In low-voltage three-phase four-wire distribution systems, disconnect switches, circuit breakers, or load switches are typically installed at the power source end of the distribution circuit. Their main functions are to disconnect or connect the circuit during normal operation, maintenance, or in the event of a fault. When selecting switches, engineers and contractors often face confusion regarding whether to use a three-pole or four-pole switch. In actual engineering design, construction, or renovation, it's common to make mistakes—sometimes selecting a three-pole switch when a four-pole switch is needed, or installing a four-pole switch in a situation where a three-pole switch would suffice. It’s crucial to choose the correct type of switch in low-voltage distribution systems. A wrong choice may result in unnecessary costs, potential safety hazards, and equipment damage, or even cause harm to people. So, how should you distinguish between when to use a three-pole or a four-pole switch? Let’s dive deeper into this issue!

How to Choose a Four-Pole Switch

A four-pole switch should meet the following requirements:

1.To ensure power transfer functions: The switch must act on all live conductors and prevent the parallel connection of power sources.

2.For TN-C-S and TN-S systems: A four-pole switch must be used for power transfer switches that disconnect both phase conductors and the neutral conductor.

3.In IT systems with a neutral conductor, and for TT or TN systems: The power transfer switch must be a four-pole switch.

4.For normal power supply and backup generator supply: The transfer switch between the two should be a four-pole switch.

5.For TT system power input switches: A four-pole switch is required.

6.In IT systems with a neutral conductor: A four-pole switch must be used.

7.For circuit breakers with earth fault protection: A four-pole switch should be used.

8.When selecting residual current protective devices (RCDs): Unless in a TN-S system where the neutral conductor reliably connects to earth potential, use protective devices that can disconnect all live conductors and protected circuits.

9.For each residential building’s main power switch: The switch must be able to simultaneously disconnect both the phase and neutral conductors.

When Should You Use a Three-Pole Switch?

If there are no specific requirements for using a four-pole switch, you should generally use a three-pole switch. A three-pole switch is typically sufficient in most cases unless there are particular safety considerations or system requirements that necessitate the use of a four-pole switch.

Precautions for Using Four-Pole Switches

Four-pole switches should be used with caution. In common TN-C-S and TN-S systems, if there are no special requirements regarding electrical maintenance safety, there is generally no need for a four-pole switch.

This is because both IEC standards and national electrical codes mandate the installation of a main equipotential bonding in buildings. In older buildings that have not implemented main equipotential bonding, the natural contact between metal structures, pipes, etc., provides some degree of equipotential bonding. Using a four-pole switch in such systems might increase the risk of improper contact between the neutral conductor’s terminal and the switch contact, which could lead to equipment damage or make the power system unreliable.