Basic Faults in an electric circuit:
- Open Circuit: Any break in the conductor will result in an open circuit by not allowing the flow of current.
- Earth Fault: If a conductor’s insulation is damaged, there are chances that the conductor touches the hull or the body of the machinery carrying the conductor. This will result in an Earth fault.
- Short Circuit: Engagement of two damaged or broken conductors will result in withdrawal of huge current which is called a short circuit.
To prevent the hazardous of electric shock and fire, all-electric components must be earthed. All non-current carrying parts of electric equipment such as body, coverings, the foundation must be earthed. The earth conductor connected from equipment to the earth is normally flowing a zero voltage. If any fault in the system it will give an alarm or trip depending upon the type of earthing system.
Generally, there are two methods of earthing.
- Insulated Neutral System.
- The machinery is insulated from earth
- Generally used in low voltage system of the range 440V AC
- Any single earth fault will not trigger any trip as and the machinery will be working normally until the next earth fault. If there are two earth faults the resulting amount will be huge. So it’ll trigger a trip to isolate the machinery.
2. Earthed Neutral System.
- The supply line neutral point is connected to the earth.
- Generally used on high voltage systems of the range >1000V AC
- Any single earth fault will trigger the trip to isolate the machinery.
Hence it is a SOLAS regulation to have an earth fault indicating system on MSBD (Main Switch Board) to monitor earth faults. Generally, there will be separate earth fault monitors for 110V/220V and 440V. As 110/220V AC earth fault is not too dangerous it can be rectified later. But if any fault in 440V system, it has to be rectified immediately to protect the personnel operating the machinery from getting a serious electric shock.
Procedures to rectify earth faults:
If there is an earth fault we’ll get an alarm in the control room.
110/220V AC System Earth fault:
- First, accept the alarm.
- Switch off and switch ON all the switches one by one at 110/220V feeder panel. The moment alarm gets cleared is the place of an earth fault.
- After locating the place we can follow the same procedure at the distribution panel to locate actual/exact equipment/circuit that is having a fault.
- Now look for the fault and rectify. Either may be a loose connection or open circuit.
440V AC System Earth Fault:
- If earth fault alarm comes from 440V feeder panel.
- Look for the machinery that has been started just before the alarm. Reconfirm the machinery by stopping and restarting it.
- Stop the machinery and rectify the problem before putting back to service.
Note: Make sure to start standby machinery before stopping the faulty machinery if it’s a critical machinery such M/E L.O Pump.
SOLAS Regulations for Emergency Generator:
- Emergency generator must automatically start and come on the load within 45 seconds in the event of a power failure.
- The source of starting an emergency generator must be capable to provide 3 consecutive starts. And the second source of starting must be available to give an additional 3 consecutive starts within 30 minutes. Unless mechanical starting can be demonstrated effectively.
- The emergency generator must be capable of starting in cold condition up to 0 degree Celsius. If not, should be provided with necessary heating arrangements.
- Fuel flash point should not be less than 43 degree Celsius.
- EG should be capable of running at full rated capacity at any condition. The ship is upright and maybe inclined with an angle of list up to 22.5 degree Celsius or may be inclined in fore or aft direction up to 10 degrees or maybe in any combination of these angles.
- Location of emergency generator must be,
- The uppermost continuous deck above waterline.
- Between forward and aft collision bulkhead.
- Away from the engine room and dangerous cargo space.
- Upon starting and coming on load, the generator must supply power to below appliances at least for a period of 18 hours
- Emergency Lightings: muster station, embarkation station, machinery space, control station, ECR, stowage location of fireman’s outfit, alleyway, stairway, emergency fire, bilge and sprinkler pump space.
- Navigational lightening and all other lighting required by international regulations for the prevention of collision at sea.
- Internal Communication Equipment, VHF radio, shipboard navigation equipment, fire detection and protection system, manually call points, daylight signalling lamp, ship’s whistle,
- Fire Fighting Services: Emergency fire pump, emergency bilge pump, sprinkler pump if provided separately, and all other electrically operated bilge valves.
- At least 1 steering gear motor.
- For a period of half an hour supply to watertight doors together with their alarm and control.
Main Switch Board safeties and protection:
- Dead front panel- You cannot open the door of MSBD panel until the power is off
- Insulated rubber mat of minimum 15mm thickness placed on the floor near the front and backside of the panel board.
- Circuit breakers to auto shutdown the system in case of overloading or short circuit etc.
- Fuses to melt and isolate the system in case of large current flow due to a short circuit.
- Earth fault indicator must be present and working properly
- No water, oil and steam pipes in the vicinity of MSBD
- Space should be ventilated properly with sufficient illumination and temperature regulation system.
- Panel doors should be interlocked to open without switching off the power.
- Appropriate fire extinguisher (CO2) must be present nearby.
- Circuit diagram should be displaced nearby the panel.
Types of circuit breakers:
- ACB (Air Circuit Breaker) – ( Rating 400 A to 2000 A)
- MCCB (Moulded Case Circuit Breaker) – (Rating 50 A to 1500 A)
- MCB (Miniature Circuit Breaker) – (Rating 5A to 100 A)
Types of Fuses:
- Cartridge Fuse
- Semi – Enclosed Fuse
- High Rupturing Capacity.
- Ac type
- DC type
- Bulb type
- Ceramic type