Separately Derived Systems and 4 Pole Transfer Switches – When and Why?

Automatic Transfer SwitchWhen reviewing emergency power system designs, I am sometimes asked about 4-pole transfer switches.  The typical question is “when should I use them?“, followed quickly by “why should I use them?“.

Since each project is likely to have special circumstances that make a generic answer impossible, this article focuses instead on providing the basic sources of information that should allow you to answer these when and why questions.

The first objective should be to have a good understanding of the National Electric Code (NEC) electrical grounding rules as they relate to a “service”, and to “load-side grounding connections”.  You must also understand the rules affecting “separately derived systems“, which are the trigger that would force you to use a 4-pole transfer switch (this article provides an in-depth discussion of separately derived systems).

What are the key sections of NEC that address grounding and separately derived systems?  These are the main ones that should get you started:

  • Article 100, Definition of a Service
  • Article 100, Definition of a Separately Derived System
  • Article 250.20(D), System Grounding – Separately Derived Systems
  • Article 250.24(A), Grounding Service-Supplied Alternating-Current Systems – System Grounding Connections
  • Article 250.30, Grounding Separately Derived Alternating-Current Systems

With an understanding of the applicable NEC rules, your next step should be to determine whether your generator is a separately derived system.  Any time that a generator is designated as a separately derived system, no shared conductors can exist between the service and the generator.  This includes any solidly connected grounded circuit conductors.

Once you have defined the generator as a separately derived system, the next step is to specify that all transfer switch equipment must be designed to switch between the service’s grounded conductor and the generator’s grounded conductor.  A 4-pole transfer switch is commonly used to accomplish this task (learn more about 4-pole transfer switch considerations here).

While the designation of separately derived system is often unavoidable due to the specifics of a facility, it is important to note that the designer can choose, for reasons of convenience or practicality, to make the generator a separately derived system.

What is your experience with separately derived systems?  Share your comments or questions below!

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