Martindale Ring Main Tester - Earth Fault?

[lenseman]

I have a steel ketch with a mains isolator and a 30mA ELCB.

I recently tested the 230volt AC mains plugs on my ketch and discovered that the Martindale gives an "Earth Fault" (all three neons lit)?

Is this possibly 'normal' considering the set up or might there be a fault?

 

[mjf107]

What type of mains isolator do you have, is it a double wound isolating transfomer ? if so a Martindale tester may give false readings. Neons will light with very little current current flow. Are you experiencing any other problems?
Regards Michael

Edit to the above 02/02/10

At the time of the above post I was not aware of the requirement / practice in this application of an isolating transformer of creating a neutral point with a connection to 'Ships' earth and one leg of the transformer secondary. With this in place the Martindale should read correct as others have stated. I also agree that it would be sensible to get the system cheked for saftey and so you know exactly what your dealing with for future maintenance

 

[VicS]

Exactly what Martindale tester are you using

This one ( a socket tester)

Shows all three neons lit when every thing is OK​

When you said "mains isolator" did you mean an isolating transformer as assumed by Michael or did you means an isolating switch or MCB

 

[rogerthebodger]

Ensure that the neutral and earth are connected on the supply side of your RCD which you call ELCD.

If you have a generator or inverter most do not have this connection.

If you have a direct connection to shore power this connection in normally at the sub station but if you have an isolating transformer this connection must be made on the boat side before the RCD.

If don't have an isolating transformer connect your GI between the steel hull and the earth line. Most recommend it to be fitted inline in the incoming earth line but after have various discussions on this fora I think it is better between the hull and earth.

I currently have this arrangement on my steel boat but will change it soon.

I also have a 3 pole mains selector switch on the incoming mains lines from shore power, generator and inverter that switches the earth due to the need to have the earth/neutral connection on the inverter/generator but not in the shore power connection in my case.

Had my boat out of the water last week and no signs of Galvanic action and my anodes can be seem working but still OK after the first year in the water but did not need changing.

 

[lenseman]

Thanks for the reply.

The isolator has no name on it anywhere and it is tucked in the dark recesses under the ops room chart table!

I removed the lid to try and find a name or manufacturer but to no avail.

It is large (9½" x 11" x 14" - 240 x 270 x 350) and gets slightly warm. I have enclosed a photo to give some impression.

 

[lenseman]

Exactly what Martindale tester are you using . . .

Interestingly, my Martindale has a differing neon configutation and when plugged into the ring main sockets showed all three neons lit up which I take to mean "Earth Fault" ?

Pontoon shore power comes onboard and straight into a 25Amp 240volt ELCB (Earth Leakage Circuit Breaker - 30mA trip current), then via this blue/grey steel 'box'.

I inherited the yacht with all fittings and it is difficult to try and read the mind of the electrician who fitted the kit all those years ago. It was installed professionally, possibly at Port Solent up the creek (which is just how I feel at the moment).

 

[rogerthebodger]

My gut feel is that the blue/grey box is an isolating transformer but without more details of the wiring could not be positive.

If it is the ELCD (RCD) should be boat side with earth/neutral connected at the isolating transformer.

You say it gets warm. Does it hum like a transformer ?

Where does the 3 core blue/green cable go and are the two plug connectors male or female and where does the cables from them go.

There looks like an earthing post next to the brown connection block what connects to that and where do the black wires go from what looks like a switch?

 

[VicS]

nterestingly, my Martindale has a differing neon configutation

That is an older type. They have been changed so that the OK situation is indicated by three neons lit to avoid the situation where a failed neon could apparently give an Ok indication.

It means ( I think) that there is no "earth" connection to your sockets.

I agree with Roger that the blue grey box is probably an isolating transformer.
( but I do wonder what all the small wires on the top are)

If that is so

As he says the incoming shore power should be connected directly to it.

The earth connection in the shore power supply should go no further than the transformer. It should not be connected to any part of the boat or the boat's wiring!

The boats own earthing system ( there should be point above any bilge water connected to the hull that forms the boats earthling point) should be connected to one side of the transformer outpet to create a neutral. It is this which may not have been done on your boat.

You should read the section on mains electricity in the tb-training website
http://www.tb-training.co.uk/MarineE11.html#MAINS ELECTRICITY
in particular the subsection on isolation transformers.

