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Ohm's Law

Whenever electric current flows through a conductor, the following three factors are present :

1. The pressure or potential difference V across the conductor (measured in volts) causing current to flow.
2. The opposition or resistance R of the conductor (measured in ohms) which must be overcome.
3. The current strength I (measured in amperes) which is maintained in the conductor as a result of pressure overcoming the resistance.

There exists a definite relationship between the three quantities involved and is known as Ohm's Law. It may be stated Thus :

"The ratio of potential difference (V) between any two points of a conductor to the current (I) flowing between them is constant, provided the temperature of the conductor doesn't change"

In other words, V/I = Constant           or      V/I = R

where R is the resistance of the conductor between the two points considered.

OR

" According to ohms law, the flowing through a conductor is directly proportional to the potential difference, across the ends of the conductor and inversely proportional to the conductor resistance."

i.e.., I α V and I α 1/R

where, I is the current, V is the voltage and R is the resistance of the conductor material.

thus

I α V/R

OR

"At constant temperature the current flowing through a conductor is directly proportional to the potential difference between its ends."

I α V ;             V α I ;            V = IR ;

Where  R is a constant known as the resistance of the conductor.

OR

" The current is directly proportional to the voltage and inversely proportional to the resistance when the temperature remains constant."

i.e.., I = V/R

Voltage Divider Formula

It's used for finding voltage drops across different resistance connected in series. Since in a series circuit, same current flows through each resistor, voltage drops are directly proportional to their resistance

R = R1+R2+R3

V1 = VxR1/R

V2 = VxR2/R

V3 = VxR3/R

V = V1+V2+V3

Kirchhoff's First Laws or Point Law or Current Law (KCL)

 These laws are more comprehensive than ohm's law and are used for solve electrical networks

It states that:

"in any network of conductors, the algebraic sum of the current meeting at a point (or junction) is zero."

OR

"the total current leaving a junction is equal to the total current entering the junction."

Incoming currents = outgoing currents

ΣI = 0   .... at a junction

Utilization Categories

 utilization categories are defined by IEC (International Electrotechnical Commission) standards and indicate the type of electrical load and duty cycle of the loads to ease selection of contactors and relays.

AC1       Non-inductive or slightly inductive loads, example: resistive furnaces, heaters
AC2       Slip-ring motors: switching off
AC3       Squirrel-cage motors: starting, switches off motors during running time
AC4       Squirrel-cage motors: starting, plugging, inching
AC5a     Switching of discharge lamps
AC5b     Switching of incandescent lamps
AC6a     Switching of transformers
AC6b     Switching of capacitor banks
AC7a     Slightly inductive loads in household appliances: examples: mixers, blenders
AC7b     Motor-loads for household appliances: examples: fans, central vacuum
AC8a     Hermetic refrigerant compressor motor control with manual resetting overloads
AC8b     Hermetic refrigerant compressor motor control with automatic resetting overloads
AC12     Control of resisitive loads and solid state loads with opto-coupler isolation
AC13     Control of solid state loads with transformer isolation
AC14     Control of small electromagnetic loads
AC15     Control of A.C. electromagnetic loads
AC20     Connecting and disconnecting under no-load conditions
AC21     Switching of resistive loads, including moderate overloads
AC22     Switching of mixed resistive and inductive loads, including moderate overloads
AC23     Switching of motor loads or other highly inductive loads
A            Protection of circuits, with no rated short-time withstand current
B            Protection of circuits, with a rated short-time withstand current
DC1       Non Inductive or slightly inductive loads, resistance furnaces, heaters
DC3       Shunt-motors, starting, plugging(1), inching(2), dynamic breaking of motors
DC5       Series-motors, starting, plugging(1), inching(2), dynamic breaking of motors
DC6       Switching of incandescent lamps
DC12     Control of resistive loads and solid state loads with opto-coupler isolation
DC13     Control of D.C. electromagnetics
DC14     Control of D.C. electromagnetic loads having economy resistors in the circuit
DC20     Connecting and disconnecting under no-load conditions
DC21     Switching of resistive loads, including moderate overloads
DC22     Switching of mixed resistive and inductive loads, including moderate overloads (i.e. shunt motors)
DC23     Switching of highly inductive loads (i.e. series motors)


The standard utilization categories defines the current making and breaking values for contactors. they depend on the following..

1. Which type of load to be controlled (squirrel cage or slip ring induction motor, resistors)
2. The operating cycle conditions - Motor running, reverse operation, stalled or starting, counter current braking.


AC-1 - Non-inductive or slightly inductive loads
Ex : resistive load, heating, distribution.

AC-2 - Starting of slip-ring motors

AC-3 - Starting of squirrel-cage motors and switching off only after the motor is up to speed. Make Locked Rotor Amps (LRA), Break Full Load Amps (FLA)
Ex : all squirrel cage motors, lifts, escalators, conveyors, bucket elevators, compressors, pumps, mixers, air conditioning units, etc.

