Electricity, protective means fuses

In order to safeguard against electric shock at the far end of the cable, where the AC root mean square voltage may exceed 50 V, the earth loop impedance must be limited to a particular value. This value is such that the fault current should only be passed by the protective device at the supply for a specific period of time, i.e. to satisfy the /-squared- criterion given in sub-section 13.1.1. The correlation of loop impedance, current and time varies with the type of protective device, e.g. fuse. 

The electricity supply company has one of its conductors solidly connected to the earth and every circuit supplied by the company must have one of its conductors connected to earth. This means that if there is a fault, such as a break in the circuit, the current, known as the earth fault current, will return directly to earth, which forms the circuit of least resistance, thus maintaining the supply circuit. This process is known as earthing. Other devices, such as fuses and residual currertt devices, which will be described later, will also be needed within the circuit to interrupt the current flow to earth so as to protect people from electric shock and equipment from overheating. Good and effective earthing is absolutely essential and must be connected and checked by a competent person. Where a direct contact with earth is not possible, for example, in a motor car, a common voltage reference point is used, such as the vehicle chassis. 

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