Terasaki - How to Choose a Circuit Breaker Discrimination

Circuit Breaker Product Selection - Load Discrimination - Withstand Criteria

Discrimination

The incoming device to the low voltage system is the first line of protection downstream of the transformer.

It protects:

• the transformer windings
• the conductors between transformer and switchboard; and
• the switchboard main busbars

A fault current in an outgoing circuit could also cause the incoming device to open, interrupting power to the entire system. To avoid this interruption the designer should ensure discrimination between the incoming device and outgoing circuit breakers.

Discrimination relies on the incoming device withstanding the fault - without opening or sustaining damage - for as long as it takes for the outgoing breaker to open. The easiest way to guarantee discrimination without further calculation is to ensure that incoming breakers can stay closed and withstand the maximum potential short-circuit current for at least one second. The protection setting should include a time-delay in the short-circuit characteristic.

Short-Circuit Withstand

Icw (kA, s) according to BS EN 60947-2

Withstand = Fault Level = Guaranteed Discrimination

Icw (ls)= lcs - Guaranteed Discrimination

It is obvious from the above why most incoming devices are ACBs! To make selection even easier, all Terasaki ACBs can withstand fault currents up to their breaking capacity (Icw, (1s) = Ics). This means that if the ACB is correctly selected according to fault level, as described above, discrimination is guaranteed with no further checks (provided the relay settings are correct.

Further Downstream .... DIN Modular Devices

Miniature Circuit Breakers (MCBs) operate on the same principles as thermal- magnetic MCCBs. Magnetic cores respond instantaneously to short-circuits and thermal elements respond to overloads. Current ratings from 0.5A to 125A with fault ratings from 6kA to 10kA are available. Terasaki's TD3 MCBs are independently tested and certified by KEMA to prove their performance.

Residual Current Breakers (RCCBs) are essential devices in many circuits: for the prevention of electric shock and reduction of the risks of fire. The use of RCCB protection in final circuits is increasing in the UK due to the requirements of the IET wiring regulations: BS7671.

TD3 Residual Current Circuit Breakers with Overload Protection (RCBO's) provide earth leakage protection and the overload and short circuit functionality of an MCB, eliminating the need for a second device in the circuit. RCBOs have the same width as a single pole MCB. This means that a single module device has the combined protection as that of an MCB and RCCB but can be easily installed in a single phase outgoing way on a distribution board.