Thursday, 14 July 2016

How do Water Ships Get Earthing?

The requirement ashore is the safety of human beings. So, in order to prevent human-electrical accidents, the neutral is earthed. The priority is neither the safety of the machinery nor the continuous necessary operation of the machinery. But the scenario on board ship is totally different. The priority is the continuous operation of the machinery which are classed "Essential".

The distribution system followed on board is "Insulated Neutral" system. The main priority on board is the safety of ship which includes navigation & fire safety. If due to earth fault, the machinery classed as "Essential" gets isolated, say for steering gear, then the safety of ship is at a question, which may lead to collision, grounding, fire etc. So the priority on board ship is to maintain the continuity of the supply to the machinery in the event of "Single Earth Fault Occurring".

A basic circuit consist essentially of Two parts:

  • The Conductor which carried the current around the circuit.
  • The Insulation which keeps the current inside the conductor.

A majority of earth faults occurs within the equipment. Generally insulation failure occurs which leads to the conductor getting in contact with the body of the metal enclosure. When such earth faults happen, the metal enclosure of the equipment if not earthed, it would cause a heavy shock, and may result in fire accidents too.

If an earth fault occurs in an "Earthed Distribution System", it would be equivalent to a "Short-Circuit" fault across the load via ship's hull. The resulting large earth fault current will immediately "blow-up" the fuses in the line. Thus the equipment is isolated from the supply and thus rendered safe. This may result in hazardous situation, if the equipment is classed as "Essential". Thus the "Earthed Distribution System" requires only one earth fault on the line conductor to cause an earth fault current to flow.

If the earth fault occurs in "Insulated Neutral Distribution System", will not cause any equipment to go out of operation and thus maintains the continuity of operation of the equipment. This point should be noted, the machinery still continues to operate. Thus a single earth fault will not provide a complete circuit for the fault current to flow. If a second earth fault occurs, then the two earth faults together would be equivalent to a short circuit fault ( via ship’s hull) thus resulting large current would operate the protection devices, cause disconnection of, perhaps, essential services creating a risk to the safety of the ship.

An insulated neutral distribution system requires two earth faults on two different lines to cause an earth fault current to flow. Thus an insulated neutral system, is therefore, more effective than an earthed system in maintaining continuity of supply to equipment. Hence it is adopted for most marine electrical systems.

Thank you!

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