In circumstances when the circuit is subjected to an overload current, this condition is intended to ensure operation of the device within a time suitable to protect the insulation against damage from any high conductor temperatures caused by the overload current. This is achieved by requiring the conductors to have suffcient current carrying capacity (Iz) to carry the rated current of the circuit protective device (In). This condition is intended to give protection under normal load conditions such that the steady state temperature of the conductors will not exceed the maximum admissible value (such as 70 ₀C for thermoplastic insulated conductors). Consequently, a device protecting such a load must be carefully selected to ensure that it will not be operated by such ‘starting currents’.Īs shown in Table 1, circuit-breakers designed to BS EN 60898-1 are classified into three distinct types – B, C, and D – according to the magnitude of current required to achieve automatic disconnection. ![]() ![]() In particular, certain types of electrical load such as discharge lighting, AC induction motors and transformers take a much higher current during starting than during normal operation.
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