Generator Bonding

Status
Not open for further replies.
EEC said:
The generator is an emergency source that is tapped into two 400 amp service rated disconnects. Then from there too two ATS, one for elevator 3 phase solid nuetral ATS and the other 4 pole switched neutral ATS for building emergency loads, ie lighting etc.

Whats wrong with bonding at the two disconnects and not at generator?
Click to expand...
And​
RUWired said:
In the situation with (1) switched neutral and (1) solid neutral, The only bonding taking place will be at the main service. The ground and neutral are seperated even at the (2) generator disconnects. Otherwise you'll have parallel paths on the ground wire with the solid neutral feeder. The fault current path with the switched neutral will flow back to the main service bond and then continue back to the generator via the neutral of the solid neutral feeder and trip the generator breaker on that feeder of the fault.
Rick
Click to expand...

EEC, Aside from the generator source are the ATS?s ?served? (utility) or ?fed? (premises distribution)?
 
I should add that if the elevator feed is only a 3-wire, no neutral, then a bonding jumper in both disconnects would be required because there would'nt be a return fault current path.
Rick
 
EEC said:
The generator is an emergency source that is tapped into two 400 amp service rated disconnects. Then from there too two ATS, one for elevator 3 phase solid nuetral ATS and the other 4 pole switched neutral ATS for building emergency loads, ie lighting etc.

Whats wrong with bonding at the two disconnects and not at generator?
Click to expand...

I just spun out :-?

You either have 2 sources with 3 feeders, or 2 sources with 1 feeder & 1 service, or 2 sources with 2 feeders?

You have to re-establish ground and bond the neutral at a SDS, ?at any point on the SDS from the source to the first system disconnecting means?, 250.30. So your ATS that opens the neutral would have to bond, but you have a 400A disconnect ahead of it?

You cannot bond ?on the load side of the service disconnecting means or on the load side of a SDS disconnecting means?, 250.142(B). So your ATS that does not open the neutral has to have a neutral separate from the EGC.
 
RUWired said:
The question here would be, can the neutral of one feeder be used as the fault current return path of another feeder.

View attachment 2117
Click to expand...

Rick, nice drawing thanks! Hopefully ECC will comment with your visual help.

The 400A disconnect for the SDS would have to be bonded and grounded to an electrode as per 250.30 as well, just like the service disconnect, so the neutral (grounded conductor) and EGC are one-in-the-same. The fault current will travel to source on the neutral & EGC that is with the circuit for the GFI breaker.

So I don?t see the neutral of a different feeder being used as the fault path but it will parallel the fault current until the circuit opens.
 
The problem with having two different types of transfer switches is., If you install the bonding jumper because of the switched neutral, you create a parallel path on the ground in the non switched feeder. In good measure, a switched neutral transfer switch should be installed in the other feeder. But due to the economy, the solid neutral switch is used. Does that make it a poor design or a code violation.
Rick
 
It will be a violation if the bonding happens at the 400A disconnects. If both of these systems were SDS?s 250.30 still requires both to bond somewhere from the source to the first disconnect. The returning neutral current (not just fault current) will travel on non-current-carrying items through the bond at the service disconnect (other source), this returning current will be dangerous touch voltage.

But bonding at the generator source and NOT the first 400A disconnect I believe would be code compliant. In this case the systems can remain as: one SDS, and one solidly grounded.

If there was no SDS and if this was not a sole source no bonding or re-establishing the ground would be required because these items would exist through the other source and the equipment grounding (EGC). In other words they would simply be 5W circuits (A, B, C, N, & GR) instead of 4W (A, B, C, N) where bonding takes place.
 
Okay I see it! :grin: Because of the tap ECC can only have two 4-pole ATS?s and can only bond at the generator, all other locations the EGC and neutral must remain separate.

Thanks again for the picture RUWire.
 
EEC said:
What is the code difference between 3 pole or 4 pole ATS? Also what code allow bonding neutral inside building at first Disconnect?
Click to expand...

