| Author |
Message |
CoryDane RTSII (66.155.188.245)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, March 04, 2004 - 8:33 pm: | |
I was looking over the manual for a Trace SW4024 and glanced at your bus wiring drawing and have come up with a question.
The manual, in the pre-configuring section, states that the BONDING between the Neutral and Ground should be from the main source of power that is the ac distribution of the Generator, Utility Grid or if none of these, in the inverter AC distribution.
Looking at your scheme, you seem to have made your bond in the RYI relay but when the inverter is on, your bond switches open.
I might be confused but the Trace Manual seems to say that the BOND is required for the inverter protective circuits to work properly but you will have none.
Also, in your subpanel, you show line two with an isolated neutral after the bond is broken.
Could you (or anyone else) explain this a little for me.
Thanks
"Imagine Your Dreams"
cd |
Sean Welsh (Sean) (64.81.73.194)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, March 04, 2004 - 10:04 pm: | |
Cory,
I think you're misreading the print -- RY1 is on the INPUT side of the SW, not the output. The bond is broken when external power is applied, such as from the shore cable. This is correct, per the Trace manual.
Where Scott has a problem, though, is that his genny neutral is connected full-time, which means he will need to (1) remove the ground/neutral bond in the genny and (2) fix RY1 so it only breaks the bond on shore input, not genny. Or else, he can go to a transfer switch that switches neutral as well as hots -- that way, RY1 will break the bond correctly when either genny or shore is connected, and no double-bonding will occur.
-Sean |
ThJerry Liebler (165.121.35.163)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, March 05, 2004 - 12:49 am: | |
Sean,
The current National Electric Code requires that ground bonding be done on each source ahead of the first disconnect means. Removing the bond in the generator violates the code. the transfer switch must switch neutral.
Regards
Jerry |
CoryDane RTSII (66.155.188.178)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, March 05, 2004 - 1:49 am: | |
Sean
I am looking at the manual for the Trace SW4024 and they have some drawings, GenSet, Inverter, Ac source. The drawing show the neutral HARD connected at all locations, that is, they are bolted in place, not switched out.
I remember the big discussion about how the neutrals have to be switched out except for the power supply device. So.... who's right, Trace? or us?
LOL, I will be installing soon so I am studying how to connect it now and I am finding lots of, err, Interesting drawings these days. Just have to decide which way is right.
"Imagine Your Dreams"
cdcd |
Sean Welsh (Sean) (64.81.73.194)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, March 05, 2004 - 2:33 am: | |
Jerry,
Please give me the code section... I can't find it in mine. Thanks.
-Sean |
Sean Welsh (Sean) (64.81.73.194)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, March 05, 2004 - 2:36 am: | |
Cory,
The neutrals are all connected, as you note. It is the connection between the neutral and ground which is being made/broken -- this is because the neutral-ground bond can only be in one place, and, when on shore power, that place is in the shore system.
-Sean |
Craig Smith (Craigs) (65.202.123.254)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, March 05, 2004 - 12:37 pm: | |
Are these discussions taking into consideration the fact that an SW4024 is for a land based installation and the SW4024MC is for a motor coach installation? I'm not sure but I think the MC unit switches the ground for you. Just more to think about.
Craig S |
Sean Welsh (Sean) (64.81.73.194)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, March 05, 2004 - 3:38 pm: | |
Craig,
The only difference between the standard and MC2 models is software (and approval labeling). Neither model includes a ground bonding relay.
For a lengthy discussion of this, see
http://ourodyssey.us/bus-e-trace.html
-Sean |
CoryDane RTSII (66.155.188.136)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, March 05, 2004 - 6:07 pm: | |
Sean, Craig and all others
Thanks, that clears it up, a bit. I recall all these lengthy discussions about this but your explanation is the simplest and best. Now I know what to do. Now I also know why this isnt shown in the book LOL.
"Imagine"
cdcd |
Jerry Liebler (165.121.32.151)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, March 08, 2004 - 2:08 am: | |
Sean,
Section 250 of the 2002 NEC discusses seperatly derived sources. It clearly says each source must bond neutral to ground ahead of the first disconect means.
Regards
Jerry |
Sean Welsh (Sean) (64.81.73.194)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, March 08, 2004 - 3:23 am: | |
Absolutely -- however, if a generator is not a separately derived source, because it is part of a system supplied by other conductors (such as shore power), then, in fact, it must not have a bond.
Code inspectors (if bus conversions were even subject to code inspection) could go either way on this -- if you have included a bond elsewhere in your system because of multiple feed issues, it is certainly safe to wire it this way (provided the generator neutral is hard-wired all the way to the bond, and the generator and any intermediate panels are clearly labeled to this effect). The intent of 250.20 on this matter is to ensure that the bond exists and is close enough to the source to safely carry any fault current back to the breaker.
Section 250.34 is worded very carefully to avoid implying that ALL vehicle-mounted generators are separately derived systems, allowing room for installations that can't work any other way. For example, a vehicle-mounted generator in a system that is also fed from utility power via a shore-cord setup, where both power sources are intended to operate together simultaneously.
