Ship Power System

[Cookie]

Is the neutral distributed (or earthed for that matter) on a ship? I keep seeing this:

 

[marconi]

The neutral in ship electrical systems is most commonly insulated and unearthed (ie not connected to the ship's hull).

At sea, in the event of an line to earth fault keeping essential equipment running is paramount. To make the electrical system tolerant to one line to earth fault the neutral is insulated and unearthed. It takes a second different line to earth fault to cause a fault current to flow to operate the automatic disconnection of supply device.

If the neutral was earthed (or not insulated so it might be accidentally earthed) then it only takes one line to earth fault to operate the ADS.

An unearthed and insulated neutral it therefore more fault tolerant and robust for the seaborne environment. When properly installed it is not less safe in this environment.

Whether the neutral is actually distributed depends on whether any marine equipment needs it; generally they are 3 phase 3 wire with control supplies powered line to line or via a step-down transformer to 24V ac.

I suspect - don't know for certain - that the 230V single phase supplies are line to line and if necessary protected by a double pole rcd - thus no distributed neutral.

I have only dealt with 440V and 115V both of which were unearthed neutral point. 440V were 3/3 and 115 was 3/3 or 3/4.

 

[Marvo]

Earthing of the neutral depends on the type of vessel. With steel hull almost always the neutral will float unearthed to reduce the hull galvanic corrosion issues. With none metalic hull it's anybodies bet whether the neutral is earthed, depends mostly on the installation requirements of that particular vessel.

 

[marconi]

Errata for my #2 - 115 Volts 3/3 derived from a closed delta secondary wiring of a 440/115 transformer. In the event of a single 115V line to earth fault the other lines are at a potential of 115v wrt to the hull - which would not be the case of it was a star secondary winding - the lines would rise to root3 x 115V = 200V.

There was a three lamp and button line earth indicator on each 115V DB.

See attachment:
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Re #3:

Galvanic corrosion does not happen with ac.

For plastic hulls, I suppose all extraneous metalwork is bonded together and the neutral insulated and not connected to the bonding to avoid the risk of electric shock between two items of electrical equipment which have earth faults to different lines, to provide robustness to one line to earth fault and ADS for two different line-earth faults.
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My #4: Ignore my remark on corrosion. As you say it can sometime happen with ac.

 

[R-fur]

With an unearthed system there is usually a requirement for an earth fault alarm and for the system to be under the direct control of a competent person.

 

[DPG]

With an unearthed system there is usually a requirement for an earth fault alarm and for the system to be under the direct control of a competent person.

I don't think it is an unearthed installation though - only the neutral is unearthed.

 

[R-fur]

So what part is earthed then?
Un-earthed systems are common on ships and drilling rigs. Basically an IT system

 

[DPG]

So what part is earthed then?
Un-earthed systems are common on ships and drilling rigs. Basically an IT system

I thought the hull of metal ships was often earthed.

 

[R-fur]

Yes the hull is earth. but you said the neutral is not earthed but it is not an unearthed system, so what part is earthed? or have I picked you up wrong. In my experience the neutral is usually earthed on low voltage systems but 600v and above are often IT.

 

[DPG]

Must admit I thought they would mostly be earthed hulls, but separate neutral - rather than an IT system. But I may be wrong - it's only from bits I've read about ships' systems.

 

[marconi]

The metal hull floating in water is at earth potential and is used as the earthing point and is the equipotential zone. Once could describe it as IT. See attachment: For 'consumer' read 'load' and the upside down pine tree is the ship's hull.

 

[R-fur]

I think low voltage systems are normally TNS. But at 600V and above they MAY be IT or conventional. I have some experience of working on drill rig supplies, they are usually 600V fed from a Delta winding with no earth connection but all metal work is bonded to the hull i.e. a IT system. An earth fault will not cause a trip, but will raise an alarm that has to be investigated. Obviously all insulation must be rated at the full line voltage.

 

[marconi]

Ship grounding - How earthing works for different types of ships? - https://www.brighthubengineering.com/marine-engines-machinery/38231-electrical-grounding-on-ships/

http://www.almet-marine.com/images/clients/EN/Aluminium-users-guide/Ch08-electrical-systems.pdf

See Section 1.2 of:

http://magelhaes.hzs.be/willem/assorted/marine_electrical_knowledge.pdf

In summary LV systems ( <1000V) have their live conductors (Lines and neutral) insulated from earth (viz the hull); HV systems are high resistance grounded to the hull at star point and have earth fault detection devices to disconnect in the event of a single line to earth fault.

 

[Lucien Nunes]

Delta power is still common for general LV applications, with a line-line voltage suitable for single-phase loads e.g. 230V in Europe. Ships built in the EU for operation in territories with US-derived wiring conventions may have multiple systems operating at different voltages.

An example of where delta can be a nuisance is entertainment installations on cruise ships. A well equipped vessel might have two dozen full height racks of audio / video / lighting control equipment, of variously EU and US spec, single and 3-phase input, sometimes 3-phase input but single-phase output, which is where chaos sets in if it is not internally reconfigurable for delta or double-pole fused. One of my colleagues has just managed to get a spec approved for all entertainment systems on a design he did last year to run from a 400/230V star supply, which simplified things his end. IIRC he had to have four additional substations put in, totalling 1.2MW on some 400 circuits.

I remember doing some specs for a similar ship 25 years ago, where we had four independent supply systems at different different voltages to deal with loads that could not be reconfigured or were not available in different voltage variants, or we wanted to use a higher voltage because of cable run lengths etc. IIRC the 120-0-120 single was separate to the 240 delta, but was also had a 208 delta. It was pre-harmonisation and we had to meet UK, Italian, Norwegian, US and Lloyds requirements at the same time. There were some issues with what the function of a black wire should be.

 

[R-fur]

Agreed, most of the 'hotel' services will have an earthed neutral but LV supplies to pumps and cranes can be IT, however I think the trend is to earthed neutrals as electronic equipment is happier with a solid reference point.

 

[Cookie]

Why do electronics like a solid earth?

 

[R-fur]

Not sure really, filters often have earth connections to reduce interference.

 

[Lucien Nunes]

There's a difference between having the equipment signal ground / casing earthed, and having its supply neutral earthed. At RF what you usually want to do is connect everything together.... whether either or both of the supply live conductors is earthed at 50Hz at the far end is not so important