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| Preferential Trip |
Preferential trip is a kind of electrical arrangement on ship which is designed to disconnect the non-essential circuit i.e. non-essential load from the main bus bar in case of partial failure or overload of the main supply.
The non-essential circuits or loads on ships are air conditioning, exhaust and ventilation fans, and galley equipments which can be disconnected momentarily and can be connected again after fault finding. The main advantage of preferential trip is that it helps in preventing the operation of main circuit breaker trip and loss of power on essential services and thus prevents blackout and overloading of generator.
Construction and Working
A method for operating all the overload type trips from one load current carrying coil uses two instantaneous trip levers.
The top lever is arranged as an instantaneous short circuit trip and opens the breaker directly through mechanical linkages.
The bottom lever closes instantly at the lower overload current setting and by doing so, completes the circuit through two (or more) non essential circuit trips and a main breaker trip, all incorporating dashpot time delay.
These relays will trip out non essentials at 5 and 10 second intervals based on their priority and finally, if the over load persists, the main breaker after 15 sees.
Warning of overload is given by the alarm. Overload protection is provided on both poles.
Working of Preferential Trip
1.The current passes through the electromagnetic coil and the linkages are kept from contacting using a spring arrangement. As soon as the current value increases the limit, the electromagnetic coil pulls the linkage up against the spring force and operates the instantaneous circuit and the alarm system. The lower linkage completes the circuit for the preferential trip circuit.
2.The current passes through the coil in the preferential trip circuit which pulls the piston in the dashpot arrangement. The movement of this piston is governed by the diameter of the orifice and the time delay made by the same.
3.The preferential trip operates at 5, 10 and 15 seconds and the load is removed accordingly. If the overload still persists, then an audible and visual alarm is sounded.
The preferential trip is one of those important electrical circuit diagrams which help in removing the excessive load from the main bus bar, thus preventing situation like blackout which is a dangerous incident to ship, especially when the ship is sailing in restricted or congested waters.
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