generators and neutral current

Hi all,
I may soon be installing a generator panel. Fact is, I've never seen one, much less installed one.
Rumor has it that during the ice storm of ???1998???, hydro workers were
shocked due to potential on the neutrals which resulted from generators hooked up to houses where the main breaker was turned off and therefore the hot wires were not energized but the neutral was not disconnected and therefore had potential (wheww, run on sentence....). Problem is, I don't understand how this could occur. Since the neutral must be grouded at the service, it cannot have potential. With the neutral tied to ground this becomes the reference voltage against which all other ungrounded conductors are measured.
I gather that generator panels disconnect the mains but not the neutral.
Two possible explanations come to mind: 1) there's no truth to the rumor 2) inadequate grounding leads to potential on the neutral
If 2) is correct,how would one best test for, and ensure, adequate neutral grounding?
Cheers, Russell Wardman
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In Canada, I believe it is illegal to disconnect the neutral conductor.
(It sounds like so much organic fertilizer to me. Go ahead and do the installation properly.)
H.

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| In Canada, I believe it is illegal to disconnect the neutral conductor.
It would be unsafe not to in certain situations. If the generator has its own ground, then a solid neutral across a transfer switch (which must have a solid ground across it anyway) results in a duplicate path for neutral currents going over ground.
The NEC allows disconnecting the neutral (not that Canada would have to follow that) but does require the connection and disconnection sequence ensure that the ungrounded conductors are connected last and disconnected first (to ensure no brief open neutral scenario).
See figure 7 on page 7 of [PDF format document]:
http://www.mikeholt.com/documents/powerquality/grounding.pdf
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| I may soon be installing a generator panel. Fact is, I've never seen one, | much less installed | one. | | Rumor has it that during the ice storm of ???1998???, hydro workers were | shocked due | to potential on the neutrals which resulted from generators hooked up to | houses where the | main breaker was turned off and therefore the hot wires were not energized | but the neutral | was not disconnected and therefore had potential (wheww, run on | sentence....). | Problem is, I don't understand how this could occur. Since the neutral must | be grouded at | the service, it cannot have potential. With the neutral tied to ground this | becomes the | reference voltage against which all other ungrounded conductors are | measured.
There is an old saying that goes like this:
"Electricity follows the path of least resistance"
*ding* False!
Electricity follows all paths. It just follows each in inverse proportion to the impedance.
If the generator, or the service entranced from the generator, has neutral bonded to the grounding conductor or a ground electrode, then any contact between the neutral and ground forms a new path back to the generator. If the intended path is very low impedance, then the higher combination of people and ground impedance will have very little return current. However, if some faulty condition exists, such as an open neutral, then the people plus ground path could be the only path, or at least the most substantial path, back to the source.
This kind of problem happens very frequently with neutrals from utility sources. The same thing can happen with a generator. And it can be worse because people are expecting no power when the utility is off.
| I gather that generator panels disconnect the mains but not the neutral.
Many are like that. And doing so means the generator is *not* considered to be a separately derived system. And a system is supposed to have just one bond between neutral and ground. But often there are two (one is at the generator or its entrance) and that leads to neutral currents going back to the selected source going back over both neutral paths from the switch, and then crossing between them at the grounding wire.
| Two possible explanations come to mind: | 1) there's no truth to the rumor | 2) inadequate grounding leads to potential on the neutral
There is no such thing as grounding that is adequate enough to eliminate potential on the neutral. The best you can do is have a sufficiently low impedance to bring ground and neutral near equipotential. But when earth ground is part of that, all bets are off.
Here's some fun reading [PDF format]:
http://www.mikeholt.com/documents/strayvoltage/pdf/MultiGroundedNeutralFinal5-3-03.pdf
| If 2) is correct,how would one best test for, and ensure, adequate neutral | grounding?
I would go with a switched neutral on the transfer switch. Each source would be separately grounded, and ONLY the grounding wires would be connected between systems (not the neutral). The code requires that such a switch ensure that with making contact, the ungrounded lines be connected last, and when breaking contact, the ungrounded lines be disconnected first (to ensure you don't have a momentary open neutral scenario).
If you are exceptionally paranoid, then don't source a neutral at all, and have a transformer after the transfer switch where you get your neutral from (and it gets bonded to the grounding conductor between the secondary winding and the secondary disconnect).
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Questions similar to yours have appeared at this ng many times. Answering your questions can be helped by reviewing some grounding drawings.These 17 drawings cover most of the problems. These drawings are from a 24 year old document. Try: http://www.electrician2.com/grounding/grounding.htm

