator Voltage
Post by: Stephen Swaffer on June 10, 2014, 10:54:23 am
Jonathan raises a very good point.  Working in a rural area, supplying outbuildings is common.  If there is a panel in the outbuilding, code requires that we install a grounding electrode system for that building.  The end result is 4 wires from supplying building-2 hots, neutral and a GEC, then a couple of ground rods at each building with a GEC to the panel.  When the buildings are close-say less than 30 or 40 feet, I have gotten away with using the same ground rods for both buildings, and just running a GEC to each.

In existing installs, I can get away without an EGC, if there are no metallic paths between buildings-but good luck convincing the AHJ that your genny on wheels is an existing install!  IMO, the EGC is mandatory, no matter how many ground rods you have the ambition to drive.

Practically, I personally would make sure the stage is bonded.  I would drive a ground rod at the genny to make sure the genny frame is at the same potential as the earth, then if it is "soupy" wet I'd be o with it.  If a situation like Jonathan mentioned were encountered, I would lean towards adding one at the stage.
Title: Re: Testing Generator Voltage
Post by: Mike Sokol on June 10, 2014, 12:12:42 pm
Practically, I personally would make sure the stage is bonded.  I would drive a ground rod at the genny to make sure the genny frame is at the same potential as the earth, then if it is "soupy" wet I'd be ok with it.  If a situation like Jonathan mentioned were encountered, I would lean towards adding one at the stage.

I agree 100%
Title: Re: Testing Generator Voltage
Post by: Jonathan Johnson on June 10, 2014, 03:40:43 pm
That's why I believe that bonding the generator ground to the stage metal is more important than the actual ground rod impedance in most cases. And perhaps a secondary ground rod at the stage could be a good idea just for the scenario you mention. For instance, I do a gig nearly every year where the big generator is parked along the access road in the gravel, while the metal stage is 100 feet away sitting on the wet grass. I certainly want a ground rod at the generator, but see the advantage of adding one to the stage as well. More to consider.
Thinking about this some more...

If the stage is bonded, that can protect those who are on the stage. FOH, on the other hand, often sits out in the grass -- bonded stage doesn't do much good. Having a ground rod at the stage MAY protect personnel at the FOH position, assuming good ground conductivity between FOH and the stage, in the event ground rod at the genny doesn't. If you really wanted to get paranoid, I suppose you could drive ground rods at every location hosting powered equipment, but considering that many people will just drive the rod below the surface rather than pull it out when they're done, the city parks department might get a bit peeved at having a bazillion ground rods driven into their lawn. And it will drive the metal detector folks crazy when they come through after the show. ;-)
Title: Re: Testing Generator Voltage
Post by: Mike Sokol on June 10, 2014, 05:05:23 pm
Thinking about this some more...

If the stage is bonded, that can protect those who are on the stage. FOH, on the other hand, often sits out in the grass -- bonded stage doesn't do much good. Having a ground rod at the stage MAY protect personnel at the FOH position, assuming good ground conductivity between FOH and the stage, in the event ground rod at the genny doesn't. If you really wanted to get paranoid, I suppose you could drive ground rods at every location hosting powered equipment, but considering that many people will just drive the rod below the surface rather than pull it out when they're done, the city parks department might get a bit peeved at having a bazillion ground rods driven into their lawn. And it will drive the metal detector folks crazy when they come through after the show. ;-)

Let's not be too paranoid. Since the mixing console and all its connected racks will be bonded to the incoming power line EGC, then it will eventually be bonded to the ground rod(s) at the stage and genny. That's good enough for me, and I suspect any inspector or code guy looking at it.
Title: Re: Testing Generator Voltage
Post by: Stephen Swaffer on June 10, 2014, 09:11:06 pm
I thought FOH personnel were expendable anyway?

Seriously, if there is enough moisture in the ground to be a hazard, 99% of the time the ground rod at the genny will be practically sufficient-and in any case driving a 2nd rod at that location makes you code compliant regardless of the ground impedance.