Introduction to the differences between 4P low voltage circuit breaker and 3P low voltage circuit breaker

There are many manufacturers, models and types of Circuit Breakers. Simply put, they are an organic whole composed of the incoming line side, the outgoing line side and the circuit breaker body. Careful electricity users or practitioners may have questions when facing different circuit breakers: why some have three terminals and some have four terminals. What are the differences and similarities between them? With this question, we enter the text of today's article and explain the differences between them in detail.

1. Appearance of 4P and 3P low voltage circuit breaker

Figure 1 Appearance of 4P low voltage circuit breaker

Figure 1 Appearance of 3P low voltage circuit breaker

A 4P circuit breaker has three phases and four poles, and four sets of terminals are arranged horizontally, among which the N line terminal may be smaller than the other three-phase terminals. The "four terminals" are intuitively visible. The casing is generally marked with L1, L2, and L3 marks, and the N (neutral line) terminal of the fourth pole usually has a clear "N" mark.

3P circuit breaker has three phases and three poles, and three sets of power terminals of the same size are arranged horizontally. The "three main terminals" are intuitively visible. The casing is generally marked with L1, L2, and L3 marks.

In fact, from the appearance point of view, except for the number and size of terminals and some logos, you can’t see the difference between them.

2. Introduction to the working principles of 4P and 3P low-voltage circuit breakers
3. Introduction to the working principle of 4P circuit breaker

There are four linked main contacts inside, three of which are connected in series to the three phase lines, and the fourth is connected in series to the neutral line. When a fault occurs (overload, short circuit) or manual opening, "four poles are switched on and off at the same time" (some can choose N not to disconnect). It has the following characteristics.

1.1. Complete safe electrical isolation

This is the most important feature of the 4P circuit breaker. It can completely separate all live conductors (including N wire) at the load end. When the "phase loss and constant zero" are completely eliminated, the risk that the N line may still carry dangerous voltages remains.

1.2. Adapt to various power supply systems

TN-S/TN-CS system: can be used as a main incoming line switch to achieve safe isolation.

TT system: The N line of the TT system must be isolated, and its incoming line switch must use a 4P circuit breaker.

IT system (when the N line is drawn out): When the IT system leads out the neutral line to form a three-phase four-wire power supply, a 4P circuit breaker must be used.

Dual power conversion system: It can prevent the neutral lines of two independent power supplies from being connected in parallel and avoid neutral line circulating current.

1.3. Can be combined with electronic trip unit to form modular protection

Since it can directly measure the currents of L1, L2, L3, and N wires, the electronic tripper can realize grounding protection by calculating the vector sum without the need for an external zero-sequence current transformer.

2. Introduction to the working principle of 3P circuit breaker

There are three linked main contacts inside, which are connected in series to three phase lines. When a fault (overload, short circuit) or manual opening occurs, the three contacts are opened at the same time, cutting off the three-phase power supply. The neutral line (N) of the circuit on the lower side of the switch is always connected to the neutral line on the power side. It has the following characteristics.

2.1. In the TN-C (three-phase four-wire) system, the PEN line (protected neutral line) is strictly prohibited from being disconnected, and a 3P circuit breaker must be used.

2.2. For equipment that does not require a neutral line (such as three-phase motors), it can provide complete overload and short-circuit protection, and the structure is the most matching.

2.3. Compared with 4P circuit breakers of the same grade, 3P circuit breakers are cheaper.

2.4. Complete electrical isolation cannot be achieved, and the N phase cannot be disconnected directly. If the system is unbalanced among the three phases or has a front-end failure, there is a risk that the neutral line will be electrified.

2.5. The method of realizing ground protection (G protection) is complicated and must rely on an external zero-sequence current transformer, which increases the installation complexity and cost.

3. Summary

Through the above introduction, readers can have a clearer understanding of 3P circuit breakers and 4P circuit breakers, including differences in their appearance, working principles, and scope of use, etc. However, there are some deeper aspects that have not been explained in detail. So here comes the question, why does N need to be disconnected in some cases, and N does not need to be disconnected in other cases? What does it have to do with specifically? We can think about this problem ourselves first.