Electric symbols, and circuits

Any electrical drawing representing an electrical installation or a circuit takes the help of specific symbols to represent various electrical devices in shorthand. This provides a quick idea to the reader about a circuit or installation, and is particularly useful while troubleshooting.

A. Electrical circuits

Electrical circuits are circuits used to interconnect different electrical equipments together to enable the working of an electrical device. Electrical schematics are commonly classified into power circuit and control circuit. A power circuit consists of the main power device (a motor, a generator, or other power devices) along with heavy power conductors, contactors, protection devices. A control circuit consists of switches, field device contacts, timers, relay coils, relay contacts, protection devices, and light power conductors.

B. Power circuits

Power circuits are required for carrying power to or from heavy electrical equipments like motors, alternators, or any electrical installation. They carry out the following functions :

• Isolation using devices such as isolators, linked switches and circuit breaks.
• Circuit control using devices such as contactors, motor circuit breakers, etc.
• Protection against overload and short-circuits using thermal overload relays,
  electro-magnetic relays, circuit breakers, with releases, fuses, etc.

Electrical devices and symbols

Power circuits have to carry heavy power and therefore, they consist of heavy conductors along with contactors used for switching the power on and off. Protection devices are also included in the same power circuit for resolving an overload condition or any other concerned faults. A Direct-on-line (DOL) starter power circuit used for a three-phase induction motor. As shown, the three-phase power input is connected to the motor through a contactor. Power is passed to the motor when the contacts (of the contactor) are in closed condition. Protection devices such as fuses, and overload relays are provided in series with power conductors to detect unhealthy conditions during operation.

Power circuit for a motor

C. Control circuit

A control circuit is for the automatic control of equipment, for safety interlocking, and sequencing the operations of the plant equipment and machines.

Control circuits hardware consists of relay contacts, wires, hardware timers, and counters, relay coils, etc. These consist of input contacts representing various conditions; the output coils are energized or de-energized depending on the input conditions represented by the control circuit.

Input contacts represent the binary state of the condition :

• True or false.
• On or off.

There are two types of contacts NO (normally open) and NC (normally closed).

• Input contact : These are contacts of relays, contactors, timers, counter, field instrument switches, pressure switches, limit switches, etc.
• Output coil: These have two states – On or Off. Output coil can be auxiliary contactor or Main contactor coil.

Article source form :

Practical Troubleshooting of Electrical
Equipment and Control Circuits

Mark Brown Pr.Eng, DipEE, B.Sc (Elec Eng),
Senior Staff Engineer, IDC Technologies, Perth, Australia

Jawahar Rawtani M.Sc (Tech), MBA,
Senior Electrical Engineer, Nashik, India

Dinesh Patil BEng,
Patil and Associates.