Protection and Wiring Practice in Electrical Substation

The entire wiring of circuitry for indications, alarms, metering and protection should be permanent wiring.

There is no place for temporary wiring or adhocism in Relay circuitry.

The leads should be identified by ferrules near terminals.

Every lead should end at a terminal point and no junctions by twisting is allowed. If two wires are to be terminated at same terminal they may be connected at two different terminals and a loop provided.

The wiring should be by copper leads for C.T secondaries for all cores (i.e.) metering as well as protection.

The wiring should be by copper leads for PT secondaries also wherever they are intended for protection.

The copper lead for above should be stranded but not single lead type.

Aluminum leads can be used for indication, alarms and PT secondaries for metering but stranded wires only are to be used. However where PTs are employed for commercial metering, stranded copper wires are to be used.

The terminations should be lugged by ring shape ‘O’ lugs. ‘U’ shape lugs should be avoided.

For CT Secondary terminations, two nuts with one spring washer and two flat washers to be compulsorily used.

The terminal strips should be stud type with nuts and not screw-in-type.

Wherever two batteries are available, the primary protection and back-up protection should be from different batteries.

Where there is only one battery at a Power Substation, the primary and back-up protections should be given D.C supply through two individual circuits with independent fuses run from D.C bus. 1.13A When CBs have two trip coils, both main protection and backup protection will energise both the trip coils.

 D.C and A.C supplies should not be taken through different cores of the same cable.

Independent D.C cables should be run to every equipment in the yard and looping of D.C supply either in the yard or in the control room from one equipment to the other is not permitted.

The D.C yard lighting for emergency lighting should be through independent cables and not mixed up with protection and other circuitry.

For indications, alarms, annunciations, controls (closing coil, trip coil, etc. negative (-ve) is always given direct and positive (+ve) is supplied only ‘on commands’ like close, trip, relay trip, etc.

Where D.C protection supply is at 24 volts or 32 volts, the battery units should be very near the equipment and not in the control rooms.

 In cases of 1.18 above, each tripping units (24 volts or 32 volts battery with charger) should not be used for more than two circuit breakers or equipment.

Standard colour codes for leads in control cable of different sizes should be as denoted on the cover page.

The lead numbers are also standardised as follows so that any MRT Engineer can easily identify the purpose for which the lead is connected by noting the lead number.