Circuit Breakers - Part 2

Trip Circuit Supervision in Circuit Breakers

Trip circuit supervision in Circuit breakers is an vital part of any protection scheme. If the trip relay fails to operate, it may result in upstream tripping or even in damage to equipment. Trip circuit supervision makes sure that the tripping coil of a circuit breaker is always in the healthy condition. 

The Trip circuit supervision is particularly important in breakers which have only one trip coil.   The Trip circuit supervision relay continually measures the resistance of the trip coil of circuit breakers. It also measures the control voltage of the trip coil and gives and alarm when the control voltage falls to low levels. 

The Trip circuit supervision relay injects a constant current through the trip coil of the breaker and measures the voltage drop across the coil. Thus, the relay is able to measure the resistance of the coil. 

The Trip circuit supervision relays can also monitor more than one breaker coil. 

If the Trip circuit supervision Relay detects a fault, it activates the breaker failure logic which can activate a backup breaker if installed or cause the tripping of upstream breakers.


Circuit Breaker Operating Time Measurement

The operating time of a circuit breaker is crucial in any protection scheme. A circuit breaker that takes too long to open will seriously compromise protection causing damage to equipment and people.

Hence, circuit breakers should be periodically tested to see whether they operate at the correct operating time specified by the manufacturer.

Being mechanical devices, circuit breaker are made of numerous springs, washers, and linkages. These components can get jammed, the lubricating oil in the linkages can evaporate or lose its properties. The lubricating oil can mix with dust and form a viscous deposit. This can cause partial seizure and is particularly true for circuit breakers which are kept in the closed or open position for very long periods of time.   

The Circuit Breaker operating time can be measured by special testing equipment known as the Time interval meters. These instruments measure the time between the signal to open and the actual interruption of the current.   The opening time of the contacts belonging to each individual phase is measured.


Making and Breaking Capacity of a circuit breaker

         Making capacity of a circuit breaker is the maximum current which the breaker can conduct at the instant of closing. The making capacity is considered to the peak value of the first cycle when there is an imaginary short circuit between the phases.

         When there is a short circuit in the line and the breaker is closed, the peak value of the first cycle is the most severe from an electrodynamic perspective. This value is in kA. The making capacity is expressed as a peak value as the dc offset during fault conditions is taken into account.

         Breaking capacity of the circuit breaker refers to the maximum current in rms value the circuit breaker can interrupt. This is also in the order of kA. 

         The making capacity of the circuit breaker is usually greater than the breaking capacity of a circuit breaker as breaking an electric circuit is difficult due to arcing which occurs and which has to be quenched.


Measuring Contact Resistance in Circuit Breaker Contacts

The contacts in the circuit breaker need to checked periodically to ensure that the breaker is healthy and functinal. Poorly maintained or damaged contacts can cause arcing, single phasing, and even fire.

The two common checks conducted on the contacts of a circuit breaker are the visual inspection check and the contact measurement check.

The Visual inspection check involves examining the contacts of the circuit breaker for any pitting marks due to arcing and worn or deformed contacts.

The second check is the contact resistance measurement. This involves injecting a fixed current, usually around 300 A through the contacts and measuring the voltage drop across it. This test is done with a special contact resistance measuring instrument.

Then, using Ohm's law, the resistance value is calculated. The resistance value needs to be compared with the value given by the manufacturer. The value should also be compared with previous records.

Both these tests need to be done together. As there are cases of contacts having good contact resistance yet being in a damaged conditions.

Thus, for a contact to be certified healthy, it needs to have a good contact resistance and should clear the visual inspection test.

Stuck breaker protection

Stuck breaker protection is a situation in which a circuit breaker fails to operate even after receiving a tripping signal from a relay or a switch. Stuck breaker can undermine the protection scheme and can cause damage to machinery and is a danger to personnel. 

Common reasons for a circuit breaker not opening are a disconnection in the trip circuit or a mechanical problem with the circuit breaker. In these conditions, there needs to be a backup protection device which can interrupt the fault and isolate the system. In some cases, the entire section of the bus to which the breaker is connected is de-energized to interrupt power. 

A simple Stuck breaker protection schemes functions by sensing the position of the circuit breaker through the limit switches in the circuit breaker. The protection system waits for the open status from the circuit breaker after the open signal has been given. If the signal is not received within a preset time, the scheme assumes that the breaker is stuck and initiates backup measures.

However, this system has its limitations. The system cannot detect a situation where the current continues to flow despite the breaker having tripped. This can occur due to situations where the arc has not been quenched (failure of the arc extinction system) and the current flows even though the contacts have mechanically separated. 

To ensure proper feedback of the interruption of the current, advanced stuck breaker schemes sense the current as well as the position contacts of the circuit. This ensures that an accurate feedback of the breaker status.