Elec. code definitions

[repairco]

This came in an E-newsletter I subscribe to. Hopefully it will be of use to
you or when you have to deal with idiot home and/or code inspectors such as
I have in the past. For those who don't recognize the name, Mike Holt is a
major author of electrical textbooks.

Dutch




Code Basics
By Mike Holt
A separately derived system is a premises wiring system that has no direct
electrical connection to conductors originating from another system [see
Art. 100 definition and 250.20(D)]. All transformers, except an
autotransformer, are separately derived because the primary supply
conductors don't have any direct electrical connection to the secondary
conductors.
A generator, a converter winding, or a solar photovoltaic system can only be
a separately derived system if the grounded (neutral) conductor is opened in
the transfer switch [250.20(D) FPN 1]. If a generator that isn't part of a
separately derived system, is bonded in accordance with 250.30(A), dangerous
objectionable neutral current will flow on the bonding paths in violation of
250.6(A).

All online UPS systems are separately derived, even if the input and output
voltages are the same. An automatic transfer switch has no effect on this
determination because an isolation transformer is provided as part of the
module.

Separately derived systems that operate at more than 50V [250.20(A) and
250.112(I)] must comply with the bonding and grounding requirements of
250.30(A)(1) through (A)(6). Bonding the metal parts of the secondary of the
separately derived system to the secondary grounded (neutral) terminal
ensures that dangerous voltage from a ground fault on the secondary can be
quickly removed by opening the secondary circuit's overcurrent protection
device [250.2(A)(3)]. In addition, separately derived systems are grounded
to stabilize the line-to-earth voltage during normal operation
[250.4(A)(1)].

For further reading on separately derived systems, visit EC&M's Web site.