Introduction

When it comes to low-voltage circuit breakers, most people are familiar with them, so we won’t go into too much detail! Low-voltage circuit breakers cover a wide range of applications and products, offering various protection functions. Generally, molded case circuit breakers (MCCBs) provide short-circuit and overload protection. Residual current circuit breakers (RCCBs) add residual current protection on top of short-circuit and overload protection. Air circuit breakers (ACBs) usually offer short-circuit, overload, and ground protection. Below, we will look at the setting principles of low-voltage circuit breaker protection.

(1) Comparison of Low-Voltage Circuit Breakers with Ordinary Circuit Breakers

Low-voltage circuit breakers typically feature two types of protection: long-time overload protection and instantaneous short-circuit protection. Ordinary circuit breakers use a thermal bimetallic strip for overload protection and an electromagnetic trip unit for short-circuit protection. Due to mechanical precision limitations, the error margin for the trip unit is relatively large, with overload protection error generally not exceeding ±10% and short-circuit protection error not exceeding ±20%.

With the advancement of electronic technology and the improvement of product reliability, many new medium and high-end circuit breakers now adopt electronic trip units using transistors or microprocessors. These improvements have significantly increased accuracy, with some manufacturers offering overload protection errors within ±5% and short-circuit protection within ±10%. Therefore, when setting and verifying circuit breaker protection, it is essential not to rigidly apply the long-standing reliability factors from design manuals, but rather choose different factors for different breakers.

(2) Setting of Long-Time Overcurrent Trip Current

The formula for setting the long-time overcurrent trip current is generally:

Where:

Ib = calculated current of the line

Kzd1 = reliability factor of the long-time trip unit, typically recommended as 1.1.
Since the error mainly comes from the trip unit, it should be determined based on the error of the long-time overcurrent trip unit.

For ordinary breakers such as CM1 and DZ20, the error margin for the long-time overcurrent trip unit is ±10%, so Kzd1K_{zd1}Kzd1 is set to 1.1. However, for digital trip units such as the IZM switch from Moller or the NS switch with STR53 trip unit from Schneider, where the error does not exceed ±5%, Kzd1K_{zd1}Kzd1 can be set to 1.05.

(3) Sensitivity Check of the Circuit Breaker Trip Unit

To ensure that the low-voltage circuit breaker reliably disconnects a ground fault, the sensitivity of the trip unit should be checked using the formula:

Where:

Izd = instantaneous or short-time delay trip current setting

Ikmin= minimum short-circuit current at the end of the protected line (generally single-phase ground fault current)

Ki = reliability factor of the trip unit, generally recommended as 1.3.

(4) Selection of Low-Voltage Circuit Breakers

1) General Principles:

The rated voltage of the low-voltage device should not be less than the operating voltage of the circuit (Ue ≥ Ug).

The rated current should not be less than the calculated working current (Ie ≥ Ig).

The breaking current should not be less than the short-circuit current (Izh ≥ Ich).

The thermal stability value should not be less than the calculated value.

The low-voltage device should be selected according to the starting conditions of the circuit.

2) General Selection of Breaker Protection, Overload, Short Circuit, and Undervoltage:

• Breaker rated voltage ≥ line rated voltage.

• Breaker rated current ≥ line calculated load current.

• Trip unit rated current ≥ line calculated load current.

• Ultimate breaking capacity ≥ maximum short-circuit current in the line.

• The single-phase ground short-circuit current at the line end should not be less than 1.25 times the instantaneous (or short-time delay) trip current setting.

• Breaker undervoltage trip unit rated voltage = line rated voltage.