Low voltage circuit breaker parameters: short time withstand current (Icw), what is this parameter used for?

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Low voltage circuit breaker parameters: short time withstand current (Icw), what is this parameter used for?
11 16 , 2021

Short-time withstand current (Icw):The ability of a circuit breaker to withstand 0.05, 0.1, 0.25, 0.5 or 1s without tripping at a given voltage, short-circuit current, or power factor.
Rated short-time withstand current, also known as thermal stable current, is the effective current that a circuit breaker or other device can withstand for a specified short period of time. Its size is equal to the rated short-circuit current and the time is usually 3 or 4 seconds.

Icw is the evaluation index of electric stability and thermal stability of circuit breaker in short delay trip. It is for class B circuit breaker. Usually, the minimum VALUE of Icw is:

When In≤2500A, it is 12In or 5kA,

When In>2500A, it is 30kA

(FOR YUW1_2000, Icw is 400V, 50kA; for DW45_3200, Icw is 400V, 65kA).

Icw rated short-time withstand current:The manufacturer claims that the RMS tolerable for a short period of time defined by current and time can be considered to be the RMS tolerable for a product for a limited time, under relevant conditions, for a complete set of equipment and electrical equipment. But the nature of various products is not the same, the definition of rated short-time tolerance current value will not be completely the same, its core is mainly the following points:


  • Ensure the reliability of system power supply;
  • On the basis of ensuring the reliability of the power supply system, the security of the product itself

For a low-voltage distribution system, the rated short-circuit current is determined by the short-circuit current that may occur in the system at the installation site and the time that the product itself and other products in the system can withstand the current.

Iec60364-4-43 standard for short circuit protection of power distribution system clearly stated: at any point in the loop due to short circuit caused by all the current, should not exceed the current, in the system conductor does not exceed the allowable limit temperature of the time for breaking.

For short circuits lasting no more than 5 seconds, the time (t) for the conductor to rise from its normal operating permissible temperature to its maximum allowable temperature can be roughly calculated by the following formula:

T = (k * S/I) 2K material coefficient, S conductor area, I short-circuit current value.

Above, select a low-pressure equipment rated short-time resistance current value, from the installation location of maximum short-circuit current, conductor or other equipment in the system from operating temperature rise to temperature can withstand the limit of time, this two aspects to consider, in determining the maximum short-circuit current installation site and the system can withstand maximum short-circuit current of time, It also determines the short-time current-time tolerance of low-voltage equipment at the installation point.

Because the structure and use of different electrical equipment are different, there are some special requirements for short-time current-time value on the basis of satisfying the system safety:


  • For electrical equipment such as busbars, low-voltage power distribution cabinets, and disconnecting switches, the short-circuit current value should not be lower than the expected short-circuit current value at the installation point, and the short-circuit current limit time should not be shorter than the operation time of protective appliances in the system.


  • For circuit breakers using class B, the short-tolerance current value shall not be less than the expected short-circuit current value at the installation point, and the short-tolerance current limit time shall not be less than the operation time of the system lower-level protective appliances, and ensure the selectivity with lower-level protective appliances.


Now, the system power supply load and expected short-circuit current increase, distribution bus system density increase, low-voltage electrical equipment miniaturization, if only the pursuit of short-term tolerance current under the long time limit of high value, in fact, there is no great practical significance.

So according to the possible maximum short-circuit current installation points and other devices in the system can withstand maximum short-circuit current of the time, reasonable selection of electrical equipment in a safe under the time limit of short-term resistance current value, in accordance with 0.5 s Icw values to examination seems to have been able to meet the requirements of system security and maximum efficiency.

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