Circuit Breakers
The main purpose of a circuit breaker is to
interrupt fault current as quickly as possible and so keep the damage to the
other pieces of equipment to a minimum.Generator circuit-breaker and other large circuit-breakers (600-6000A) on board ship are usually of the air break type. This means that the circuit-breaker contacts separate in air. (Ashore, comparable size circuit-breakers are often immersed in oil (OCB) and larger circuit-breakers for high voltage operation are either air blast, or have a special gas filing or a vacuum break).
Air Circuit Breakers (ACB) are mounted on special rails in the main switchboard cubicle, and must be racked out and isolated from the bus-bars for maintenance and testing. The ACB and its slide rails are usually mounted in a special cassette bolted into the switchboard cubicle and electrically connected to the bus bars. If repair demands that the ACB be completely removed from its cassette, then usually a special hoist or ‘fork-lift’ is required for large, heavy-duty breakers.
The action of withdrawing the ‘breaker’ causes a safety shutter to cover the live bus-bar contacts.
Mechanical linkage in the circuit-breaker is quite complex and should not be interfered with except for maintenance and lubrication as specified by the manufacturer.
The main fixed and moving contacts are of copper (sometimes of special arc resistant alloy or silver tipped) and most often silver coated. Main contacts should not be scraped or filed. If the main contacts suffer severe burnings they probably require realignment as specified by the manufacturer. Arcing contacts normally suffer burning and may be dressed by a smooth file as recommended by the manufacturer. Carborundum and emery should not be used – the hard particles can embed themselves in the soft copper contacts and cause future contact troubles.
The arc chutes or arc splitter boxes confine and control the inevitable arc, and to accelerate arc extinction. These must be moved and inspected for broken parts and erosion of the steel splitter plates.
The circuit breaker must b closed against powerful ‘throw-off’ springs which are later used to open the contacts in the tripping operation. In addition, if the circuit breaker is closed onto a fault, the electromagnetic effect of the fault current will attempt to open the contacts and will therefore act in opposition to the closing force. For safe operation when closing against a fault the contact should fully close before opening.
Consequently, as the fault rating of the circuit breaker increases, the closing force must also be increased. For high fault level equipment an operator may be unable to produce the force necessary to ensure correct closure, and all modern designs of ACBs now use either springs or solenoids.
Various types of closing mechanism may be fitted.
(a) Independent Manual Spring – 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.(b) Motor Wound Stored Charge Spring – Closing springs are charged by a motor/gearbox unit. Spring recharging is automatic following closure of the ‘breaker’. Breaker closure is operated by a push button. This may be a direct mechanical release of the charged spring or it may initiate an electrical release via a solenoid latch.
c) Hound Wound Stored Charge Spring – This is similar to (b) but with manually charged closing springs.
d) Solenoid – The ‘breaker’ is closed by a dc solenoid energised from the generator or bus bars via a transformer/rectifier unit, contactor, push button and, sometimes, a timing relay.No index entries found.
WARNING – Circuit breakers store energy in springs;
(a) in store-charge mechanism in the closing springs and
(b) in 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 when 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 a 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 by:
(a) Manually – a push button with mechanical
linkage trips the latch.(b) Undervoltage trip coil (trips when de-energised).
(c) Overcurrent/Short-circuit trip device (trips when energised).
(d) Solenoid trip coil – when energised by a remote switch or relay (such as an electronic overcurrent relay).
Mechanical interlocks are fitted to ACBs to prevent racking out if still in the ON position. Care must be taken not to exert ‘undue force’ if the breaker will not move – otherwise damage may be caused to the interlocks and other mechanical parts. Dangers of explosion and fire may also result from such action.
Electrical interlock switches are connected into circuit-breaker control circuits to prevent incorrect sequence operation, e.g. when a shore-supply breaker is closed onto a switchboard. The ship’s generator breaker are usually interlocked OFF to prevent parallel running of a ship’s generator and the shore supply.
Even an experienced operator can sometimes attempt to carry out these operations in an incorrect manner and interlocks are usually provided to prevent this.
Local and remote electrical indication will often be employed to show the state of a circuit breaker. However, whether or not electrical indication is provided mechanical indicators should be included in the circuit breaker design. These are used to show whether the circuit breaker is open, closed or isolated.
This function is sometimes provided by minim diagrams on the front of the switchboard an arrangement which is preferred by many engineers. These diagrams are of single line schematic type, showing the particulate circuit with ‘windows’ at the breaker position. A mechanical indicator behind the window shows the state of the circuit. When a spring charging mechanism is used the state of the spring should be indicated: either ‘charged’ or ‘free’.
For medium size motors moulded case circuits breakers (MCCB) are used. These are small, compact air circuit-breakers fitted in a moulded plastic case. They have a lower current rating (30 – 1500A) than air circuit breakers and generally a lower breaking capacity.
MCCBs usually have an adjustable thermal overload setting and an adjustable or fixed magnetic overcurrent trip for short-circuit protection built into the case. An undervoltage trip coil may also be included within the case.
MCCBs are usually closed by a hand operated lever but motor closing can also be fitted. MCCBs are claimed to be reliable, trouble free and require negligible maintenance. If the breaker operates in the ON position for long periods it should be tripped and closed a few times to free the mechanism and clean the contacts. Terminals should be checks for tightness otherwise overheating damage will develop. The front cover of larger MCCBs (around 400A rating) can usually be removed, interior dust blow out and the contacts dressed with a file if required. Following tripping under a short-circuit fault the breaker should be inspected for damage, checked for correct operation, and its insulation resistance measured. A reading of at least 4MΩ - 5MΩ is usually required. Any other faulty operation usually requires replacement or overhaul by the manufacturer.
MCCBs can be used for every application on board ship from generator breakers to small distribution breakers. The limited breaking capacity may demand that ‘back-up’ fuses be fitted for very high prospective short-circuit fault levels.
For small loads as is usually found in lighting distribution boards, and the like, miniature circuit breakers (MCB) are used.
MCBs are very small air circuit breakers fitted in moulded cases. They have current ratings of 5-100A and generally thermal overload and magnetic short-circuit protection. They have a very limited breaking capacity (3000A) and are commonly used in final distribution boards instead of fuses. The distribution board is supplied via a fuse of MCCB with the required breaking capacity.
MCBs must be replaced if faults develop – no maintenance is possible.
Handles for opening the doors on switchboard cubicles are usually linked (or interlocked) to an isolating switch. This ensures that supplies to components in the cubicle are switch off before the door can be opened.
Fused isolators are isolating switches that incorporate fuses. The action of opening the switch isolates the fuses so they can be replaced safely.
Fused isolators can also be interlocked to the cubicle door handle. Motor starters frequently incorporate this arrangement
One type of interlocked fused isolator can be completely withdrawn and removed to ensure complete safety when carrying out maintenance on equipment.
Maintenance on fused isolators consists of periodically checking the operating mechanism. Contacts must be inspected for damage and lightly greased with an electrical lubricant. The interlock mechanism (if fitted) should also be examined for correct, safe operation.
