Sometimes we have seen that a ceiling fan suddenly starts moving in opposite direction with slow speed, what could be the defect?

The starting winding circuit is open-circuited may be due to burnt winding or damaged capacitor or the running winding is damaged. The fan is running only on one winding. The blades and thrust of air in the room accelerated the blades in opposite direction. As we know that the single phase motors are not self-starting, they will continue to move in the direction in which these are initially accelerated.

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What are the Functions of Substation Equipment?

Functions of Substation Equipments:

Sl. No.	Equipment	Function
	Bus-Bar	Incoming & outgoing circuits Connected to bus-bar
	Circuit Breaker	Automatic switching during normal or abnormal conditions
	Isolators	Disconnection under no-load condition for safety, isolation and maintenance.
	Earthing switch	To discharge the voltage on deadlines to earth
	Current Transformer	To step-down currents for measurement, control & protection
	Voltage Transformer	To step-down voltages for measurement, control & protection
	Lightning Arrester	To discharge lightning over voltages and switching over voltages to earth
	Shunt reactor	To control over voltages by providing reactive power compensation
	Neutral-Grounding resistor	To limit earth fault current
	Coupling capacitor	To provide connection between high voltage line & PLCC equipment
	Line –Trap	To prevent high frequency signals from entering other zones.
	Shunt capacitors	To provide compensations to reactive loads of lagging power factors
	Power Transformer	To step-up or step-down the voltage and transfer power from one a.c. voltage another a.c. voltage at the same frequency.
	Series Capacitor	Compensation of long lines.

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What is REF relay?

It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

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What are the errors in Current Transformer?

(a) Ratio error
Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100
The value of transformation ratio is not equal to the turns ratio.

(b) Phase angle error:
Phase angle =180/π[(ImCos δ-I1Sin δ)/nIs]

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What is an arc? Give the two methods of arc interruption?

Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or short circuit condition.

Methods of arc interruption: 

1. High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished 
2. Current zero method:-The arc is interrupted at current zero position that occurs 100 times a second in case of 50Hz power system frequency in ac.

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