Switch Neutral Transfer Switch?
I find it truly amazing that I cannot
find a definitive basis for selecting a separately derived system vs. a nonseparately
derived system when using a generator. Certainly, IEEE, reference books, vendors, etc.
parrot NEC Article 250, but they have no discussion concerning the reason for selecting
Comment: For the generator to be considered a separately derived system,
the supply to the transfer switch must include a grounded (neutral) conductor and
the transfer switch must switch the grounded (neutral) conductor (example a 4-pole
transfer switch for a 3-phase, 4-wire system). If there is no grounded (neutral) conductor
run to the transfer switch or if the transfer switch does not switch the grounded
(neutral) conductor, then the generator is not a separately derived system.
Separately Derived System.
If the generator is a separately derived system, then the grounded (neutral) conductor/terminal
of the generator MUST be bonded to the metal parts of the electrical installation
in accordance with 250.30(A)(1) and the generator is REQUIRED to be grounded in accordance
Nonseparately Derived System.
If the generator is not a separately derived system, then the grounded (neutral) conductor/terminal
of the generator MUST NOT be bonded to any metal parts of the electrical installation
and the generator is NOT REQUIRED to be grounded to any electrode.
Mike, I have done a search on your site
for "separately derived systems" and yes the references do an outstanding job
in the discussion the grounding and bonding requirements for separately derived systems
and it seems to concentrate on the dangers of an improperly grounded and/or bonded system.
By inference, the solidly grounded nonseparately
derived system (no switched neutral) seems to be the logical choice and a separately derived
system (switched neutral) is only required for special applications. I am curious about
which applications would demand the use of a separately derived system.
Did I miss some pearl of wisdom concerning
this subject in your older columns?
Comment: Dick you are correct, all of my article focus on when a system
is considered separately derived by the NEC (Article 100) and how the NEC requires
them to be grounded and bonded. I also focus on the dangers of electric shock to fire
safety from an improperly grounding or bonding system. But I have not written any
articles on when a separately derived system (generator supplying a switched neutral
transfer switch) is required or preferred. But your question is going to over 20,000
electrical professionals and I'm sure we'll get lots of positive feedback.
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