Separately Derived System?

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BAHTAH

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I install a 480-120/208 3ph4w transformer. Primary is either metallic conduit or PVC with EGC run from main service switchboard (where neutral and ground are bonded together) to transfomer. Transformer secondary neutral is bonded to the transformer enclosure and neutral. Is the metal conduit that is the EGC (250.118) or the EGC in PVC considered a solid connection of the transformer secondary Grounded Circuit Conductor to the primary system Grounded Circuit Conductor? Is this a Separately Derived System according to Art100?
 
Re: Separately Derived System?

Yes it is still a SDS and there is no way to avoid all the Eq Grounding Conductors being Bonded thru the Transformer Case.

Charlie
 
Re: Separately Derived System?

cpal, I think the NEC has had problems with their definition of SDS for a long time. If they would add an exclusion for the EGC/bonding conductors it would remove anything but the actual system conductors from the definition. Just a thought.
 
Re: Separately Derived System?

Well I have heard that article 250 is most likely the least understood article in the book. I thought I had a handle on it for reference then the 99 code came along!!

In general the NEC considers equipment grounding (soon to possibly become bonding) is a good thing. There are many instances where it is recommended to tie all the equipment grounds together, in fact you might say that the article takes pains in indicating were to lift the equipment grounds.

I'm not aware of a section that allows this transformer to isolate the Eq Ground from the supply and the Eq Ground at the secondary.


Charlie
 
Re: Separately Derived System?

Grant you must think of fault path when applying this. The secondary of a transformer's fault path will only be to the secondary winding or the X0 tap (neutral) (transformer's are isolating) The path of the primary will be to the source which will be back at the panel bond and even to the transformer supplying the service. With the secondary X0 bonded to equipment grounding conductors that is also bonded to the supply grounding at the main service the transformer is protected from a primary to secondary fault. This is why it is required to re-ground and bond after each and every transformer. Or SDS in this case.

[ February 14, 2005, 07:40 PM: Message edited by: hurk27 ]
 
Re: Separately Derived System?

Grant the transformer is considered SDS and requires a ground electroded connection is addition to the EGC ran with the primary circuit.
 
Re: Separately Derived System?

cpal, once again the English language gets in my way. I did not mean to imply isolation of the EGC and bonding just trying to figure a way of identifying SDS in Art100 by somehow defining the EGC as NOT BEING a solid connection of the grounded circuit conductors from each system used in the definition. All the bonding and EGC are solid connections but not really meant to be part of the definition even though they do make a solid connection between system neutrals. Maybe if the NEC stated that there are to be no solidly connected grounded circuit conductors between the primary and secondary except EGC and Bonding.
 
Re: Separately Derived System?

Grant the only transformer that would not be an SDS would be an autotransformer. If a shortcircuit was to happen on an auto transformer that fault would pass to the primary OPCD's but with any SDS this does not happen all that happens is the load of the fault passes to the primary which could open the primary OCPD but then again maybe not. Code cannot depend on that. Any transformer with or without a neutral tap that is isolating is a SDS. Even if this neutral does connect between the primary and the secondary neutrals it still is a SDS because any fault current on the secondary will only return to the secondary and not to the primary source. If you didn't bond the X0 to the grounding system then any secondary fault to this grounding would not open any circuit breakers on the secondary side of this transformer.
 
Re: Separately Derived System?

Thanks hurk27, now I can sleep. I just could not figure out why the neutral needed to be in the definition of SDS. Forgot all about autotransformers. Thanks again.
 
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