What's the difference between 4-pole transfer switches and 3-pole transfer switches

4 pole STS are usually used for 3P4W system where the neutral is also switched when transitioning from source 1 to source 2. Each pole represents an individual switch contained in the breaker, which corresponds to a different wire or phase of electricity. 4-pole device can protect four circuits.3 pole STS are usually used for 3P4W system where the neutral is not switched when transitioning from source 1 to source 2.  3-pole device can protect three circuits. 

While both types swtiched are common in three-phase electrical systems, the differences between a 3-pole and 4-pole transfer switch are as below.

• When neutral is switched, it's 4 pole switch; When neutral is not switched, it's 3 pole switch.

• In separately-derived system it requires 4 pole switch; In non separately-derived system 3 pole switch is ok.

• 4-pole switches used in scenarios where there is a greater potential for a fault on the neutral wire.

• 4-pole switch correctly isolates ground currents during a ground fault. Any switching transients generated across the neutral pole of a 4-pole switch are minor.