Page 322 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Current setting

Usually achieved by the use of tapping’s on the relay operating coil.
Pick-up current = Rated secondary current of C.T x Current setting
Plug-Setting Multiplier (P.S.M.)
It is the ratio of fault current in relay coil to the pick-up current i.e.

Fault current in relay coil
P.S.M. =
Pick - up current
Functional relays
• Induction type overcurrent relays
• Induction type reverse power relays

• Distance relays
• Differential relays
• Translay scheme
Equipments used in substations

Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage
caused by over current or short circuit. Its basic function is to interrupt current flow after protective relays detect a
fault. A circuit breaker can be reset (either manually or automatically) to receive normal operation.
Once a fault is detected, a circuit breaker contact must open to interrupt the circuit, some mechanically stored
energy contained within the breaker is used to separate the contacts, although some of the energy required may
be obtained from the fault current itself. The contacts must carry the load current without excessive heating and
must also withstand the heat of arc produced when interrupting the circuit.
Different types of breakers use vacuum, air, insulating gas(SF6) or oil as the arc quenching medium.
Lightning arrestor
A lightning arrestor is a device used on power systems above 1000 V to protect other equipment from lightning
and switching surges. It is essentially a collection billions of microscopic junctions of metal oxide grains that turn
ON and OFF in micro seconds to form a current path from the top terminal to the ground terminal of the arrestor.
Underground cables

1 These cables give greater safety to the public and better out look to the city.
2 These are used in submarine crossings.
3 These cables provide less interference with amenities.
1 Classification of cables
a The type of insulating material used by the manufacturer.
b The voltage for which they are used manufactured.

2 Types of cables for three phase service
In practice underground cables are generally used to deliver 3-phase power. For the purpose three core cable or
three single core cables may be used. For voltage up to 66kv 3-core cable is preferred due to economic reasons.

The following cables are generally used for three phase service are:
a Belted cables – up to 11kv.
b screened cables – from 22kv to 66kv.
c pressure cables – beyond 66kv.

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CITS : Power - Electrician & Wireman - Lesson 106-116 CITS : Power - Electrician & Wireman - Lesson 106-116