Power Switching

1. Do I specify a three-pole, or a four-pole transfer switch?
2. How many transfer switches does an emergency power supply system require?
3. Does my transfer switch need to be Service Entrance rated?
4. What about UL?
5. Why can't I size a transfer switch to match my generator?
6. Which enclosure should I specify?
7. What is "bypass isolation," and why is it so expensive?
8. How often should a transfer switch be tested?
9. What is a "closed transition" transfer switch?
10. Won't my closed transition switch back-feed the utility?
11. I have another question

Do I specify a three-pole, or a four-pole transfer switch?

A fourth (neutral) pole should be specified when the standby power source will be a "separately derived system" as defined by the National Electrical Code (NEC). In other words, a switched neutral pole should be used whenever ground fault protection is required anywhere in the system. A three pole transfer switch with a solid (not switched) neutral connection should be used for three-phase-four-wire service where there is no ground fault protection.

How many transfer switches does an emergency power supply system require?

According to the 2002 version of the National Electrical Code, any transfer switch that serves "Emergency" class load must serve no other loads. If loads other than Emergency class are to be served by a standby power source, at least two transfer switches will be required. Furthermore, a larger number of smaller-ampacity transfer switches provides greater protection against branch circuit failures, so "more and smaller" is a good general principle.

Does my transfer switch need to be Service Entrance rated?

ASCO and other transfer switch manufacturers offer a SUSE ("suitable for use as a service entrance") rating as an optional feature for transfer switches. This option should ONLY be specified if the building is to use only one transfer switch, installed at the building service entrance.

What about UL?

The UL standard for both automatic and non-automatic transfer switches is UL 1008. According to the 2002 version of the NEC, any device used for switching an electrical load between two sources of power must bear a UL 1008 label.

Why can't I size a transfer switch to match my generator?

This is a common mistake. Since a transfer switch is used with a generator set, many people assume that the switch can simply be sized to match the output of the generator. In reality, the transfer switch is a conductor that carries power to a load, sometimes from utility power, and sometimes from the generator. It should be sized to match the larger of the two sources. In many cases the generator output is actually smaller than the normal power source.

Which enclosure should I specify?

These are the NEMA definitions of the five most commonly specified enclosure classes:

• NEMA 1 "General Purpose - Indoor"
• NEMA 3R "Rainproof and Sleet-Resistant (Ice Resistant) - Outdoor"
• NEMA 12 "Industrial Use - Dusttight and Driptight - Indoor"
• NEMA 4 "Watertight and Dusttight - Indoor and Outdoor"
• NEMA 4X "Watertight, Dusttight and Corrosion-Resistant - Indoor and Outdoor"

Example: A common mistake is to specify NEMA 12 or NEMA 3 (rather than 3R) for outdoor/raintight applications. Neither is appropriate for this duty. The correct selection would be NEMA 3R.

What is "bypass isolation," and why is it so expensive?

Since a transfer switch is a conductor, everything downstream of the switch is de-energized when the transfer switch is de-energized for scheduled maintenance or emergency repairs. Any circuit that will require power on a 24/7 basis should have a transfer switch with a fully-rated bypass isolation device. This allows power to be bypassed around the automatic transfer switch, and allows the auto switch to be isolated from the circuit for safe maintenance, without any interruption in power to the load. Since all components in the bypass and isolation devices need to carry the same ratings as the automatic transfer switch, the price of a bypass-isolation equipped switch is roughly 2-1/2 times the price of a non-bypass transfer switch.

How often should a transfer switch be tested?

According to NFPA 110 "The monthly test of a transfer switch shall consist of electrically operating the transfer switch from the standard position to the alternate position and then a return to the standard position." Monthly transfer testing is the minimum, but some facility operators choose to test highly critical transfer switches on a weekly basis.

What is a "closed transition" transfer switch?

A "closed transition" transfer switch (also known as a "make-before-break" transfer switch) transfers power between the normal and standby sources with no interruption in power to the load. It does this by connecting to both sources in parallel for a fraction of a second. Closed transition transfer switches are used on circuits for which any interruption in power could cause some inconvenience.

Won't my closed transition switch back-feed the utility?

This is a common misconception caused by the name "closed transition." In reality any UL 1008-labeled closed transition switch is designed to operate in both open and closed transition modes. It could more accurately be called a "dual mode transfer switch." Any time the normal power source fails, this switch operates in a conventional break-before-make fashion, disconnecting completely from the dead utility source before connecting to a live generator source. The make-before-break mode is only utilized when switching between two acceptable live sources, as is done during monthly testing.

I have another question

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