01 Introduction

In the past three decades, domestic electrical manufacturers have designed and produced various types of four-pole electrical appliances (including four-pole knife switches, four-pole circuit breakers, four-pole contactors, etc.). Both universal (frame-type), plastic casing circuit breakers, and miniature circuit breakers have products with four-pole configurations available in the market. Determining the suitable application places for four-pole circuit breakers is an essential consideration for electrical professionals.

02 Current Structural Types of Four-Pole Circuit Breakers

Currently, there are six types of plastic casing four-pole circuit breakers available in the domestic market:

(1) The neutral pole (N pole) of the circuit breaker does not have an overcurrent release, nor does it have dynamic or static contacts. The N pole always remains connected and does not switch together with the other three phase poles;

(2) The N pole of the circuit breaker does not have an overcurrent release, but it has dynamic and static contacts. The N pole switches together with the other three phase poles;

(3) The N pole of the circuit breaker has an overcurrent release and dynamic and static contacts. The N pole switches together with the other three phase poles;

(4) The N pole of the circuit breaker has an overcurrent release, but it does not have dynamic or static contacts. The N pole always remains connected and does not switch together with the other three phase poles;

(5) The N pole of the circuit breaker has a "neutral wire disconnection protector," dynamic and static contacts. The N pole switches together with the other three phase poles;

(6) The N pole of the circuit breaker has a "neutral wire disconnection protector," and it always remains connected without dynamic or static contacts. Types (1) through (4) are more commonly produced, and many manufacturers define these four types as Type A, Type B, Type C, and Type D. When the N pole type is (1) or (2), which corresponds to Types A and B, respectively, the N pole does not provide protection; when the N pole type is (3) or (4), corresponding to Types C and D, respectively, the N pole provides protection. Types (5) and (6) are less commonly produced and used nowadays.

03 Application Places of Different Structural Types of Four-Pole Circuit Breakers

According to relevant national standards, professional standards, and specifications of the IEC and China, various departments, especially in building electrical systems, have reached a basic consensus on the application of four-pole switches (circuit breakers):

(1) At the incoming end of the power supply:

① For the main switches of users (floors) and single-phase main switches of each user, for safety during maintenance (to prevent electric shock), four-pole or two-pole switches (circuit breakers with both phase and N poles having dynamic and static contacts) should be used.

② For single-phase circuits, to prevent confusion between phase and neutral lines, two-pole switches (with both phase and N poles having dynamic and static contacts) should be used for terminal single-phase electrical equipment. When cutting off power, both the phase and N lines are disconnected simultaneously to prevent accidents from occurring.

(2) Switching between two power sources: Four-pole circuit breakers should be installed at places where two power sources are switched. If the switch (circuit breaker) for switching between two power sources is a three-pole circuit breaker, a fault voltage from the neutral line (if any) may still enter the electrical equipment of the disconnected power circuit, posing a risk of electric shock to maintenance personnel. Moreover, in asymmetrically loaded circuits, there are zero-sequence currents on the N line, and parallel current paths can lead to unequal total current vectors in each line, causing heating of cable armor or conduit walls due to eddy currents. Therefore, four-pole circuit breakers should be installed to disconnect both the phase and neutral lines simultaneously. In systems where the neutral and PE lines are combined into one (TN-C or TN-C-S), four-pole circuit breakers should not be installed.

(3) In TN-C systems, due to the presence of current in the PEN line (which is actually the N line) and the excessive voltage to ground due to load imbalance or harmonic currents, four-pole circuit breakers can also be installed, but the N pole should not switch together with the other three phase poles.

(4) When installing residual current operated protective devices (RCDs, RCCBs, relays, etc.), according to IEC 364 532.2.1, "The residual current protective device must ensure that all live conductors (wires) in the protected circuit are disconnected" (the neutral line is also a live conductor). If the neutral line is not disconnected, current may flow through it (from the neutral line of the upstream circuit), posing a risk of electric shock if a person touches this neutral line. In a system with dual power sources, failure to disconnect the neutral line may cause the RCD to trip incorrectly or fail to trip. Therefore, the RCDs installed on three-phase circuits (three-phase four-wire) or single-phase circuits (single-phase two-wire) should have four-pole or two-pole configurations with both phase and N poles having dynamic and static contacts. However, it is well-known that TN-C systems cannot accommodate four-pole or two-pole RCDs due to specific reasons.