This becomes an issue if you have, for example, a Trace SW4024 or SW2512 and you want to use the built-in 60-amp transfer switch. The SW series have internally ganged neutrals, so the only way to safely connect both a shore source and a generator source to this device, and have it do the transfer internally, is to have an unbonded generator set, and a bond relay that lifts when shore is connected. The SW series are UL-listed "multiply fed" devices -- connecting shore, generator and inverter together like this mandates that the generator not be a "separately derived system" as that is defined in 250.20.
250.20(D) FPN1 covers this case explicitly for fixed-installations, but, of course, is not applicable for a vehicle-mounted application because the "service-supplied system" is not "solidly interconnected," leaving those of us designing vehicle-mounted applications to establish through engineering (as that term is used in the code) that the grounding requirements of the code have been met in our implementations.
This is an interesting dilemma, in the true sense of that word, because such "engineering" in a fixed installation, to meet code requirements in most jusridictions, would require the stamp of a PE, however, the fixed installation case is explicit in the code and no engineering is required. By contrast, the mobile installation, which ought to require some sort of blessing by someone with the qualifications to declare it safe, is not subject to any sort of regulatory approval in most jurisdictions (unless being implemented by a "manufacturer")
I have no qualms whatsoever about using a switchable ground bond in a mixed shore/generator/inverter installation so long as it is electrically impossible for ground and neutal to become unbonded at any time, and any unbonded generator is so labeled. I believe such an installation to be in full compliance with 250.20 of the '02 code.
Incidentally, by the way, a more common problem with relay-bonding arrangements is that the shore cord is connected, but the shore line is not hot (e.g. due to breaker being off at the pole, etc.). Since most such designs lift the bond upon detection of live power at the shore inlet, such an installation will actually have a double bond. This is also forbidden by the code, and can be dangerous in its own right, though clearly this is less of an issue than a bond that is inadvertently missing altogether.
-Sean |
Jerry Liebler (165.121.34.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, March 09, 2004 - 12:15 am: | |
Sean,
I respectfully dissagree with your compliance assessment.
While the trace 4024 you keep refering to may be UL listed If I were King it'd be declared in violation of the code, it's transfer switch should switch the neutral as many others do (even Tripplite got this right). I do think this unsafe product will sooner or later result in a big product liability suit. A generator, even a synchronus one, is as much a seperatly derived source as a transformer which is the more common use of this section.
Why not do it right rather than the creative mickey mouse reinterpet the code work around for a deficient product. If every transfer switch switches neutral every source can have it's neutral bonded to a common ground in full compliance with the letter of the code, with no hazardous ground bonding relays.
Regards
Jerry |
Sean Welsh (Sean) (64.81.73.194)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, March 09, 2004 - 1:08 am: | |
Jerry,
If part of your installation is a feed-through inverter (any inverter, irrespective of whether or not it has a correctly designed transfer switch), you still need a relay to bond ground to neutral in the event that no other source (generator or grid) is connected. Granted, a well designed inverter will include this as an internal component, rather than forcing you to install it externally as do the SW units. I don't see any way to avoid this -- could you please explain your reasoning?
I could not agree with you more that Trace ought to have gotten this right and included both an internal bonding relay as well as a transfer switch that switches both hot and neutral. But that point is moot in the case where an SW4024 is already part of the installation, as I believe is the case with the system we were originally discussing. I do consider this to be a serious deficiency of the SW product, and therefore I generally recommend professional installation of this unit to ensure the implementation is a safe one. That being said, I own one myself, because it has many other things to recommend it.
With regard to your statement that "a generator, even a synchronus one, is as much a seperatly derived source as a transformer..." the code does not dictate that. It merely stipulates different requirements when one is separately derived vs. when one is not. It is far, far more common for backup generators to be integral, rather than separately derived. Very few large, 3-phase backup generators, for example, are connected through neutral-switching transfer gear.
I am generally the first person to jump all over someone when they violate the code. That having been said, as you well know, the code does not and can not contemplate and cover every possible scenario that can be built -- the code by itself is not a substitute for application of good judgement.
respect,
-Sean
p.s. How's your coach coming along? |
Sam Sperbeck (204.248.119.254)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, March 09, 2004 - 4:48 pm: | |
Hi Jerry and Sean,
As yet, I don't have an inverter so I have no personal knowledge of them, but I have read of some inverters having the capability of supplementing shore power. If you are pluged in to a 15 amp GFCI outlet and the inverter is on, it can provide power if you exceed the 15 amps you are hooked up to. In that case, since you have two sources of power, would both the sources have to have a neutral/ground bond? If they do, what keeps the GFCI from triping when the inverter tries to supplement the shore power? This electrical stuff can be SO confusing!
Thanks, Sam Sperbeck
La Crescent,MN |
Sean Welsh (Sean) (64.81.73.194)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, March 09, 2004 - 5:09 pm: | |
Sam,
When the inverter is connected to grid power via the shore cord, the gound-to-neutral bond is in the shore power system (likely at the transformer powering the campground) and must not be in the coach itself.
When the shore power is disconnected, a ground-to-neutral bond must be made inside the coach. Many inverters do this automatically (by means of an internal bonding relay), however some models do not, and it is up to the installer to add this capability. The Xantrex/Trace SW series is one example (and this applies, by the way, to both the household and RV-specific models) of a unit that requires an external bonding relay.
-Sean |
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