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Chris Peters (Chris_85_rts)
Registered Member
Username: Chris_85_rts
Post Number: 6
Registered: 11-2006
Posted From: 68.205.99.121
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 06, 2007 - 10:34 pm: | |
So the SW4024 thread below sure brought out alot of passion on this issue. I thought I had a complete understanding of the issue and still think I do, but I wanted to investigate further. I joined a web forum which is for electricians, contractors, inspectors and generally people in the electrical trade.
I posted a simplified version of the discussion which I show below:
quote:I was under the assumption that Neutral and Ground should be bonded in only 1 place, and that place is the source of power. However, most RV's can have three power sources, 1) Shore Power, 2) Genset Power and 3) Inverter Power. (How you switch between them is not at issue here yet.)
1) Shore Power. With out a doubt it is agreed that when connected to shore power, neutral/ground is bonded at the shore panel, not inside the RV. You are in effect a sub panel. Your breaker panel in the RV has isolated neutral and ground.
2) Genset Power. I used to think this was clear cut, but recent discussion has clouded the issue. For simplicity sake, suppose the shore cord (4 conductor L1-L2-N-G) is unplugged from the shore power and plugged into the genset. Some contend that in this case you should not bond neutral and ground, and others contend you must. They fire NEC sections at each other and then start saying that in 2005 things changed or were clarified.
3) Let's not even go here. Whew.
So, let's focus on number 2 if we can. Can anyone say with certainty which way it should be? It sure would be nice if I could refer people to certain text which would support one method over the other. This seems to be a very passionate issue that keeps coming up over and over again.
So far things are leaning toward agreeing with my scenario as presented. How you make or break the various connections is a separate topic altogether as long as it accomplishes the correct bonding.
I encourage anyone still interested in this topic to read the thread here:
http://www.mikeholt.com/code_forum/showthread.php?p=652251 |
Jerry Liebler (Jerry_liebler)
Registered Member
Username: Jerry_liebler
Post Number: 241
Registered: 3-2005
Posted From: 67.140.170.107
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 06, 2007 - 11:48 pm: | |
For the case 2 above the neutral to ground bond should be at the generator, before any circuit breakers. The cord to the RV panel (really still a sub panel) when pulgged into either source gets the neutral bond from the source ONLY. Nothing changed in the code, it was clarified however. BTW The fact that the SW inverters do not switch the neutral in their internl transfer switch means external contactors are needed if both generator and shore are going to be wired to the inverter.
Regards
Jerry 4107 1120 |
Richard D. Bishop (Rdbishop)
Registered Member
Username: Rdbishop
Post Number: 8
Registered: 1-2007
Posted From: 24.243.23.195
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 07, 2007 - 12:47 am: | |
Would that mean,contactors before the inverter? |
James Stacy (Jimstacy)
Registered Member
Username: Jimstacy
Post Number: 31
Registered: 1-2001
Posted From: 75.14.4.87
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 07, 2007 - 10:48 am: | |
Just to throw some more dust into this discussion, (and I don't have my NEC book with me) do I remember correctly that ground connections (as opposed to neutral) were not to go through a contact?
Both transfer relays and converters depend on a relay contact for grounding.
I don't see an obvious way to acomplish this without running the genset and converter each to their dedicated receptacle and manually plugging in the RV power cord. Relays are much more convenient and passive. Hopefully my memory is clouded (again). |
Jerry Liebler (Jerry_liebler)
Registered Member
Username: Jerry_liebler
Post Number: 242
Registered: 3-2005
Posted From: 67.140.170.107
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 07, 2007 - 12:08 pm: | |
Richard,
Yes, the only way to, in compliance with the code, to use both inputs to an SW inverter is to have a 2 pole contactor ahead of the inverter's AC1 (shore) and AC2(generator) inputs and the coresponding neutral wires. These contactors must be 'interlocked' so only one is closed at any time. In addition a neutral bond, to ground, must exist at the inverter whenever niether contactor is closed. The bond, when no contactor is closed, is done through normally closed contacts of an appropriatly rated relay that is energised whenever either contactor is closed. I did the interlocking of the contactors and the selection of the power source for the neutral bond relay with a 4pdt relay. Depending on how one wires the 4pdt relay one can have either shore or generator priority but generator priority is desireable to fully exploit the inverter's features.
Regards
Jerry 4107 1120 |
Jerry Liebler (Jerry_liebler)
Registered Member
Username: Jerry_liebler
Post Number: 243
Registered: 3-2005
Posted From: 67.140.170.107
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 07, 2007 - 12:14 pm: | |
James,
In my transfer switch/inverter installation, ground is never through a contact, however, neutral runs through a contact to connect to ground.
Regards
Jerry 4107 1120 |
Duane Kaler (Duane)
Registered Member
Username: Duane
Post Number: 24
Registered: 5-2006
Posted From: 69.19.14.31
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 07, 2007 - 5:50 pm: | |
Jerry,
Is it possable to get a diagram and relay part numbers of your design.
Thanks, Duane |
Sean Welsh (Sean)
Registered Member
Username: Sean
Post Number: 523
Registered: 1-2003
Posted From: 67.142.130.41
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 07, 2007 - 9:52 pm: | |
We seem to go around and around on this topic, even though the code (if you know your way around it) is clear: every system must have exactly one ground-neutral bond. Multiple bonds are dangerous, and no bond at all is even more dangerous.
BTW, Chris, I made extensive use of Mike Holt's site during my design process.
Jim Stacy: As Jerry says, you can make a safe and code-compliant installation using relays or contactors, provided that you DO switch the neutrals, but DON'T switch grounds. You are correct that all grounds must be continuous, uninterrupted, and unswitched. (Well, OK, there is an exception for cord connections; obviously, the ground wire disconnects along with the rest of the cord.)
I would add a further recommendation on top of Jerry's regarding contactors ahead of the SW (or just building an ATS, for that matter): Get contactors that are mechanically interlocked. They also need the electrical interlock that Jerry describes, but the mechanical one protects you in the event that one or more contacts "welds" closed on one of the contactors.
-Sean
http://OurOdyssey.US |
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