LV generator circuit-breakers and other large distribution circuit-breakers (600-6000 A) on board ship are traditionally of the air break type called ACB (air circuit breaker).
This means that the circuit-breaker contacts separate in air.
High voltage (HV) installations e.g. at 6.6 kV and 11 kV generally use the vacuum interrupter type or gas-filled (sulphur hexafluoride – SF6) breakers.
In a vacuum interrupter the contacts only need to be separated by a few millimetres as the insulation level of a vacuum is extremely high. The quality of the vacuum in the sealed interrupter chamber is checked by applying a short duration HV pulse (e.g. 10 kV for a 6.6 kV breaker) across the open contacts in the gas breaker the contacts separate in a special interrupter chamber containing SF6 gas typically at 500 kPa (5 bar) at 20C.
The operating mechanism for vacuum and SF6 breakers is similar to that employed for an ACB.
Various types of circuit breaker closing mechanism may be fitted:
Independent Manual Spring braker on ship
The spring charge is directly applied by manual depression of the closing handle. The last few centimetres of handle movement releases the spring to close the breaker. Closing speed is independent of the operator
Motor Driven Stored Charge Spring (most common type for marine applications) braker on ship
Closing springs are charged by a motor-gearbox unit. Spring recharging is automatic following closure of the breaker which is initiated by a push-button.
This may be a direct mechanical release of the charged spring, or more usually, it will be released electrically via a solenoid latch.
Manual Wound Stored Charge Spring braker on ship
This is similar to above method but with manually charged closing springs.
Solenoid braker on ship
The breaker is closed by a d.c. solenoid energised from the generator or bus-bars via a transformer/rectifier unit, contactor, push button and, sometimes, a timing relay.
WARNING: Circuit breakers store energy in their springs for:
Store-charge mechanisms in the closing springs.
Contact and kick-off springs.
Extreme care must be exercised when handling circuit breakers with the closing springs charged, or when the circuit breaker is in the ON position.
Isolated circuit-breakers racked out for maintenance should be left with the closing springs discharged and in the OFF position.
Circuit-breakers are held in the closed or ON position by u mechanical latch. The breaker is tripped by releasing this latch allowing the kick-off springs and contact pressure to force the contacts open.
Tripping can be initiated:
Manually a push button with mechanical linkage trips the latch.
Undervoltage trip coil or relay (trips when de-energised).
Overcurrent/short-circuit trip device or relay (trips when energised).
Solenoid trip coil – when energised by a remote push-button or relay (such as an electronic overcurrent relay).
Mechanical interlocks are fitted to main circuit breakers to prevent racking-out if still in the ON position. Care must be taken not to exert undue force the breaker will not move, otherwise damage may be caused to the interlocks and other mechanical parts.
Electrical interlock switches are connected into circuit-breaker control circuits to prevent incorrect sequence operation.
When a shore-supply breaker is closed onto a switchboard.
The ship’s generator breakers are usually interlocked OFF to prevent parallel running of a ship’s generator and the shore supply.