All the boats wiring should then be connected via an RCD in the same way as it would be if connected directly to the shore supply.

With a isolation transformer there is then no need for a galvanic isolator as no part of the boat including its anodes are connected to the shorepower earth conductor.

 

[Darby10]

This post is written with the assumption that the grey box pictured above is an isolation transformer (which it looks like it is).

The RCD (or ELCD) should be on the services (boat) side of the transformer or it simply won't offer any protection other than for a fault on the transformer itself. This is dangerous.

The earth path should allow the current, in the event of a fault, to return to the negative of the supply (in this case the isolating transformer). If the earth and negative aren't connected on the services side of the transformer then you have no earth. This again is dangerous.

To address this issue, the first step is to confirm that the grey box is indeed an isolation transformer. As Rogershaw asked, does it hum?

I hope this helps.

 

[savageseadog]

This post is written with the assumption that the grey box pictured above is an isolation transformer (which it looks like it is).

The RCD (or ELCD) should be on the services (boat) side of the transformer or it simply won't offer any protection other than for a fault on the transformer itself. This is dangerous.

The earth path should allow the current, in the event of a fault, to return to the negative of the supply (in this case the isolating transformer). If the earth and negative aren't connected on the services side of the transformer then you have no earth. This again is dangerous.

To address this issue, the first step is to confirm that the grey box is indeed an isolation transformer. As Rogershaw asked, does it hum?

I hope this helps.

It isn't dangerous to have an unearthed supply after a mains isolating transformer, it's perfectly safe. As the mains is fully floating, ie not earth referenced it would be possible to touch live or neutral without receiving a shock (but don't touch both at the same time!).
There are definite benefits to a floating supply on a boat in regard to corrosion.

Before you go any further though, unless you really know what you are doing you should have an electrician test it for you. An improperly wired mains system is a danger. Be aware that isolated supplies aren't dealt with in the regs (?) and many electricians won't have encountered them in their normal course of work. A true isolating transformer should be certified to some standard or another, it should be flash tested to x(?) KV and so on.

 

[VicS]

The earth path should allow the current, in the event of a fault, to return to the negative of the supply (in this case the isolating transformer). If the earth and negative aren't connected on the services side of the transformer then you have no earth. This again is dangerous.

What is this reference to "negative". AC supplies do not have a "negative".
Your post should refer to the "neutral" but even then the "neutral" is only created by earthing one side of the output from the transformer.

"negative" only applies to a DC system and at this point we are not discussing the DC system and to do so would complicate the issues unnecessarily.

It isn't dangerous to have an unearthed supply after a mains isolating transformer, it's perfectly safe. As the mains is fully floating, ie not earth referenced it would be possible to touch live or neutral without receiving a shock (but don't touch both at the same time!).
There are definite benefits to a floating supply on a boat in regard to corrosion.

If the supply is not earthed as above to create live and neutral the safety benefits of an RCD in the supply will be lost! In this respect it is dangerous to have floating mains system. Especially I would think on a metal hulled boat.
The usual corrosion problem associated with shore power is eliminated by using an isolating transformer or more specifically by not having a connection to the shorepower earth.


I would agree that the OP should have the system checked by a qualified electrician who is familiar with the specific requirements of mains power systems on boats.
There are issues that may require addressing which have not been covered by this thread.

Reference should also be made to ISO 13297, the international standard for AC power systems on small craft

 

[Darby10]

What is this reference to "negative". AC supplies do not have a "negative".
Your post should refer to the "neutral" but even then the "neutral" is only created by earthing one side of the output from the transformer.

Oops, bit of a brain stall moment there!

If the supply is not earthed as above to create live and neutral the safety benefits of an RCD in the supply will be lost! In this respect it is dangerous to have floating mains system. Especially I would think on a metal hulled boat.
The usual corrosion problem associated with shore power is eliminated by using an isolating transformer or more specifically by not having a connection to the shorepower earth.

Quite right. There is no protection and will be no indication of an earth fault occuring. Should a second earth fault occur, dangerous currents could flow through the hull (or the crew!).

 

[savageseadog]

If the supply is not earthed as above to create live and neutral the safety benefits of an RCD in the supply will be lost! In this respect it is dangerous to have floating mains system. Especially I would think on a metal hulled boat.
The usual corrosion problem associated with shore power is eliminated by using an isolating transformer or more specifically by not having a connection to the shorepower earth.