If the generator has OCP on it like a breaker this is considered the first disconnect so bonding cannot take place past it, 250.30, Post #28.
Print RUWire?s picture and consider utility ?commercial? power is being used; returning neutral current will travel back to utility source and also through the bond at the generator to equipment frames to source, this would be in violation of 250.30 and 250.142(B).
 
tryinghard said:
Because of the tap ECC can only have two 4-pole ATS?s and can only bond at the generator,
Click to expand...

I'm not totaly convinced that the 4-pole is required and not just the cadillac of systems. As it is with the two types of switches, there are no parallel paths present in either the normal or emergency operation. Article 250.35 doesn't clearly say what to do if there is a combo of switches. Putting both switched transfer switches in would sure make it clearer.(are there parallel paths, no)( is there an effective ground fault current path, yes.)
 
RUWired said:
I'm not totaly convinced that the 4-pole is required?Article 250.35 doesn't clearly say what to do if there is a combo of switches?
Click to expand...

This generator supply system can be two the same of one or the other, 3-pole or 4-pole, but it cannot have one of each. The tap eliminates these sharing because the SDS becomes grounded through the required system-bonding jumper at the generator, causing a parallel neutral current path on non-current carrying items if a 3-pole is use with this.

As soon as the 4-pole ATS is used it causes the system to be separately derived and ground must be re-established as well as ground fault path to source. A short requires the bonding jumper to enable an effective ground fault path. By using one 4-pole ATS the bonding jumper must be installed at the generator and not past the first disconnect, 250.30.

Circle the N & G together at the generator on your drawing to represent the bonding jumper. Now draw arrows returning on the neutral to the service disconnect powered by utility source. The current will also return parallel through the generator bond to frame ground and back to utility source on equipment frames and all conductive items.

250.35 (2008) cannot say what to do with one each because it would intentionally install current on non-current carrying items.

This is a brain twister! :grin:
 
I think the statement above should read: "The tap eliminates these sharing because the bonding jumper at the generator causes a parallel return path for unused current on non-current carrying items."
 
tryinghard said:
This is a brain twister! :grin:
Click to expand...

Here is another example of using a grounded conductor of one feeder to carry a fault in another. The one feeder is a 3-phase 3-wire solid neutral transfer switch. The other feeder is a 3-phase 4-wire solid neutral transfer switch. So in this case the generator is a non SDS set. Zero parallel paths are present. This another example of economy design or code violation? I would think a switched neutral switch would be a nice design, but not required.

View attachment 2131
 
RUWired said:
Here is another example of using a grounded conductor of one feeder to carry a fault in another. The one feeder is a 3-phase 3-wire solid neutral transfer switch. The other feeder is a 3-phase 4-wire solid neutral transfer switch. So in this case the generator is a non SDS set. Zero parallel paths are present. This another example of economy design or code violation? I would think a switched neutral switch would be a nice design, but not required.

View attachment 2131
Click to expand...

This drawing is not ECC?s case, ECC requires neutrals.

It also includes two 3-pole feeds (non-SDS) from one generator supply; it is actually three grounded systems (XFMR 1, XFMR 2, & Gen set) and a duel fed service as well.

Returning current will safely travel to either source without paralleling on non-current carrying items, not the case with one 3-pole and one 4-pole.

I?m not sure an AHJ would agree to this because the effective ground-fault path is reliant on a jumper in a non-related piece of equipment (bldg 2 disconnect) for the 3W feeder to bldg 1.

With regards to equipment grounding it seems to be in violation of 250.118 in that ?the equipment grounding conductor run with or enclosing the circuit conductors? is not happening for the 3W feeder because it is routing through bldg 2?s disconnect.
 
I realize the second drawing is not ECC's case. I wanted to point out that in either drawing, one feeder is relying on the other feeders grounded conductor to return the fault back to the source. In both drawings, a switched neutral transfer switch would make it (a) a better system or (b) a code violation? I should point out that the second drawing is a system that i am working on that has been approved by the AHJ and since it has been approved, would ECC's setup also be approved.
Rick
 
Rick, if the AHJ does this for ECC?s design it will be in error and a violation. Given ECC?s need for the neutral switched because of GFI protection I believe two 4 pole ATS?s must be used.

Your first drawing needs to have the bond at the generator because of the 4 pole so the fault current is not relying on the other neutral, it is only paralleling on it.
 
Status
Not open for further replies.
Top