Services      About Us      Blog      Careers     Contact Us     


Understanding the Purpose of Electrical Relays, Their Working and Selection

By Anzar Hasan, Chief Inspector

Electrical relays are simple switches that basically have an electromagnet and set of contacts, which are operated both electrically and mechanically. The main operation of a relay comes in places where only a low-power signal can be used to control a circuit. It is also used in places where only one signal can be used to control a lot of circuits. Relays are safety devices that protect a circuit from advertent operation or an electrical fault in the circuit. Advertent operation could be described as the usage of Relay 25 used in the generator circuit in power stations. To reduce the possibility of catastrophic mechanical failure of the generators from inadvertent attempts by operators to synchronize the generators out of phase, a synchronization check relay, device number 25 is installed. If it is not installed a recommendation is warranted.

The relay primarily has four main components:

• Electromagnet
• Movable Armature
• Switch point contacts
• Spring

IEEE Device Numbers
1 Master Element
2 Time Delay Starting or Closing Relay
3 Checking or Interlocking Relay
4 Master Contactor
5 Stopping
6 Starting Circuit Breaker
7 Rate of Change Relay
8 Control Power Disconnecting Device
9 Reversing Device
10 Unit Sequence Switch
11 Multi-function Device
12 Overspeed Device
13 Synchronous-speed Device
14 Underspeed Device
15 Speed – or Frequency, Matching Device
16 Data Communications Device
17 Shunting or Discharge Switch
18 Accelerating or Decelerating Device
19 Starting to Running Transition Contactor
20 Electrically Operated Valve
21 Distance Relay
22 Equalizer Circuit Breaker
23 Temperature Control Device
24 Volts Per Hertz Relay
25 Synchronizing or Synchronism-Check Device
26 Apparatus Thermal Device
27 Undervoltage Relay
28 Flame detector
29 Isolating Contactor or Switch
30 Annunciator Relay
31 Separate Excitation
32 Directional Power Relay or Reverse Power Relay
33 Position Switch
34 Master Sequence Device
35 Brush-Operating or Slip-Ring Short-Circuiting Device
36 Polarity or Polarizing Voltage Devices
37 Undercurrent or Underpower Relay
38 Bearing Protective Device
39 Mechanical Condition Monitor
40 Field (over/under excitation) Relay
41 Field Circuit Breaker
42 Running Circuit Breaker
43 Manual Transfer or Selector Device
44 Unit Sequence Starting Relay
45 Abnormal Atmospheric Condition Monitor
46 Reverse-phase or Phase-Balance Current Relay
47 Phase-Sequence or Phase-Balance Voltage Relay
48 Incomplete Sequence Relay
49 Machine or Transformer, Thermal Relay
50 Instantaneous Overcurrent Relay
51 AC Inverse Time Overcurrent Relay
52 AC Circuit Breaker
53 Exciter or DC Generator Relay
54 Turning Gear Engaging Device
55 Power Factor Relay
56 Field Application Relay
57 Short-Circuiting or Grounding Device
58 Rectification Failure Relay
59 Overvoltage Relay
60 Voltage or Current Balance Relay
61 Density Switch or Sensor
62 Time-Delay Stopping or Opening Relay
63 Pressure Switch
64 Ground Detector Relay
65 Governor
66 Notching or Jogging Device
67 AC Directional Overcurrent Relay
68 Blocking Relay
69 Permissive Control Device
70 Rheostat
71 Liquid Level Switch
72 DC Circuit Breaker
73 Load-Resistor Contactor
74 Alarm Relay
75 Position Changing Mechanism
76 DC Overcurrent Relay
77 Telemetering Device
78 Phase-Angle Measuring Relay or “Out-of-Step” Relay
79 AC Reclosing Relay
80 Flow Switch
81 Frequency Relay
82 DC Reclosing Relay
83 Automatic Selective Control or Transfer Relay
84 Operating Mechanism
85 Communications, Carrier or Pilot-Wire Relay
86 Lockout Relay
87 Differential Protective Relay
88 Auxiliary Motor or Motor Generator
89 Line Switch
90 Regulating Device
91 Voltage Directional Relay
92 Voltage and Power Directional Relay
93 Field Changing Contactor
94 Tripping or Trip-Free Relay
95 For specific applications where other numbers are not suitable
96 Busbar Trip Lockout relay
97 For specific applications where other numbers are not suitable
98 For specific applications where other numbers are not suitable
99 For specific applications where other numbers are not suitable

Suffixes & Prefixes

Suffix letters or numbers may be used with device numbers. For example, the suffix “ N” is used if the device is connected to a neutral wire, hence 59N is a relay used for protection against neutral displacement & suffixes X, Y, Z are used for auxiliary devices. Similarly, the “G” suffix is used to denote a “ground”, hence “51G” is a time overcurrent ground relay. The “G” suffix can also mean “generator”, hence “87G” is a generator differential relay while “87T” is a transformer differential relay. “F” can denote “field” on a generator or “fuse”, as in the protective fuse for a transformer.

Suffix numbers are used to distinguish multiple “same” devices in the same equipment such as 51-1 & 51–2. Device numbers may be combined if the device provides multiple functions, such as instantaneous & inverse time overcurrent relay denoted as 50/51.

« »

Global Engineering Excellence

Part owned by GAPS, an XL Group company