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Active Contributor

Join Date: Jun 2011
Posts: 22
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Ungrounded Generator Neutrals

12/02/2011 11:15 AM

What are the pros and cons on U.S. shipboard installations of grounding the neutrals of 480 Volt wye connected generators. Lighting being supplied by 480V/208V-120V, delta/wye transformers. The transformer secondary neutrals being grounded. IEEE Standard 45-2002 and the U.S. Coast Guard by 46 CFR 111.05-15 only requires a power distribution system having a neutral to be grounded.

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Pathfinder Tags: Marine generators
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#1

Re: Ungrounded Generator Neutrals

12/02/2011 11:27 AM

All ac distribution systems on U.S. Navy ships are ungrounded. However, certain avionics shops, aircraft servicing systems, and landing craft air cushion (LCAC) servicing outlets are supplied from three-phase, four-wire systems that are supplied by transformers with a grounded, wye-connected secondary and single-phase, three wire grounded outlets with two pole ground fault current interruption (GFCI) protection are provided where required by the user equipment. Only one ground connection point is allowed for each grounded power system. Ungrounded electrical systems have no intentional connections between the electrical conductors and the ground (ship's hull). These systems were chosen for their higher reliability because grounding of a single conductor will not typically produce enough fault current to interrupt power. Grounds must be removed as soon as possible to maintain this advantage however, because a second ground on another phase will cause a power loss and could endanger personnel.

U.S. Navy reference: Naval Ships' Technical Manual, Chapter 320, Electric Power Distribution Systems, S9086-KY-STM-010/CH-320R2

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Active Contributor

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#2
In reply to #1

Re: Ungrounded Generator Neutrals

12/02/2011 12:00 PM

Cuba pete: Thank you for the U.S. Navy reference.

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#3
In reply to #2

Re: Ungrounded Generator Neutrals

12/02/2011 4:45 PM

and very deserving of a GA!

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#4

Re: Ungrounded Generator Neutrals

12/03/2011 5:55 AM

There has not been a fully correct answer to the OPs question up to now to my mind.

1) A ship's steel hull IS ground, unless it is in dry dock maybe.....but even then, the way a ground is used functions even without needing a possible connection to planet earth.Though I would expect this still to be made to keep safe any dock workers standing in the dry dock and say painting the hull.

2) The question about grounding Neutral, grounding Neutral is simply not required, in fact Neutral is not normally available on ship's generators, or better said, not fed out as all the transformers are delta/star and the Delta side ONLY has 3 connections, one per phase.

3 phase motors only need the 3 phases and a ground connection to work properly and safely.

On the Star/Wye side a neutral is possibly available for any single phase loads encountered, but generally the phase to phase voltage from the transformer is either already 240 or 120 VAC. Therefore no neutral needed......so no need to link neutral and earth.

Special earth chasing equipment is at each main switchboard and also each Delta/Wye transformer fuse box, to allow regular checks for earth contact to any phase.

On a big ship, there are usually many false earth connections from both sides of the supply at any one time. If the in circuit resistance is low enough, fuses will blow, bringing the problem to the attention of the people concerned.....otherwise regular checks must be made and logged for each and every earth testing position.

It is almost impossible to have a completely earth free big steel hulled ship 100% of the time. Also due to the inductivity of cables lying for great distances a few centimeters away from the steel hull, there is a pseudo earth connection similar to a transformer effect......

Neutral at the ship's high (alternator) voltage is never needed in my experience (maybe someone else can give an example where a ship needs it), so not feeding it all over the ship saves on both cost & weight.

Conclusion, no neutral available as none is ever needed on a ship.

I hope this helps......

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#5
In reply to #4

Re: Ungrounded Generator Neutrals

12/03/2011 6:31 PM

In response:

1) You are correct on a metal ship the hull of a ship afloat is recognized as a substitute for earth or ground. When a ship is in drydock it is required to be grounded to a shore ground. IEEE Std 45-2002 Section 5.9.7.5. Requires wood and composite ships to have a grounding conductor terminated at a copper plate of area not less than 0.2 square meters fixed to the keel that is immersed under all conditions of heel, to facilitate the grounding of non-current-carrying exposed metal parts.

2) The following regulations for building ships: IEEE Std 45-2002 Section 5.9.6.5 and USCG 46 CFR 111.05-15 require power distribution systems having a neutral bus or conductor to have the neutral grounded at a single point. A 208/120 V four wire system, with three phase conductors and a neutral conductor is required to have the neutral grounded.

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