Fuji Electric – How do you determine the breaking capacity of a circuit breaker?

(1) Is there a difference between ICU and ICS in the sample?

(2) What is breaking capacity? How do I choose a breaking capacity?

(3) Why must the circuit breaker use 0.8-0.9 times the rated current value?

(4) How to determine the sensitivity current value?

(5) Does the insulation space of the circuit breaker have to exist?

Is there a difference between ICU and ICS in the sample?

Both represent the breaking capacity of the circuit breaker, and the differences are as follows:

ICU

Indicates the limit short-circuit breaking capacity

The number of breaks is 2 times

ICS

Indicates the rated operating short-circuit breaking capacity

The number of breaks is 3 times

When selecting a circuit breaker, if the breaking capacity of the equipment is clear, the system will usually use the ICU for the short-circuit current.

What is Breaking Capacity? How to choose?

When calculating the short-circuit current of a circuit, it is necessary to select a breaking capacity greater than this value.

So, what is breaking capacity?

Breaking capacity

The capacity to cut off the flow of large currents (short-circuit currents) to the accident point in the event of an accident.

In simple terms, the breaking capacity is the maximum capacity of the current that the circuit breaker can cut off without a fault.

Failure to cut off the short-circuit current without selecting the appropriate breaking capacity can cause great damage to low-voltage electrical appliances both in the panel and under the circuit breaker.

Here's an example

For example, the breaking capacity of the backbone circuit breaker of the mechanical device installed in the factory (see point C in the figure below) needs to be selected according to the short-circuit current value of the sum of all switches below point C.

Fuji Electric – How do you determine the breaking capacity of a circuit breaker?

Why does the circuit breaker have to use 0.8-0.9 times the rated current?

The reason is related to the following 2 points:

1. The current rises due to load aging

2. Changes in electrical characteristics caused by changes in ambient temperature

The aging of the motor and pump causes the resistance to rise, which leads to an increase in the current used.

Therefore, if the value is set to the limit, the circuit breaker may trip by mistake:

How to determine the sensitivity current value?

There are selected criteria for sensitivity currents, which can be referred to the following electric shock impact rating table:

Deterioration of equipment and wiring reduces the insulation resistance value, so electric shock accidents can occur when leakage current flows through a place where there is no current.

However, in devices equipped with noise filters and high-frequency inverters, the leakage current value becomes high, so it is necessary to reconsider the sensitivity current value or use a dedicated circuit breaker instead.

In addition, when choosing a product, you can also consider the following two special risks:

1. If you are more inclined to reduce the risk of electric shock

Please select a high-sensitivity product (15mA-30mA).

2. If you prefer to prevent leakage fires

Please select a medium sensitivity product (50mA-).

Is it necessary to insulate the circuit breaker?

The circuit breaker has a gas jet, and in order to prevent secondary accidents and give full play to the performance of the circuit breaker, the insulation space is necessary

This is because when the circuit breaker cuts off the short-circuit current, free gas is discharged from the exhaust port on the power side of the circuit breaker, and the conductive gas sometimes causes interphase short-circuits or grounding accidents between bare charging sections or between installed metals.

Fuji Electric – How do you determine the breaking capacity of a circuit breaker?

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