Portable generator bonding queston

[don_resqcapt19]

For services 800 amp or more the neutral must be switched due to GFI protection. For smaller systems the only hazard would be the parallel neutral caused by bonding at the gen. and not switching the neutral. If you lost the neutral from the gen. then you would put unwanted current back on the normally non-current carrying parts. As a hazard to linemen there wouldn't be any as the return current on the generator wants to return to its source.
The real world facts are it is not allowed, as you well know, the same as not allowing the same redundant bonding in sub-panels.

It would be for service disconnectes rated 1000 amps or more for the ground fault protection, but I really doubt we are talking about generators of that size in this thread.
As far as the unwanted current on normally non-currrent carrying parts, the code requires us to create that type of situtation for services, so I don't see a real world issue with this application....just a code issue.

 

[kwired]

For services 800 amp or more the neutral must be switched due to GFI protection. For smaller systems the only hazard would be the parallel neutral caused by bonding at the gen. and not switching the neutral. If you lost the neutral from the gen. then you would put unwanted current back on the normally non-current carrying parts. As a hazard to linemen there wouldn't be any as the return current on the generator wants to return to its source.
The real world facts are it is not allowed, as you well know, the same as not allowing the same redundant bonding in sub-panels.

Not only would it be for 1000 amp services or more but is also only required for solidly grounded wye sytems operating over 150 volts to ground.

 

[joeba]

I didn't ask what are the code rules.
I asked what is the real world hazard when a portable generator has its neutral conductor bonded at the generator and is conneted to the building wiring system with a transfer switch that does not switch the grounded conductor?

This came up about 5 years ago in an internet newgroup and this is the best explanation of a potential hazzard of using a nbg generator with a transfer switch that does not switch the neutral...

Question:
The transfer switch I have (Connecticut Electric 10-7501) appears not to switch
neutral. My B&S generator, bonds the neutral to the ground, as do most portable
generators. For reasons that I don't quite understand, this is not a safe combo
since neutral is grounded in 2 places, the service panel and the generator.

Answer:
If you are going to use it only for power failure and unplug it the rest
of the time then you don't need to make any change in the generator (unbound the neutral). If
it will be left connected to the transfer switch nearly all of the time
then you do have to have a means to open the bond when you leave it
connected to the house when using the utilities power.

The reason that having a bond on the load side of the service
disconnecting means is a bad thing is that in the event of a failure of
continuity of the neutral conductor the neutral current would be flowing
on the exterior conductive surfaces of appliances, metal face plates,
metal framed tools, raceway, etc.. A second failure in the continuity
of the equipment grounding (bonding) conductor would raise the surface
of all conductive parts of the system to the applied circuit voltage.

Remember that utility power cannot return to the home until the transfer
switch is thrown back to public power if it is a whole house switch.
If it is a circuit by circuit switch assembly then if the two defects
are present the generator remaining connected would complete a fault
path to the normally dead conductive surfaces of the electrical system.
If you shut down and disconnect the generator prior to throwing the
transfer switch then there will be no danger. The danger can only arise
with the generator connected with public power on and two separate
defects in the wiring of of a circuit that is supplied through the
transfer switch.

The "Service Disconnecting Means" is the first disconnect of any kind
between the utility supply and the homes wiring. It can take the form
of between one and six switches, fuse pullouts, circuit breakers or a
combination of the three.

The Main Bonding Jumper" is the conductor that connects the grounded
current carrying conductor, that most of us call the neutral, to the
Service Equipment enclosure and the Equipment Grounding Conductors. The
only place the "Main Bonding Jumper" is supposed to be connected is in
the Main Disconnecting Means enclosure/s. The "Grounding Electrode
Conductor/s" connect the grounded current carrying conductor to the
grounding electrode system.

If the neutral is bonded or faulted to the Equipment Grounding Conductor
of a circuit and the neutral of that circuit fails open between that bonding point and
the main bonding jumper andthe Equipment Grounding Conductor is also open or high resistance
between the bonding point and the main bonding jumper then the surface of normally deenergized conductive parts of the circuit in question will develop a touch potential that is equal to the supplied
voltage of the circuit in which all of of those faults exist.
The Situation is unlikely but because it is possibly fatal it is thought
to be worth guarding against by keeping the neutral and the EGCs
insulated from each other at all points on the load side of the Service
Disconnecting Means.

 

[don_resqcapt19]

This came up about 5 years ago in an internet newgroup and this is the best explanation of a potential hazzard of using a nbg generator with a transfer switch that does not switch the neutral...

Question:
The transfer switch I have (Connecticut Electric 10-7501) appears not to switch
neutral. My B&S generator, bonds the neutral to the ground, as do most portable
generators. For reasons that I don't quite understand, this is not a safe combo
since neutral is grounded in 2 places, the service panel and the generator.

Answer:
If you are going to use it only for power failure and unplug it the rest
of the time then you don't need to make any change in the generator (unbound the neutral). If
it will be left connected to the transfer switch nearly all of the time
then you do have to have a means to open the bond when you leave it
connected to the house when using the utilities power.

That is what happens with most portable generators used to power a building.

The reason that having a bond on the load side of the service
disconnecting means is a bad thing is that in the event of a failure of
continuity of the neutral conductor the neutral current would be flowing
on the exterior conductive surfaces of appliances, metal face plates,
metal framed tools, raceway, etc.. A second failure in the continuity
of the equipment grounding (bonding) conductor would raise the surface
of all conductive parts of the system to the applied circuit voltage.

Where do you draw the line? Two different failures, the failure of the EGC and a hot to case fault results in the same hazard. Are these two fault more unlikely than the failure of both the grounded conductor and the EGC?

Remember that utility power cannot return to the home until the transfer
switch is thrown back to public power if it is a whole house switch.
If it is a circuit by circuit switch assembly then if the two defects
are present the generator remaining connected would complete a fault
path to the normally dead conductive surfaces of the electrical system.
If you shut down and disconnect the generator prior to throwing the
transfer switch then there will be no danger. The danger can only arise
with the generator connected with public power on and two separate
defects in the wiring of of a circuit that is supplied through the
transfer switch.

In general the electrical system is not designed to be safe with more than one fault. Multiple faults almost always result in a safety issue.

The "Service Disconnecting Means" is the first disconnect of any kind
between the utility supply and the homes wiring. It can take the form
of between one and six switches, fuse pullouts, circuit breakers or a
combination of the three.

The Main Bonding Jumper" is the conductor that connects the grounded
current carrying conductor, that most of us call the neutral, to the
Service Equipment enclosure and the Equipment Grounding Conductors. The
only place the "Main Bonding Jumper" is supposed to be connected is in
the Main Disconnecting Means enclosure/s. The "Grounding Electrode
Conductor/s" connect the grounded current carrying conductor to the
grounding electrode system.

If the neutral is bonded or faulted to the Equipment Grounding Conductor
of a circuit and the neutral of that circuit fails open between that bonding point and
the main bonding jumper andthe Equipment Grounding Conductor is also open or high resistance
between the bonding point and the main bonding jumper then the surface of normally deenergized conductive parts of the circuit in question will develop a touch potential that is equal to the supplied
voltage of the circuit in which all of of those faults exist.
The Situation is unlikely but because it is possibly fatal it is thought
to be worth guarding against by keeping the neutral and the EGCs
insulated from each other at all points on the load side of the Service
Disconnecting Means.

I still don't see these two faults as any more likely than having an ungrounded condutor fault to a normally non-current carrying part and at the same time having an open EGC.