Electrical Safety

[barry1971]

My surveyor suggested fitting a RCD to my shore power circuit as the previous owner has wired through a domestic style fuse. Can anyone suggest which type/rating of rcd is suitable for the marine environment?

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[tome]

They work by sensing an imbalance between the current in the live and neutral wires caused by earth leakage.

The MK Sentry units are widely available (B&Q etc) and trip at 30mA imbalance. The rating of the current trip should match your load, probably a 16A unit will be suitable. You need to purchase the enclosure and the RCD, which should be a 2 pole switch to fully protect. Place the unit upstream of any mains sockets or outlets on board, as close to the input socket as possible but in a dry location.

Once installed don't forget to test it regularly using the test button - this will prevent it becoming sticky and failing to protect when you need it to.

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[Fill]

You don't need to purchase the enclosure if you'll be content with an old one I've got. Drawback is you'll need to pick it up from me. Send me a pm to check if you're based close enough to make it economic.

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[VicMallows]

Try Screwfix 63138--73 £24.99 IP55 enclosure; 30mA RCD; 6A and 16A MCBs. Best price I've seen to date. (Garage cunsumer unit).

Vic

(if you don't know of Screwfix PM me and I'll claim my reward when you order!!).

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[Guest]

I\'ve always wondered....

If an RCD works by detecting a small electrical leak to earth, will it work when the boat is chocked on the hard with the keel resting on blocks of wood the thickness of railway sleepers? I mean would these blocks not act as perfect insulators?

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[Superstrath]

Re: I\'ve always wondered....

Most of the consumer units sold for boats, caravans or other applications where you connect via lead have a small light that is wired from the negative to earth. The idea is that if the incoming polarity is wrong, due to a wrongly made-up lead, the light comes on.
Since there is no other way to tell quickly whether or not shore-power is on, I thought the light could be wired from positive to earth, that way the light would come on if power was on and the correct polarity. BUT - is the current used by the light, returning via the earth wire, going to be enough to trip the shore rcd? Anyone know?
If the light was wired accross the positive and negative, it would come on regardless of polarity.

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[Trevethan]

Re: I\'ve always wondered....

Don't forgott hat the earth also runs back along the cable to the shore -- my 240 vsystem has no earth connection with the boats system and as long as you check the thing by pressing the test switch which shorts the circuit and it works then you are safe.

I asked about dual earthing to the boat, but the consensus seemd to be that it was not worth doing and caused lots of problems re: stray current.



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[Guest]

Re: I\'ve always wondered....

Aaaah....

Thanks

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[petery]

RCD + galvanic isolator

I'm currently re-doing my electrics. If I have a galvanic isolator in the earth lead from the shore, do I need an RCD. If I do where do I position it?

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[MedMan]

Re: I\'ve always wondered....

My Dis Board has both - a red one between pos & neg to show that there is power and a yellow one between pos & earth to show that the polarity is correct. The former cannot affect the RCD as it does not create and imbalance but, like you, I have often wondered about the polarity light.

Though we have both called them lights, they are, of course, (on my board anyway) LEDs. I suspect that the current draw of a typical LED is well below the 30mA needed to trip the RCD or even the 15mA required by the more sensitive RCDs sometimes fitted. Like you, I would appreciate confirmation that this is so.

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[catmandoo]

Re: I\'ve always wondered....

My understanding is that for the system to work you must have an earth and that when pressing the test button you initiate a tiny current leak to earth thus causing a current difference between live and neutral. So if boat insulated on chocs and marina earth which you are supposed to be connected to is faulty then no earth unless you have an aluminium ladder to the ground and it is wet. In former case it would mean that ELCB would not work because although it is supposed to measure difference between positive and negative if there was no place for current to go ie from you to boat to earth or from fault to boat to earth then there would be no current loss. I have a Maplin tester which I plug into every 240 V sockets with boat in water ,press a button to activate and rotate a knob to test if it trips at 30 milli amps or not . Nothing happened until I tied boat's 240 V earth to boats DC earth. I was always suspicious that Italian marina earth was faulty . I assume that the path in this case was from boat to sea and not from boats earth to marina earth. In any case marina polarity was also reversed in adjacent sockets so how safe can you be without a proven decent earthpath ? in any case I only link to marina supply when on the boat.

So I would suggest that people also test the continuity of their earth connections at all locations. Or am I mistaken ? Any one feel qualified to comment ?

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[MedMan]

Re: I\'ve always wondered....

So I would suggest that people also test the continuity of their earth connections at all locations.

Very wise advice.

When on the hard in Almerimar folk all around were complaining about strange things happening with the electrics. Whilst we were on board a visitor touched our propellor and received a violent shock. We had noticed nothing unusual aboard. I asked the visitor to remove our shore power cable and I went to investigate. There was a substantial voltage between the yard's earth cable and true earth. (I stuck one probe into the ground) At that time I had the DC Negative and AC earth bonded together. I promptly removed the bonding and have never replaced it as this experience could just as easily have been fatal.

The electricity supply along the side of Lazaretto Creek in Malta also has (or at least had when I was there in 2000) some interesting characteristics. It appears to be fed through a centre-tapped transformer so that both Live and Neutral are 120v relative to earth but 240v relative to each other. It's a safe system, but it plays havoc with my polarity warning light!

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