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generators and neutral current

R

Russell

Jan 1, 1970
0
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
 
R

Rowbotth

Jan 1, 1970
0
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.
 
J

John C

Jan 1, 1970
0
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
 
S

SQLit

Jan 1, 1970
0
Russell said:
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?

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.
 
Russell said:
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,
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.
 
G

Gerald Newton3

Jan 1, 1970
0
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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