(5) In TN-S systems, for circuits with both three-phase and single-phase loads, or heavily unbalanced three-phase loads, four-pole circuit breakers should be used to eliminate unbalanced voltages on the neutral line during faults.

(6) If the cross-sectional area of the neutral line is half or one-third of the phase line, and in the event of overload on the neutral line (current density exceeding that of the phase line), the overcurrent protection on the phase line may not protect the neutral line. In this case, overload protection should be installed on the neutral pole, but the neutral line (pole) may remain connected (i.e., continuously connected).

(7) For TT, TN-S, and TN-C-S systems, if there are stage dimmers (using a large number of thyristors), electronic ballasts, coupled with unbalanced three-phase loads, varying power factors in three phases, where the maximum current in the neutral line can exceed twice the rated current of the phase line, and the cross-sectional area of the neutral line is smaller than or equal to that of the phase line (not twice as large as the phase line), four-pole circuit breakers should be used. The N pole should have an overcurrent release, and it should switch together with the other three phase poles.

(8) In IT systems, if there are three-phase four-wire or single-phase voltage circuits with neutral lines, all switchgear should disconnect the N line along with the phase lines. In such cases, four-pole circuit breakers (with the N pole having an overcurrent release and switching together with the other three phase poles) should be installed.

(9) In TN-C-S, TN-S, and TT systems with dual power source switches but minimal zero-sequence currents in three-phase four-wire distribution circuits, four-pole circuit breakers do not need to disconnect the N line. In this case, the N pole may not have an overcurrent release, and it may remain connected without switching together with the other three phase poles.

(10) In TN-C and TN-C-S systems, when installing neutral line disconnection protectors, all phase lines along with the PEN line must be disconnected in the respective circuit, and the PE line should be connected to the N line at the load end of the protective device.

According to GB 51348-2019 "Code for Design of Electrical Installations in Residential Buildings," Clause 7.5.3 stipulates that the selection of four-pole switches in three-phase four-wire systems should comply with the following provisions: ① Functional switches for power conversion should act on all live conductors without paralleling the connected power sources; ② Switches for power conversion in TN-C-S and TN-S systems should disconnect both phase and neutral conductors; ③ Switches for power conversion between IT systems with neutral conductors and TT systems should disconnect both phase and neutral conductors; ④ Switches for power conversion between normal power sources and standby generators should use four-pole switches; ⑤ In TT systems, when the power supply has a neutral conductor, four-pole switches should be used; ⑥ Circuit breakers with ground fault protection (GFP) functionality should be four-pole.

Based on the above provisions, clients (design institutes) should carefully consider the grounding system and load functionality when designing, avoiding indiscriminate use of four-pole switches (circuit breakers) to prevent unintended faults and losses.

04 Issues with Poor Contact of Neutral Line (N Pole) Dynamic and Static Contacts (Knife Contacts)

Some users and electrical designers are concerned that poor contact of the N pole's dynamic and static contacts may result in the actual disconnection of the N line. Indeed, such problems have occurred in practice. However, if there is poor contact on the phase pole, it may lead to phase disconnection. In the event of phase disconnection while a motor is running, increased currents may flow through the remaining phases, and if a phase is disconnected before starting, the motor may fail to start due to phase loss. Therefore, manufacturers need to conduct type tests, periodic tests, and factory tests on their three-pole or four-pole circuit breakers. For the former two, thousands to tens of thousands of endurance (energized and de-energized) tests should be conducted, while factory tests should include 5 to 10 continuity tests and synchronous contact tests per unit to avoid (or detect) poor contact of dynamic and static contacts. Another possible cause of poor contact of the neutral line is related to user wiring (installation) or the mechanical strength of selected conductors. When users wire, the torque of their screws (bolts) must comply with national standards, such as a tightening torque of ≥6N·m for M8 screws (bolts) and greater than 10N·m for M10 screws, to avoid neutral line disconnection or spark generation due to loose installation, leading to phase-to-phase short circuits and electrical fires.

The choice between three-pole and four-pole switches can affect the reliability, safety, and economy of the system. The best balance point should be considered during design. The rational selection of four-pole circuit breakers is crucial for the safety of power systems. On the one hand, as live conductors, four-pole circuit breakers may cause electric shock accidents in certain situations due to the non-disconnection of the neutral line; on the other hand, if four-pole circuit breakers are used where they are not needed, issues such as "loss of neutral" and other safety problems may arise. Therefore, the selection of four-pole switches should be cautious.