1Commonly used safety measure: “Earthed Equipotential is at ion & Automatic Disconnection of Supply”
2Inspection and verification of L.V electrical system as per IEC 60364-6
Safety is achieved by (1) Separation, (2) Double / reinforced insulation (class 2 equipment), (3) Earthed equipotentialisation and automatic disconnection of supply and (4) using E.L.V network. Out of these, commonly used safety measure in buildings is Automatic Disconnection of Supply.
Fire in buildings due to electrical short circuit is common from the day of invention of electricity. Statistics shows more than 40 % fires in buildings are due to electrical reason, mostly assumed due to short circuit.
In TN network disconnection of power supply need to be achieved under the following condition
Zs XIa 80 OR Zs 80 / Ia
Where Zs = fault loop impedance, Ia = current ensuring
the automatic operation of disconnecting device &
8o = conventional voltage limits. In a TT network the
condition is RA X Ia 80
Where RA = resistance of the earthed system for exposed conductive parts, Ia = operating currents of the disconnecting series device or settings of shunt UHOD\V, & 8o = conventional voltage limit.
Earthing is simple, but an important subject. Mis-interpretations of IS3043 and IS 732 cause hazard.
Some common myths are
|Earthing means Connection
to a vertical electrode such as a pipe or a rod
|Connection of the exposed conductive parts of an installation to the main earthing terminal of that installation. (ref IS 732 – 2016 draft)|
|Electricity Goes to ground / Earth||Current leave the source and return back to the source. In some case earth/soil is used to return the earth fault current back to the source.|
|Double earthing – Two connection to two vertical earth electrode in soil.||Two distinct and separate fault return path designed to reduce fault loop impedance and ensure safety by “Automatic Disconnection of Power supply” during an earth fault.|
|Safety is increased by reducing the earth electrode resistance to soil||Electrical Safety is achieved by
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For the purpose of Automatic Disconnection of Supply during earth fault by RCD in a TT network, connection to a vertical earth electrode is necessary. In TN network with PME (Indian TN-C-S / Indian TN-S as per IS 3043) connections to earth electrodes (in addition to the protective conductor) is necessary to reduce fault voltage and increase safety.