Neutral on a ship with neutral insulated networks?

[dnpdnp]

Hello,

first of all thank you for your help in advance..,

I would like to ask somebody to verify me the following:
in a 3 phase 3 wire system ship power distribution, the neutral of the generator is not connected at all..
in order to power 1 phase loads though (lights etc..) a neutral is needed.
Is the neutral then "created" at the step-down transformer? furthermore, are tripping safety devices (30 mA) used in the low V distribution panels that accomodate neutral?

Thank you very much,

dnp

 

[waross]

A neutral is not needed for single phase loads. On delta systems single phase equipment is purchased with the same voltage rating as the line to line voltage and connected line to line. Usually the generator will be connected for 240 volts and all equipment will be 240 V rated. I have seen small delta generators configured for 120 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dnpdnp]

Bill,
first of all, thank you very much for the prompt response..
do you know if the safety tripping device (30 mA) used in residential supplies can also apply in this network? I should assume that it can work, am I right?
the ground line needed for the 1 phase network (i.e. power supply 230V sockets in acommodation areas) is locally created at each 1-phase sub-panel (or in the main 230 V distribution board)?
Again, thank you for your response,
dnp

 

[waross]

Hi dnpdnp;
First of all, where are you and what codes do you work under?
Second, what are your voltages? Both supply and devices.

the ground line needed for the 1 phase network

This is a code issue. You don't need a ground or neutral for functionality. The ground is for safety. You don't need a neutral for single phase. There are single phase systems where the grounded circuit conductor is called a neutral even though the system does not have a true neutral.
You may have a three wire, three phase system without a neutral and supply single phase loads without needing a neutral.
What is your generator voltage and what is the rated voltage of your single phase equipment?
This is an international forum and there are many systems in use onshore in the world. When you get into marine applications, you may find ungrounded systems in use that would not be allowed on shore. Marine applications may develop hull currents that can lead to rapid corrosion and loss of a vessel in a matter of months due to accelerated corrosion. That is part of the reason that many vessels choose to run ungrounded systems.
Re: Ground fault breakers and devices, there are 240 volt rated two pole breakers available in North America that would provide protection on an ungrounded circuit.
Be aware that if you are using an ungrounded supply you should be using two pole breakers for single phase feeds.
If the system is not grounded, it may not be a good idea to introduce grounds on any panels.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dnpdnp]

Bill,
first of all, thank you for your time..
I'm in Europe and the code applying is 98/18 Eurosolas (marine code). The project I'm working about concerns a small two-way RORO/Pass.
Two main generators will be at 400 kV together with a step down transformer (?/?) to create also a 240 V line for basically 1-phase loads. The 400V network will be for 3-phase machinery while the 240 V will be for lighting, WC fans, battery charging etc..
As you already mentioned the 240V / 1 phase supply can be provided from the 2 phases of the 3 phase 240V line (coming out of the transformer).
My concern is basically about some equipement in the accomodation-saloon area where grounding may be needed (for example, there should excist about six to eight 240V/1 phase power sockets for passengers).
I assume, that if I provide a ground line coming from the main 240V panel (the line will be connected only to the hull at the panel location) should be ok, since I still won't have an earthed neutral system.
The ground at the lower 1-phase 240V network will only provide a path for potential leaking current (equipement fault) to the sea (earth) through hull. Is this correct?
Thank you very much again,

dnp

 

[waross]

If someone has experience with your code, their comments will be more accurate than mine.
I would suggest leaving the system ungrounded. Supply 240 from either a single phase or three phase panel. Use two pole breakers. Look for two pole ground fault protection breakers.
Install a ground detection system.
Should the vessel ever go on shore power ground...
Yes I know, the vessel will never go on shore power, and the check's in the mail.
Someday the ship will be alongside for repairs and someone will object to running the gen-sets for a small lighting load. Orders will be given to run a shore line and connect it directly to a lighting panel. Should the lighting panel be grounded the result may be sharing the current on the grounded conductor with the ground path. There doesn't have to be much current developed voltage difference on a ground path through the hull to overcome the voltage difference generated by the sacrificial zinc electrodes. Rapid, severe, and expensive hull corrosion may be the result. Rapid consumption of the zinc electrodes is also possible.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Kiribanda]

dnpdnp,

The DELTA 240 V electrical network in your ship seems to be "equivalent" to an ungrounded 240 V WYE network.Therefore,in your accomadation/saloon area all 240 V socket outlets are connected to TWO wires from the 240 V system and hence "usual" neutral conductor/ terminal is not available.Also the earth terminal of each socket should be connected to the earth (PE) wire.Eventhough your 240 V network is "so called ungrounded",still you SHALL provide some method of earth leakage protection to the socket outlets.For that you have to install 30 mA earth leakage circuit breakers (ELCBs defined in IEC60364/BS7671)to each socket outlet feeder circuit.I think 10 mA ELCBs which are nomally used for swimming pools are too sensitive for your application.But please refer your ship electrical code(s) for the correct rating.Then for any insulation leakage more than 30 mA,the ELCB will trip & isolate the faulty socket outlet circuit.Also you may encourgae to use "Double insulated electrical equipment" made for IEC60364 in the accomadation/saloon area as supplementry earth leakage protection.

 

[dnpdnp]

Bill and Kiribanda,
thank you both for your helpfull advices..
I think that probably the wisest way is to keep all network ungrounded, except the 2-3 subpanels for lighting (subpanels providing all the 1-phase 230 V supply in general) where I should install earth leakage circuit breakers at all feeds for tripping any leaking line occuring.
In that way, there should be no connection between the ground conductor in the subpanel and the ground path through the hull when shore supply is provided at that panel, since the leaking equipement will be isolated.
As for the protecting devices,Residual-Current Devices (RCDs) can probably be used instead of the Earth-Leakage Circuit Breakers (smaller cost..)
Thank you again,

dnp

 

[waross]

except the 2-3 subpanels for lighting

You seem to be fixated on a nonexistent need for grounding.
IF THE SHIP'S SYSTEMS ARE UNGROUNDED DON"T GROUND ANY CIRCUIT CONDUCTORS AT ANY PANELS.
You may (and should) use a grounding system to ground the ground terminals of receptacles and devices. Equipment grounding conductors may be run back to the panels and connected to a grounding bus that is in turn connected to the hull but not to any circuit conductors.
In a system of any great physical size there will be enough capacitive coupling to ground to provide a nasty shock from even an ungrounded system. RCBs and GFCIs will provide protection even though the system is not grounded.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dnpdnp]

"..Equipment grounding conductors may be run back to the panels and connected to a grounding bus that is in turn connected to the hull but not to any circuit conductors..."

Bill,
I think we 're saying the same thing since this is what I meant in my last post.I didn't mean to ground the neutral inside the specific panels (or the panels themselves) but to provide a grounding bus inside (which is connected to the hull) only for the equipment grounding.
thank you again for your help,

dnp