I respectfully suggest it isn't dangerous to float mains supply, quite the opposite in fact. An RCD is only beneficial in normal domestic (or boat) mains circuitry because the mains live is reference to earth via neutral. If the live is floating then the RCD isn't necessary with the caution that neither the live or neutral should contact earth in which event the other pole of the mains becomes earth referenced. That could be taken care of with an RCD so the system would benefit from both being a floating one and protected in the event of a contact from "neutral" or "live" to earth. As I say one could freely take hold of a live wire in an isolated system without harm.

 

[VicS]

I respectfully suggest it isn't dangerous to float mains supply, quite the opposite in fact. An RCD is only beneficial in normal domestic (or boat) mains circuitry because the mains live is reference to earth via neutral. If the live is floating then the RCD isn't necessary with the caution that neither the live or neutral should contact earth in which event the other pole of the mains becomes earth referenced. That could be taken care of with an RCD so the system would benefit from both being a floating one and protected in the event of a contact from "neutral" or "live" to earth. As I say one could freely take hold of a live wire in an isolated system without harm.

Your beliefs on this are not reflected by the ISO standard.

I would recommend to anyone that their system should conform to the standards. In the event of a fatality they would be in a very unenviable position if it did not.

 

[rogerthebodger]

savageseadog is correct in the fact that an float mains supply can be considered "safe" but if either the of the supply lines cone in contact with the hull of a metal boat there this would now make the supply non floating and thus if a person cane into contact with the non shorted line this would make it very dangerous as the short would not be detected as in the case of a neutral/earth system.

That if probably why the ISO standards specify neutral/earth connection and thus the need for a RCD.

IMHO on a GRP boat float mains supply would be much safer than on a metal boat and could be used due to the much lower possibility of a short from one of the supply line to a conduction surface that a person would come into contact with.

Just a not that in the case of a float mains supply there is now not a live and neutral and thus the switching must now be on both supply lines i.e. double pole mains switches not the common single pole switches that are fitted to most mains power outlets and appliances.

 

[lw395]

....
If the supply is not earthed as above to create live and neutral the safety benefits of an RCD in the supply will be lost! In this respect it is dangerous to have floating mains system.....

I don't think the benefit of the RCD will be completely lost.
An RCD works by residual current, any difference between the live and return currents trips the device.
Because the transformer output is floating, there is no low impedance path to ground, but in practice the capacitance across the transformer windings will probably give sufficient coupling to earth to trip the rcd. There may also be some filtering that creates such a path, with a controlled impedance.
There is not much point in an isolating transformer if you then earth the return line with a low impedance.
I don't know the standard practice for transformers in boats, but I have worked in a high voltage lab where we had some carefully thought out systems. A lot of stuff was floating!

I would suggest the key thing is to understand exactly what you have on your boat and why it is that way and stick rigidly to one system. Or hire someone to sort it to a defined standard. A mixture of approaches could be dangerous.

PS Remember to test RCD's regularly. Like anything else in a marine environment, they can jam, seize, corrode, get full off crumbs, dead spiders etc etc and not work when required.
An isolating transformer has the beauty of no moving parts, which is probably why it's stood the test of time as building site equipment.

 

[savageseadog]

savageseadog is correct in the fact that an float mains supply can be considered "safe" but if either the of the supply lines cone in contact with the hull of a metal boat there this would now make the supply non floating and thus if a person cane into contact with the non shorted line this would make it very dangerous as the short would not be detected as in the case of a neutral/earth system.

That if probably why the ISO standards specify neutral/earth connection and thus the need for a RCD.

IMHO on a GRP boat float mains supply would be much safer than on a metal boat and could be used due to the much lower possibility of a short from one of the supply line to a conduction surface that a person would come into contact with.

Just a not that in the case of a float mains supply there is now not a live and neutral and thus the switching must now be on both supply lines i.e. double pole mains switches not the common single pole switches that are fitted to most mains power outlets and appliances.

I did say that there was a possibility of a ground fault on one of the legs in which case an RCD would then protect from a user contact to the other leg or other fault condition which is why I suggested it. Why the ISO should suggest a clamp to ground for the "neutral" I don't know, was this to do with fully isolated supplies? I would have said two lines of safety are better than one and the benefits of a fully isolated system on a boat are substantial.