Fault Clearing Time

Definition

The time from the occurrence of the fault to the final arc interruption in the circuit breaker

Mathematically,

Fault Clearing Time = [Relay Time] + [Breaker Time]

Relay Time: Time from the fault occurrence to the closing of relay contacts

Breaker Time: Time from the closing of relay contacts to the final arc extinction in the circuit breaker

Significance of Fault Clearing

• The rapid clearing of faults minimises the damage to the system
• Slow relays and circuit breakers should not be preferred for protection where stability is important

Time Lag in Fault Clearing

Although fast fault clearing is desirable, time lag is purposely required for the following reasons,

• To discriminate between primary and backup protection
• To prevent the operation of the relay during the transients, starting currents and permissible load fluctuations

Cycles of Operations

Relay Cycles

The relay time of fast relays is of the order of a few cycles, and that of inverse time relay can be adjusted between 6 sec to 60 sec

Circuit Breakers

• Slow Circuit Breaker -------- 5 cycles
• Fast Circuit Breaker --------- 2 cycles to 3 cycles

Protection System and Protection Scheme

Protection System

The protection system is named according to the principle of operation and abnormal conditions. Some of the examples are;

• Overcurrent Protection
• Differential Protection
• Distance Protection
• Under frequency Protection

Protection Scheme

It comprises of group of protection systems. It is named after the protected equipment.

• Generator Protection
• Transformer Protection

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