Electrical Earthing and Circuit Protective Devices

Electrical Earthing and Circuit Protective Devices Reasons For Earthing

There are four basic reasons for earthing an electrical system:

REDUCTION OF SHOCK HAZARD

An earthed chassis will provide a low impedance path to ground for any active voltage that appears on the case or metalwork.

SAFE DISCONNECTION OF SUPPLY

If the exposed metalwork of an appliance were connected by a fault condition to an active conductor, the active current path needs to be able to flow to ground via a low impedance path so an excessive current will flow that will cause a protective device such as a fuse or circuit breaker to open circuit, thus prenting an electric shock situation.

MINIMISE THE DANGER OF FIRE HAZARD

The earth wire should always be in good contact with all metal parts of any appliance the user may touch. Should the equipment become faulty by the active wire touching the case, then current will rush to earth, smoke will rise from the equipment, and if the current is very high the fuse in the active line will blow, disconnecting mains power from the faulty appliance. Without the earth wire, this fault would go undetected, allowing high current to flow through the patient whenever the patient formed a circuit to ground. Standards limiting current in any circuit are vital to prevent excessive heating of the power supply wires leading to fires.

VOLTAGE STABILITY

Neutral earthing : Star point of Wye connected transformers and alternator windings are earthed primarily to provide voltage stability to system on account of imbalance in loading resulting in neutral current flow. Neutral point allows vector sum of three phase currents displaced at 120 degrees nullifying it to zero. Secondary purpose served is detection of earth faults or earth leakages and allow the protective gear to operate in desired time.

In the view of high harmonic contents found in modern facilities, the zero sequence or triplen harmonic currents overload the neutrals if not adequately sized. This is because these are in phase and hence do not cancel out at neutral point. So neutrals are required to be sized equal to or sometimes double the size of line conductors to carry "algebraic sum" of triplen components in the lines.

Neutral earthing for applicable to low voltage distribution systems upto 1000 V ac comprise of solid earthing with no intentional resistance connected in the path.