Switchboard & Safety Flashcards
- With reference to marine electrical distribution systems:
a) State the meaning of the term insulated neutral. (8)
b) Briefly describe how the location of an earth fault can be found. (4)
c) State the safety precautions to be taken before investigating faulty electrical machinery. (4)
a) An insulated distribution system is one in which main generators neutral point is kept away from the ships hull. Insulated neutral systems are preferred on low voltage ships operating at 440V AC.
b) Earth faults can be located by the process of elimination. Power is switched off to a certain area in a pre-determined order – personnel in the area must be alerted before doing so – and someone keeps an eye on the fault monitor until it clears. Once the general location has been found, go there and conduct an insulated resistance test to locate the exact location.
c) Risk assessment must be conducted by the competent person to put in place control measures.
Electrical circuits to be investigated will be safely isolate.
Safe isolation means switching off the power, locking it in place with a safety lock and posting appropriate notices; caution if dead and danger if live work.
- a) List FIVE protective devices fitted to a main electrical switchboard. (10)
b) Explain the meaning of the term earth fault and state why it is undesirable in electrical installations. (6)
a) Reverse power trip relay. Overload trip relay. Preferential trip relay. Under/Over frequency trip relays. Under/Over voltage trip relays. Short circuit protection. b) An earth fault is when electrical current flows to the ships hull. This is due to a weakness in insulation whereby insulation can no longer contain the electrical current. When two earth faults occur, it can lead to a short circuit which may lead to fire and explosions.
- a) List FIVE precautions that should be taken before any work is carried out on electrical equipment. (10)
b) Identify THREE additional precautions to those in Q4(a), if it has become essential to work near to live equipment. (6)
a) Risk assessment must be conducted by the competent person to put in place control measures.
Appropriate PPE worn.
Electrical circuits to be investigated will be safely isolate.
Safe isolation means switching off the power, locking it in place with a safety lock and posting appropriate notices. Caution if dead and danger if live work.
Equipment is proved dead.
b) Appropriate barriers put in place.
Rubber matts under feet.
Another competent engineer is present.
- With reference to the emergency generator:
a) state the checks required prior to starting the engine: (6)
b) describe the routine testing. (10)
a) The fuel tank level.
Check the batteries.
Check the lubricating oil level.
Check cooling water level.
Check turbocharger oil level.
Check for any general leaks.
b) Start emergency generator and allow it to run up.
Check the generator for any abnormal behaviour such as leaks, vibrations and noise. Check the running parameters for the prime mover and alternator.
Simulate a black out to check the generator I connected with the emergency switchboard in no more than 45 seconds.
A portion of emergency load (say 50%) is put on the generator and is allowed to run for a given period (30 mins) to see if the generators running parameters are holding up.
- a) State THREE underlying causes of electrical causes of electrical equipment failure. b) State the precautions to be taken before working on the switchboard.
a) Overloading
Insulation failure
Misuse
b) You decide whether it is possible to make the equipment dead.
If not possible, do a risk assessment to put in place appropriate control measures to identify hazards.
Appropriate PPE must be used.
A competent engineer must be present with the person working.
Safety barriers must be put in place.
Rubber matts are put in place.
Appropriate notices are placed to warn other personnel.
A permit to work must be issued before work can begin.
- With reference to electrical distribution systems:
a) State the meaning of the term earth fault; (2)
b) State FOUR different possible causes of an earth fault occurring in an electric motor; (4) c) State THREE undesirable consequences of earth faults; (3)
d) Describe ONE method of detecting earth faults. (7)
a) When current flows to earth, which on ship is the hull.
b) Excessive damage.
Water entering motor.
Insulation damage.
Surface dirt.
c) The ships structure has become “live” risking the potential for a dead-short between multiple phases leading to a potential black-out situation.
High Oscillating currents throughout the ships structure, may cause over- heating and sparking very dangerous on ships with dangerous cargos.
Reduced effectiveness of over-current protective devices, due to a reduced potential difference
d) Earth faults can be located by the process of elimination. Power is switched off to a certain area in a pre-determined order – personnel in the area must be alerted before doing so – and someone keeps an eye on the fault monitor until it clears. Once the general location has been found, go there and conduct an insulated resistance test to locate the exact location.
- Explain why EACH of the following protective devices are fitted to a main electrical switchboard. i) Reverse power.
ii) Under voltage.
iii) Main circuit breaker overcurrent.
iv) Preference trips.
i) Reverse power trip: is fitted to avoid the loss of electrical power and possibly damage to the generating set. Reverse power – also known as motoring – occurs when generator instead generating power consumes power. In case of a generator motoring, the healthy generator will not be able to cope with the electrical load of the motoring generator AND ship’s electrical load and therefore due to overload healthy generator will tend to trip.
ii) Under voltage: protection is fitted to prevent the circuit breaker of a dead generator to be closed on to a live bus bar. Closing a dead generator on a live bus bar will be equal to a three phase short circuit fault.
iii) Main circuit breaker overcurrent: protection is fitter to prevent the generator from damages of overcurrent – current above full current rating of the generator. iv) Preference trip: protection is fitted to prevent generator from overloading by shedding the generator off electrical non-essential load in a predefined manner. In case of an overload, after a time delay, preference trip relay will switch off predetermined set of non-essential loads.
