Earthing is the process of connecting the current carrying and non-current carrying metal parts of an electrical system to dissipate the currents to the ground. Even though earthed system has so many advantages, the unearthed system is preferred for safety-critical applications, such as intensive care units, where a failure of the power supply would have disastrous consequences.
IS 3043 explains about the factors influencing the choice of earthed or unearthed system and the details are as follows.
IS 3043 explains about the following factors for comparing the earthed and unearthed systems. They are,
The main objective of earthing is to ensure the safety of operating person & equipment, reference for the system voltage, dissipating the impulse lightning current and discharge the static charges.
Some of the more common sources of over-voltages on a power system are the following:
b) Switching surges,
d) Contact with a high voltage system,
e) Line-to-ground fault,
f) Resonant conditions, and
g) Re-striking ground faults.
Lightning is a natural source of over voltage. Surge arresters are recommended for lightning protection. Neutral grounding is effective in reducing transient build up by reducing the neutral displacement from ground potential and the destructiveness of any high frequency voltage oscillations following each arc initiation or restrike.
In an unearthed system resonant over-voltages and arcing, re-striking or vibrating ground faults can produce surge voltages as high as 6 times the normal voltage.