Electrical Workers Registration Board (EWRB) Theory Practice Test

The term "cut off current" in the context of HRC (High Rupturing Capacity) fuses refers to the specific fault current level at which the fuse will effectively break the circuit to protect the electrical system. When an overcurrent situation occurs, such as a short circuit, the fuse must quickly detect this excess current to prevent potential damage to the electrical equipment.

HRC fuses are designed to operate under high fault conditions, and the cut off current signifies the threshold at which the fuse's element melts or blows, disconnecting the circuit and thus safeguarding downstream devices. This action occurs faster than other protective devices would react, ensuring minimal damage in the event of a short circuit.

In contrast, the maximum current that can be handled continuously relates to the fuse's sustained operational capacity rather than its protective capabilities during fault conditions. The current required for operational efficiency does not coincide with the protective function of the fuse, nor does the initial surge current during a motor startup, which is often significantly higher than normal operating currents but does not relate to the fuse's cut-off function.