CODE OF PRACTICE : PROTECTION
1.00 Circuitry
1.01 The entire wiring of circuitry for indications, alarms, metering and protection should
be permanent wiring
1.02 There is no place for temporary wiring or adhocism in Relay circuitry
1.03 The leads should be identified by ferrules near terminals.
1.04 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.
1.05 The wiring should be by copper leads for C.T secondaries for all cores (i.e.) metering
as well as protection.
1.06 The wiring should be by copper leads for PT secondaries also wherever they are
intended for protection.
1.07 The copper lead for 1.05 & 1.06 above should be stranded but not single lead type.
1.08 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.
1.09 The terminations should be lugged by ring shape ‘O’ lugs. ‘U’ shape lugs should be
avoided.
1.10 For CT Secondary terminations, two nuts with one spring washer and two flat washers
to be compulsorily used.
1.11 The terminal strips should be stud type with nuts and not screw-in-type.
1.12 Wherever two batteries are available, the primary protection and back-up protection
should be from different batteries.
1.13 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.
1.14 D.C and A.C supplies should not be taken through different cores of the same cable.
1.15 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.
1.16 The D.C yard lighting for emergency lighting should be through independent cables
and not mixed up with protection and other circuitry.
1.17 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.
1.18 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.
1.19 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.
1.20 Standard colour codes for leads in control cable of different sizes should be as denoted
on the cover page.
1.21 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.
1.22 CTs with 1 amp secondary rating should be used compulsorily where meters,
protective devices etc. are remotely situated with reference to equipment.
1.23 The CT ratios available and adopted with number of cores shall be displayed on each
panel as follows: (with underlined position as adopted)
400 - 200 - 100 / 1-1-1
1.24 Wherever CT cores are not used “SHORTING LOOPS” should be provided near CT
secondary terminals and not in marshaling boxes or at panels
1.25 The Cable entries near equipment, marshaling boxes and panels should be by use of
appropriate size glands.
1.26 The Wiring inside the panels should be clear and neatly fastened avoiding loose wires.
1.27 All wires not in use should not only be disconnected but removed from panels
.
1.28 PT secondaries should have group MOCBs with D.C alarm. Fuses at different panels
should not be used.
1.29 Few cells from a battery of cells should not be used for separate low voltage D.C
circuits. D.C - D.C converters only should be employed utilising full D.C voltage of
the entire battery as input.
2.00 STANDARD LEAD NUMBERS
Certain lead numbers are standardised as follows and should be compulsorily adopted
with ferrules at terminations of leads.
J1 - Positive
J2 - Negative
Controls & Alarms
Remote Close : K15R
Remote Trip : K5R
Local Close : K15L
Local Trip : K5L