How The Earth Protects
In a system with a grounded (earthed) neutral, connecting all non-current-carrying metallic parts of equipment to earth ground at the main service panel, will ensure that current due to faults (such as a "hot" wire touching the frame or chassis of the device) will be diverted to earth. In a TN system where there is a direct connection from the installation earth to the transformer neutral, earthing will allow the branch circuit over-current protection (a fuse or circuit breaker) to detect the fault rapidly and interrupt the circuit.
In the case of a TT system where the impedance is high due to the lack of direct connection to the transformer neutral, an RCD (Residual-Current Device, sometimes known as a Residual Current Circuit Breaker or Ground Fault Circuit Interrupter) must be used to provide disconnection. RCDs are also used in other situations where rapid disconnection of small earth faults (including a human touching a live wire by accident, or damage) is desired.
Read more about this topic: Electrical Bonding
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