Code of practice for power system protection

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

02 - The leads should be marked and identified by ferrules near terminals.

03 - Every lead should end at a terminal point and no junction by twisting is allowed.

04 - The wiring should be by copper leads for C.T secondary for all cores i.e. metering cores as well as protection cores and for PT secondary for protection core.

05 - The wiring should be by copper leads 1.07. The copper lead for 1.05 & 1.06 above should be stranded but not single lead type.

06 - Aluminum leads can be used for indication, alarms and PT secondary for metering but stranded wires only are to be used. But copper leads are always preferable for these said purposes.

07 - The terminations should be lugged by ring shape ‘O’ lugs. ‘U’ shape lugs should be avoided since ‘U’ shape lugs may slip if terminal is loosen.

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

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

10 - Wherever two sets of battery are available, the primary protection and back-up protection should be from different battery .

11 - Where there is only one battery at an Electrical 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.

12 - When CBs have two trip coils, both main protection and backup protection will energize both the trip coils.

13 - D.C and A.C supplies should not be taken through different cores of the same cable. Totally different cables should be used for DC and AC supplies.

14 - Independent D.C cables should be run to each equipment in the yard and looping of D.C supply from one equipment to other is not permitted.

15 - The D.C emergency lighting in substation should be through independent cables and not mixed up with protection and other circuitry.

16 - Standard color codes for wires in control circuit of different sizes should be as follows,

Indication, Alarm, trip, close etc 1.5 mm2 Gray
Red Phase Metering PT Circuit 1.5 mm2 Red
Yellow Phase Metering PT Circuit 1.5 mm2 Yellow
Blue Phase Metering PT Circuit 1.5 mm2 Blue
Red Phase Protection PT Circuit 2.5 mm2 Red
Yellow Phase Protection PT Circuit 2.5 mm2 Yellow
Blue Phase Protection PT Circuit 2.5 mm2 Blue
Red Phase Metering and Protection CT Circuit 2.5 mm2 Red
Yellow Phase Metering and Protection CT Circuit 2.5 mm2 Yellow
Blue Phase Metering and Protection CT Circuit 2.5 mm2 Blue
Phase for auxiliary AC supply 2.5 mm2 Red
Neutral for auxiliary AC supply 2.5 mm2 Black
Common star point of CTs 2.5 mm2 Black
Common star point of Protection PTs 2.5 mm2 Black
Common star point of Metering PTs 1.5 mm2 Black
Earthing Connection 2.5 mm2 Green

17 - The lead numbers are also standardized as follows so that anyone can easily identify the purpose for which the lead is connected.

Alphabet Series Purpose Example
J Series D.C Incoming J1, J2, etc.
K Series Control - Closing, Tripping, etc. K1, K2, K3 etc.
L Series Alarms, indications and annunciations L1, L2, L3, etc.
M Series Motor Circuit M1, M2, etc.
E Series Potential transformer secondaries E1, E2, E3, etc.
H Series LT A.C Supply H1, H2, H3, etc..
A Series C.T secondary for special protection A1, A2, A3, etc.
B Series Bus bar protection B1, B2, B3, etc..
C Series Protection Circuits C1, C2, C3, etc.
D Series Metering Circuit D1, D2, D3, etc.

18 - 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
19 - Wherever CT cores are not used “SHORTING LOOPS” should be provided in CT secondary terminals and not in marshaling boxes or at panels.

20 - The Cable entries in the equipment, marshaling boxes and panels should be through appropriate size of cable glands. No other means are allowed.

21 - PT secondaries should have group MOCBs with D.C alarm.

22 - Few cells from a battery set should not be used for separate low voltage D.C circuits. Here D.C - D.C converters may be employed for utilizing full D.C voltage of the entire battery as input.

Standard Lead Numbers used in Control Circuit of Protection of Power System

Certain lead numbers are standardized as follows and should be compulsorily adopted with ferrules at terminations of leads.

Main DC Positive supply – J1

Main DC Negative supply – J2

DC Positive bus inside panel – K1

DC Nagetive bus inside panel – K2

Remote Close - K15R

Remote Trip - K5R

Local Close - K15L

Local Trip - K5L

Metering CT secondaries – D11, D31, D51, D71 etc.

Protection CT secondaries – C11, C31, C51, C71 etc.

Special Protection CT secondaries – A11, A31, A51, A71 etc.

PT scondaries - E11, E31, E51, E71 etc.

Different Relay Device Number used in Protection of Power System

Mark Number Name of the Device
2 Time delay relay
3 Checking or Interlocking relay
21 Distance relay
25 Check synchronizing relay
27 Under voltage relay
30 Annunciator relay
32 Directional power (Reverse power) relay
37 Low forward power relay
40 Field failure (loss of excitation) relay
46 Negative phase sequence relay
49 Machine or Transformer Thermal relay
50 Instantaneous Over electric current relay
51 A.C IDMT Over electric current relay
52 Circuit breaker
52a Circuit breaker Auxiliary switch “Normally open” (‘a’ contact)
52b Circuit breaker Auxiliary switch “Normally closed” (‘b’ contact)
55 Power Factor relay
56 Field Application relay
59 Overvoltage relay
60 Voltage or electric current balance relay
64 Earth fault relay
67 Directional relay
68 Locking relay
74 Alarm relay
76 D.C Over electric current relay
78 Phase angle measuring or out of step relay
79 AC Auto reclose relay
80 Monitoring loss of DC supply
81 Frequency relay
81U Under frequency relay
81O Over frequency relay
83 Automatic selective control or transfer relay
85 Carrier or pilot wire receive relay
86 Tripping Relay
87 Differential relay
87G Generator differential relay
87GT Overall differential relay
87U UAT differential relay
87NT Restricted earth fault relay
95 Trip circuit supervision relay
99 Over flux relay
186A Auto reclose lockout relay
186B Auto reclose lockout relay

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