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Why do ships mostly use 3 phase 3 wire systems?
Why do ships use 3 Phase 3 wire instead of 3 Phase 4 wire and questions on EMI/EMC?

Hi all,

I have the following doubts:

[1] In the past 23 years of my service, I have only seen every ship being built in my yard with 3 Phase, 3 wire system with the neutral isolated. why is it that 3 phase 4 wire not used on a ship like in a land based power system?

[2] Earlier we used to procure Galley equipment for such a system and off late we are procuring COTS (commercial off the shelf) items which require a Neutral. This is being achieved with a local Delta / Star transformer with its neutral grounded to ships hull. The Galley equipment get their supplies from the secondary phase to this neutral. What effect could this have on the EMI signature of the hull .

[3] is this forum appropriate to ask questions on EMI/EMC?

By the way we also connect every equipment body to the hull with a bonding strap to maintain an equipotential surface. Anyone have any ideas? Anything historical or is it to do with protection and telemetry aspects. An underlying physics would be helpful to me in understanding this.

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Regards,

1 out of 1 members thought this post was helpful...

Ungrounded systems like the one you describe are typically used in critical installations where a power loss can be catastrophic or, at least, very dangerous. Like you, I've seen them shipboard where very bad things can happen if you lose the plant (up to and including the ship sinking in rough weather).

In a normal grounded system, the first ground fault will cause the circuit protective device to trip. Aboard ship, this can be a really big deal, depending on which breaker trips and how the systems are divided up in the switchgear. In an ungrounded system, the first fault will be tolerated. Such systems are equipped with a ground detection system so that qualified persons on staff will know and can respond in an orderly fashion with a controlled shut-down. A ground fault on a second phase will cause a phase-to-phase short, so the first ground fault must be corrected as soon as possible.

Actually there are insulation monitors int he switchboard and these are programmed to give Audio visual alarms.

Next what do you think is the effect of using a local D/Y Transformer ( for the Galley) on the system in terms of defeating this protection or in terms of any circulating currents? Is there any paper or book on this aspect published anywhere which examines grounded and ungrounded systems in more detail? TIA.

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Regards,
Surya

It would seem that a Delta Y transformer would be ok. The Galley is hardly a critical application, a short to ground say on one of the ranges would not effect the rest of the ship.

Having said that I am not a marine electrician, surely there is a code that applies to that industry.

Dear CSA,

Can you throw some light in [1] below. I understand you have good ship building experience.

>[1] In the past 23 years of my service, I have only seen
>every ship being built in my yard with 3 Phase, 3 wire
>system with the neutral isolated. why is it that 3 phase 4
>wire not used on a ship like in a land based power system?

--
Regards,
Surya