10.11 Short-Circuit Withstand Strength

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Short circuit flashThis is where a circuit is made between one phase and another, or earth, without there being a load between them. This enables a high magnitude of current to flow between the phases or down to earth. When testing to IEC Standards the short circuit is required to be consistent and repeatable, therefore a bolted connection is made. Depending on the short circuit current rating of the circuit, either solid bars or cables are used to connect the phases, or earth, together.

There are two short circuit ratings that can be considered within an assembly, Icc and Icw:

  • Icc – rated conditional short circuit current. This is the rating that is associated with a SCPD; the short circuit current will initially flow and then be cleared by the protective device (this test is not given a time duration). Because of the complexities of ensuring appropriate selection is made, the second alternative rating, the Short Circuit Withstand (Icw) is often specified.
  • Icw – rated short time withstand current. This is the rating of current that the assembly can withstand for a set period of time without the aid of a SCPD. The rating is made up of 2 parts: the RMS rating in kA and the duration. There is no international standard for the timing, however times of 0.5, 1 and 3 seconds are commonly used. Icw includes the Rated Peak Withstand Current (Ipk), which is a surge of current that occurs on one phase of the system in the first full cycle, as well as the changes of electro-magnetic forces that occur through the number of cycles seen during the test. The number of cycles is dependent on the frequency of the supply and duration of the test. For example, if the test supply is 50Hz and the duration is 3 seconds, there will have been 150 cycles.

Clause 10.11 is split into tests for the main circuits:

  • Outgoing circuits
  • Incoming circuit and main busbars
  • Connections to the supply side of outgoing units (e.g. distribution busbars)
  • Neutral conductor
  • Protective circuit

Depending on the design of the assembly and the certification required, a combination of tests or all tests may be performed. See below for the Test Requirements of the assembly.

During short circuit testing, the locking means and hinges of the doors should remain intact; deformation may result in the test failing to meet the requirements and may alter the IP rating of the assembly.

Test Requirements

The assembly will require to be fitted with a set of incoming connection points L1, L2, L3, N & PE outside the panel to facilitate the connection of the incoming test supply to the termination points. The connection terminals required to connect to the short circuit test laboratory supply are shown on this drawing. These are generally a set of removable terminals connected to the end of the panel near the incomer.

The test connections consist of a three phase link on the outgoing circuits (one of each circuit rating) and a phase to earth link on one outgoing circuit (the largest outgoing circuit). A three phase link and a phase to neutral link on the incoming terminals or busbars will be fitted as required.

For an extendable panel main busbar must be a minimum of 1.6m long and contain at least one joint, otherwise the panel is classed non-extendable.


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