AC-4 - Starting of squirrel-cage motors with inching and plugging duty. Rapid Start/Stop. (Make and Break LRA)

AC-11 - Auxiliary (control) circuits

ANSI Device Numbers

In the design of electrical power systems, the ANSI (American National Standards Institute) Standard Device Numbers (ANSI /IEEE Standard C37.2) denote what features a protective device supports (such as a relay or circuit breaker). These types of devices protect electrical systems and components from damage when an unwanted event occurs, such as an electrical fault. Device numbers are used to identify the functions of devices shown on a schematic diagram. Function descriptions are given in the standard.

List of device numbers and acronyms

1 – Master Element



Any initiating Device will be term as master element. Control Switches, voltage relays, Current Sensors etc. This device serves may be directly or it will operate some other protective device and will place equipment in or out of protection.

Ex: TNC switch ( Trip Neutral Close) to act as breaker control switch. So TNC is master element for Breaker.

2 – Time Delay Starting or Closing Relay

3 – Checking or Interlocking Relay

4 – Master Contactor

5 – Stopping Device

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 Device

32 – Directional 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 or "Out-of-Step" 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

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 – For specific applications where other numbers are not suitable

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

AFD – Arc Flash Detector

CLK – Clock or Timing Source

DDR – Dynamic Disturbance Recorder

DFR – Digital Fault Recorder

ENV – Environmental Data

HIZ – High Impedance Fault Detector

HMI – Human Machine Interface

HST – Historian

LGC – Scheme Logic

MET – Substation Metering

PDC – Phasor Data Concentrator

PMU – Phasor Measurement Unit

PQM – Power Quality Monitor

RIO – Remote Input/Output Device

RTU – Remote Terminal Unit/Data Concentrator

SER – Sequence of Events Recorder

TCM – Trip Circuit Monitor

SOTF – Switch On To Fault

A suffix letter or number may be used with the device number. for example, suffix N is used if the device is connected to a Neutral wire (example: 59N in a relay is used for protection against Neutral Displacement) and suffixes X,Y,Z are used for auxiliary devices. Similarly, the "G" suffix denotes a "ground", hence a "51G" is a time over current ground relay. 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 the instantaneous/time-delay AC over current relay denoted as 50/51.

For device 16, the suffix letters further define the device: the first suffix letter is 'S' for serial or 'E' for Ethernet. The subsequent letters are: 'C' security processing function (e.g. VPN, encryption), 'F' firewall or message filter, 'M' network managed function, 'R' router, 'S' switch and 'T' telephone component. Thus a managed Ethernet switch would be 16ESM.

Types of Relay

First Letter - Operation Quantity

A - Phase angle comparison

B - Balanced Current

C - Current (Amps)

D - Differential

E - Direction

F - Frequency

K - Rate of rise of current

M - Manual

O - Oil Pressure

P - Poly phase VA

R - Reactive VA

S - Slip Frequency

T - Temperature

V - Potential

W - Watts

X - Reactance

Y - Admittance (Conductance)

Z - Impedance

Second Letter - Measurement

A - Attracted armature

B - Buchholz

C - Induction

D - Induction Disc

G - Galvanometer

I - Transmitter

J - Mixed Types

M - Magnet (polarized)

P - Plug

R - Rectifier

S - Synchronized Motor

T - Transistor

W - Weight (Gravity)



Third Letter - Application

A - Auxiliary

B - Testing

C - Carrier or Counting

D - Directional

E - Earth/Ground

F - Plug or Alarm Indicator

G - General or Generator

H - Harmonic Restricted

J - Tripping

JE - Tripping (Electrical Reset)

JM - Tripping (Manual Reset)

JS - Tripping (Self)

JC - Tripping (Conductor)

K - Check Alarm

L - Local Limiting

M - Same Phi or motor

N - Negative Sequence

O - Out of Step

P - Potential Failure

R - Re-closing

S - Synchronizing

T - Timer or Transformer

U - Definite Time

V - Voltage Resistant

W - Piolet Wire

WA - Interposing

WS - Inter-tripping

X - Supervisory

Y - Flash Back (Black Fire)

Z - Special Application

Z0 - Zero Sequence

Examples :

CDG - Current (operation), Disc(Measurement), General Purpose(Application)

CAG - Current (operation), Attracted armature(Measurement), General Purpose(Application)

VAJ - Potential (operation), Attracted armature(Measurement), Tripping(Application)

VTT - Potential (operation), Transmitter(Measurement), Transformer(Application)

VAZ - Potential (operation), Attracted armature(Measurement), Special Application

VAA - Potential (operation), Attracted armature(Measurement), Auxiliary (Application)