UL 508 A: Industrial Control Panels

UL 508A - Current Limiting Devices

Short-Circuit Current Limiting Devices in Feeder Circuit

The devices for which a limiting effect on the short-circuit current is expected, if they are in the feeder, are the following:

  • Power transformers with isolated secondary (their short-circuit current is limited by the longitudinal impedance representing the copper losses in the windings); the method to be followed is provided by Section SB4.3.1);
  • Listed circuit breakers marked as "current limiting"
  • Class CC, G, J, L, RK1, RK5 or T fuses.

For example, if a fuse is used in the feeder as main protection, UL 508A states that its SCCR is equal to: 

(SB4.3.3)

a) The breaking capacity (or interrupting rating) of the fuse when the SCCR of the single components in the branch is greater than the "let through current" of the fuse (i.e. the components downstream of the fuse must be able to carry the current the fuse lets through before breaking). Moreover, the breaking capacity of the BCPDs downstream of the fuse or the SCCR of the motor controller + BCPD combination must be greater than the breaking capacity of the fuse. In other words, although the SCCR of the motor controller can be increased by means of the coordination with the magnetic protection of the BCPD, the SCCR of the BCPD can not be increased because of the presence of the fuse in the feeder. If the breaking capacity of the BCPD is greater than that of the fuse, then the SCCR of the fuse is considered (this is the only case).

b) The smallest SCCR of all BCPDs downstream of the fuse if condition (a) is met but the breaking capacity of any of the BCPDs (or the SCCR of the motor controller + BCPD combination) is less than the breaking capacity of the fuse.

(c) The smallest SCCR among all the devices in the branches downstream of the fuse if none of the above conditions are met.

In conclusion, according to UL 508A, the use of any limiting device in the branch (whether fuse or current limiting circuit breaker) does not increase the SCCR of the terminal block, which therefore, as the "weakest" component in the control panel, lowers the SCCR of the entire branch circuit. The only way to increase the SCCR of the Branch Circuit, avoiding any constraint due to the terminal block, is represented by the proper choice of a current limiting device in the feeder (following the conditions listed above).

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