The AHJ over what we do installing generators at communication sites are now wanting to see nothing but switched neutrials. This would make the gen. a separately derived system. Two questions
Can any one see any benefit from switching the neutrials in these cases? I cannot. They say it is for safety of the utility side as to not induce voltage on the utility during an outage. From what little I know if there is an outage and the gen. is running the return voltage will seek it source ( the gen) and flow very little or nothing on the utility.
The other thought. You have the main service disconnect, the neutral and the GEC are bonded. The phase conductors, neutral and an EGC go to the ATS. The neutral lands on the utility side of the pole it the switch. The EGC is bonded to the housing. Now the neutral and GEC are bonded in the generator, phase conductors, neutral and EGC come into the gen. disconnect. neutrals are isolated from can and EGC is bonded to can. From there they go into ATS. Neutral lands on E side of pole, EGC bonded to can. Now would the bond of these EGC in essence still tie the neutrals together through the housing? And if you were to have a line to ground fault on the gen. side you could,for a mill-sec., induce voltage on the utility side?
I can't understand how these (or any AHJ for that matter) can ever think that you can get current through a single wire back to the utility system? imagine all the light bulbs we could light up with just one wire???
just because the neutral doesn't get switch and is still connected to the MGN at the utility transformer current can not flow on it, the transfer switch has disconnected the utility ungrounded conductors and there is no other pathway for current to flow from the generator that would form a complete circuit to the utility side of the transfer switch.
This is simple basic electrical theory, draw out a simple line diagram showing the transfer switch in the backup position and you can see you will only have one conductor (the neutral) connected to the utility side.
I try to never switch a neutral, why put another possible failure point in a conductor that will cause allot of damage if that pole fails to make contact?
We all know that damage that a loss neutral can cause.
Even with a GFP as long as the neutral is not re-grounded anywhere after the GFP current transformers (such as in the generator) then a 3-pole (3-phase) or 2-pole (single phase) transfer switch will work just fine.
In many service larger than 1ka most GFPs are installed at the main distribution panel and the transfer switch will be ahead of the GFPs anyway so the GFP wont see the bonding ahead of it.
as far as to what is called a SDS or a NON-SDS?
A secondary ground fault will trip a GFP ahead of a NON-SDS such as an autotransformer, but in a SDS a ground fault on its secondary will not trip a GFP ahead of it, why? simple current returns to source, the ground fault current on the secondary side of an SDS will only produce a line to line current on it's primary, so to me since current cant flow on the grounded conductor, there is no electrical connection between the primary and secondary side grounded conductors.
Lets try this, if there is no electrical circuit (current flow as shown above) then why would there be a difference in they way the code address the circuit? why does it matter on a NON-autotransformer? the current will still only flow to its source, the point of having a neutral bonded to the supply neutral is mute, a ground fault after this transformer will only flow on this bond if a secondary circuit faulted to a conduit of a circuit from the supply ahead of this transformer, this still doesn't change the way the circuit functions, and or why it would be called a NON-SDS, or the requirements needed for a NON-SDS?
This is only my opinion of what an SDS or NON-SDS is. right or wrong?? who's to say.
I think over the years the definition in the NEC got clarified in the wrong direction, but thats just my opinion.:roll:
