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Author Topic: Testing Generator Voltage  (Read 2550 times)

Mike Karseboom

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Re: Testing Generator Voltage
« Reply #10 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?
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Tim McCulloch

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Re: Testing Generator Voltage
« Reply #11 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.
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Stephen Swaffer

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Re: Testing Generator Voltage
« Reply #12 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!
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Mike Sokol

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Re: Testing Generator Voltage
« Reply #13 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.   

Mike Karseboom

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Re: Testing Generator Voltage
« Reply #14 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? 

« Last Edit: May 22, 2014, 11:09:00 am by Mike Karseboom »
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Tim McCulloch

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Re: Testing Generator Voltage
« Reply #15 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.
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BobWitte

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Re: Testing Generator Voltage
« Reply #16 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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Stephen Swaffer

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Re: Testing Generator Voltage
« Reply #17 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.
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Jonathan Johnson

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Re: Testing Generator Voltage
« Reply #18 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.
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Mike Sokol

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Re: Testing Generator Voltage
« Reply #19 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.   
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