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Author Topic: Testing Generator Voltage  (Read 2419 times)

Mike Karseboom

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Testing Generator Voltage
« 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.



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Ray Aberle

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Re: Testing Generator Voltage
« Reply #1 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
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Mike Karseboom

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Re: Testing Generator Voltage
« Reply #2 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?
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Tim McCulloch

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Re: Testing Generator Voltage
« Reply #3 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.
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Ray Aberle

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Re: Testing Generator Voltage
« Reply #4 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
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BobWitte

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Re: Testing Generator Voltage
« Reply #5 on: May 20, 2014, 12:26:21 pm »

Just to be clear and state the obvious, the pin-out looks like this:





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.

Mike Karseboom

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Re: Testing Generator Voltage
« Reply #6 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.
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Ray Aberle

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Re: Testing Generator Voltage
« Reply #7 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
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Mike Sokol

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Re: Testing Generator Voltage
« Reply #8 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.

Ray Aberle

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Re: Testing Generator Voltage
« Reply #9 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
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