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I have an imediate concern when reading this article.
IEE regs ask for complete disconnection from supply ( phase and neutral ) and a connection for the gen set from a changeover type arrangement.
The other statement about utility workers being shocked from 'live' neutrals happens all to frequently. That is why when working live we use PPE and when not live we short and earth the conductors.
Sorry for the rant....
JC
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These drawings are from the IEEE 1980 Orange Book on Emergency and Standby Power (ANSI/IEEE Std 446-1980.) I was reluctant to put this drawings online for many years (for fear of copyright infringement), but am finding a complete lack of understanding of how generators are supposed to be grounded when transfer switches are used. I have used these drawing many times to correct engineer's and electrician's mistakes in the field. I have seen many commercial and industrial systems grounded according to these drawings. Our local utility has gone to great lengths to produce specific drawings for standby power for residential systems that require that the grounded conductor be transferred with he transfer switch. This is not the case with many commercial and industrial systems. The key is the definition of a Separately Derived Alternating-Current Systems. If the neutral is not transferred the generator is not a separately derived system and the bonding of the grounded conductor at the generator or at the generator first disconnect is not done to prevent objectionable fault current from taking parallel paths as illustrated in drawing Fig 64. However, the generator frame is always grounded. I have seen it done both ways in the field. Sometimes the neutral is transferred and sometimes it is not. The drawings are for 3-phase 4-wire systems but the same rules apply to premised 1-phase 3 wire systems in which case we would be talking about a 3 pole transfer switch for transferring the grounded conductor (neutral) and a two pole transfer switch where the neutral or grounded conductor is not transferred. I SHOULD POINT OUT THAT MANY UTILITY COMPANIES REQUIRE THAT THE GROUNDED CONDUCTOR OR NEUTRAL BE TRANSFERRED. IT IS RECOMMENDED THAT ANYONE INSTALLING A STANDBY GENERATOR THAT IS GOING TO BE CONNECTED TO A UTILITY SUPPLIED PREMISE WIRING SYSTEM CHECK WITH THE LOCAL UTILITY FOR THEIR REQUIREMENTS BEFORE INSTALLING THESE SYSTEMS!!!!!!!!!!!!!!!!
Ref: 2002 NEC
250.20(D) Separately Derived Systems. Separately derived systems, as covered in 250.20(A) or (B), shall be grounded as specified in 250.30. FPN No. 1: An alternate ac power source such as an on-site generator is not a separately derived system if the neutral is solidly interconnected to a service-supplied system neutral. FPN No. 2: For systems that are not separately derived and are not required to be grounded as specified in 250.30, see 445.13 for minimum size of conductors that must carry fault current.
250.6 Objectionable Current over Grounding Conductors. (A) Arrangement to Prevent Objectionable Current. The grounding of electrical systems, circuit conductors, surge arresters, and conductive non-current-carrying materials and equipment shall be installed and arranged in a manner that will prevent objectionable current over the grounding conductors or grounding paths.
250.4(B)(4) Path for Fault Current. Electrical equipment, wiring, and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit from any point on the wiring system to the electrical supply source to facilitate the operation of overcurrent devices should a second fault occur on the wiring system. The earth shall not be used as the sole equipment grounding conductor or effective fault-current path.

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must
this
There is truth to the issue that is why most utilities I have worked around require complete disconnection, commonly called "islanding". 20+ years ago we installed for an commercial building using 3 phase an 3 pole transfer switch. About 10 years ago there were 4 pole transfer switches coming to market. (reduce number by one for single phase). They disconnect the neutral and work (in my experience) a whole lot better for sensitive electronics.
You are assuming that your ground is the lowest ohm path to ground. That is not necessarily true. The lowest ohm path to ground could be some distance away. One of the reasons I test all ground systems before doing any additional work on the electrical system. I have seen older grounds that were close to infinity, (no ground). I found one set of transformers in an central plant for an large business that had no relationship to ground at all. These transformers were installed in the middle 1950's.
Call your serving utility and ask the regs before you proceed.
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Russell wrote:

I can't say whether the rumors are true or not. Older premises systems weren't grounded, and connecting a generator to such a system could energize the neutral with respect to ground.
A "hydro worker" getting shocked, however, means he/she wasn't qualified or made a mistake. Following proper procedures keeps them safe, regardless ot the wiring system and/or violations/ wiring errors/misuse/whatever that exist on the premises.
So, if a "hydro worker" did indeed get shocked, the problem is more than a generator connected to a non-disconnected neutral. I put "hydro worker" in quotes because I don't know if the power utility sent out unqualified persons as part of the manpower doing the repairs.

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