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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Chris Delcambre on September 20, 2013, 02:12:47 pm

Title: 2 bonded neutrals
Post by: Chris Delcambre on September 20, 2013, 02:12:47 pm
Hi there,

I understand alot of electrical stuff and how to hook something up. However I do not understand the workings of electricity. Can there be 2 bonded neutrals in a system???
Example:
My main portable power distro has a bonded neutral and then I connect it to a generator that also has a bonded neutral. What would the result be????

Thanks,
Chris
Title: Re: 2 bonded neutrals
Post by: TJ (Tom) Cornish on September 20, 2013, 03:29:50 pm
Hi there,

I understand alot of electrical stuff and how to hook something up. However I do not understand the workings of electricity. Can there be 2 bonded neutrals in a system???
Example:
My main portable power distro has a bonded neutral and then I connect it to a generator that also has a bonded neutral. What would the result be????

Thanks,
Chris
The result is any ground conductors before the second bond (your distro) potentially become current-carrying - there are effectively two neutral wires returning current - the neutral, and the ground.  The amount of current flowing on each of these wires will be in proportion to the path resistance, and in the event of a neutral wire failure, all return current would be on that ground wire.

Your portable distro should not have a ground-neutral bond unless that's the only bond in the system.
Title: Re: 2 bonded neutrals
Post by: Rob Spence on September 20, 2013, 03:56:28 pm
Hi there,

I understand alot of electrical stuff and how to hook something up. However I do not understand the workings of electricity. Can there be 2 bonded neutrals in a system???
Example:
My main portable power distro has a bonded neutral and then I connect it to a generator that also has a bonded neutral. What would the result be????

Thanks,
Chris

There can as you say in your setup (and as we see around here, often does) but there should NOT be!

Sub panels should not bond neutral to safety ground.
Since the generator has the main panel (think of it as the service panel), the bond should only be there.

You should remove the bond in your sub panel (distro).

Edit: clarify the wording

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Title: Re: 2 bonded neutrals
Post by: John Roberts {JR} on September 20, 2013, 04:09:19 pm
Indeed it is incorrect to short ground to neutral in multiple places. I once read about an unusual (studio) wiring scheme where a local independent ground system was tied to neutral at some distance from the main panel (trying to clean up studio ground noise). I sent an email to that author questioning the safety of that wiring and he never responded to me.

The major concern about commingling neutral and ground paths is that a loose or open connection of both back to the generator could make your distro ground and any plugged in product chassis grounds hot....  if ground was not connected to neutral at your sub panel, the hot neutral would be relatively harmless to humans in the area.

Admittedly this anticipates multiple faults at the same time but in life stuff happens. 

JR

Note: I am not an electrical wiring expert, so take my advice with a gain of salt. but i do grok how the electrons can misbehave.
Title: Re: 2 bonded neutrals
Post by: Mike Sokol on September 20, 2013, 04:16:46 pm
It can (and as we see around here, often does) but should NOT!

Sub panels should not bond neutral to safety ground.
Since the generator has the main panel (think of it as the service panel), the bond should only be there.
Yes, that's correct according to the NEC. However, be aware that many (if not most) smaller (under 5 KW) inverter generators have a floated G-N bond. That includes all modern Yamaha and Honda generators up to 5,000 watts. In that case, you'll want to create a G-N bond in your distro (think of it as a sub-panel).

Now, this G-N bond doesn't have to be a complicated thing. Here's an article I wrote specifically for RV owners who want to run their trailers and motor homes from a Honda/Yamaha portable generator. Since those generators have a floated neutral, and their RVs have isolated neutrals, then adding a simple jumper plug at the generator itself is an easy and safe solution. See http://www.noshockzone.org/generator-ground-neutral-bonding/ for the article.

So, if you isolate the G-N bond in your distro (it's supposed to be isolated anyways for connecting to building power that DOES have a G-N bond), then that will work as-is with any large generator (over 5 KW) which should already have a grounded neutral. However, if you're plugging into a 3KW Honda Inverter generator, just stick your G-N bonding plug in the unused generator outlet, and it will provide the neutral to ground bond.

How to tell if your generator has a G-N bond? Just plug in a simple 3-light cube tester and see if it shows a proper ground. If it shows open ground, then plug in your G-N bonding plug and try it again.
Title: Re: 2 bonded neutrals
Post by: Jonathan Johnson on September 21, 2013, 01:22:24 am
The result is any ground conductors before the second bond (your distro) potentially become current-carrying - there are effectively two neutral wires returning current - the neutral, and the ground.  The amount of current flowing on each of these wires will be in proportion to the path resistance, and in the event of a neutral wire failure, all return current would be on that ground wire.

Your portable distro should not have a ground-neutral bond unless that's the only bond in the system.

In this scenario (where the generator has bonded neutral and ground) you have two options:

It may be an option to install an SPDT switch to selectively bond your load grounds to either the neutral or the upstream ground connection. Or you could just make up a "bonding adapter" as Mike suggests and use it when needed, leaving the ground and neutral unbonded in the distro.
Title: Re: 2 bonded neutrals
Post by: Jerome Malsack on September 27, 2013, 08:36:07 am
You should look carefully at the switch before you install that because It will need to handle the 20 amps current that can be there.   
Title: Re: 2 bonded neutrals
Post by: Chris Hindle on September 27, 2013, 08:58:11 am
... or It may be an option to install an SPDT switch to selectively bond your load grounds to either the neutral or the upstream ground connection. Or you could just make up a "bonding adapter" as Mike suggests and use it when needed, leaving the ground and neutral unbonded in the distro.

Switches = BAD
Adapters = Useable

A switch can be set to the "wrong" position too easily.
Adapters, while sometimes clunky and able to get "lost", are put in place when needed, and not likely to get installed/removed by accident.
The real question is, will the fire marshall/inspector accept you workaround, or declare it N/G ??
Title: Re: 2 bonded neutrals
Post by: Jonathan Johnson on September 27, 2013, 05:10:43 pm
The real question is, will the fire marshall/inspector accept you workaround, or declare it N/G ??

Always comply with the code enforcement officer. If that is not acceptable, present the code enforcement officer with engineering drawings (from an electrical engineer licensed in the state or jurisdiction where the installation is located) showing your preferred method. Then comply with the code enforcement officer. They will usually go with the engineer's specs, but they do have the power to override the engineer.
Title: Re: 2 bonded neutrals
Post by: Craig Hauber on October 05, 2013, 03:53:18 pm
Yes, that's correct according to the NEC. However, be aware that many (if not most) smaller (under 5 KW) inverter generators have a floated G-N bond.

What's "floated" mean?  Is the ground simply not connected to anything but the metal of the generator housing? 

I have an older style pullstart Generac and haven't actually checked it out but need to use it soon for a homecoming float.  (which brings up a whole other grounding set of questions as the metal framed float trailer is grounded to the vehicle's battery and the whole assembly is on rubber tires.  what happens when generator-run equipment is tied to inverter-run equipment (or car-audio equipment) through the grounds of audio cables?
Title: Re: 2 bonded neutrals
Post by: Phil Graham on October 16, 2013, 05:51:35 pm
What's "floated" mean?  Is the ground simply not connected to anything but the metal of the generator housing?

Craig. On a "floating neutral" generator the ground is only attached to the metal of the generator, and it is specifically design to operate not terminated to earth. Further, there is no neutral - ground bond point on these generators.

I wrote a detailed sidebar on the specific topic of floating neutral generators in an article for a major industry trade rag, I'm happy to send you the text of that sidebar via email if you PM me.

Cheers,