Steaming/Deck Light wiring

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fortynine

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I am trying to wire a steaming /deck light. There are three cables red, green and black. I assumed that the black would be the common return and that the red and green would be live but when wired like that the fuse blew. Before I resort to buying a load of fuses to try different permutations until one works can any of you elecrickery experts tell me which colour cable is which. Thanks.
 
fortynine said:
I am trying to wire a steaming /deck light. There are three cables red, green and black. I assumed that the black would be the common return and that the red and green would be live but when wired like that the fuse blew. Before I resort to buying a load of fuses to try different permutations until one works can any of you elecrickery experts tell me which colour cable is which. Thanks.
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If the fuse blew, there must be a short. It is possible that the light is earthed to the mast so applying volts to what turns out to be the common return would blow a fuse.

You need a multi meter - cheap as chips for a minimum one.

Another trick would be to connect volts via another (12v) lamp. If it glows brightly, you have found a short (and saved a fuse).
 
Thanks Graham. I have a multimeter and have identified the live and return wires at the boat end, it's the triple wires from the mast that I am trying to identify.
 
fortynine said:
I am trying to wire a steaming /deck light.
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Applying power to any two should not have blown the fuse. There must, as Graham says, be a short.

In the absence of a short you'd expect one lamp or the other to light, or both dimly.

A Multimeter on a low ohms range or Graham,s trick with a bulb should identify which two are shorting.

Worth checking the below deck wiring separately from the mast wiring to begin with to narrow the location of the fault down a bit. ( Oh youve done that!)

Measure the resistance between all three combinations of two out of three. The pair that gives you zero resistance is the pair shorting .... all three could be of course.

Once you have cleared the fault you should get two low readings and one that is equal to the sum of the other two.... those are the two positives and the other wire the common negative.
 
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Red/green/black is the usual set of colours for marine three-core cable. There's no particular standard for how to use them, though if I was wiring such a light I'd use black as negative and a colour for each bulb, as you assumed.

You don't say whether you're looking at the light, the cable poking out halfway up the mast, or the bottom end of the cable behind the electrics panel. So it's rather hard to advise.

Pete
 
prv said:
There's no particular standard for how to use them, though if I was wiring such a light I'd use black as negative and a colour for each bulb, as you assumed.

Pete
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That's quite interesting. By default I choose green as the common! (maybe influenced by US practice where black is regularly the 'live' conductor :D). Just goes to show you should never assume anything on an unknown boat!
 
PRV/ VicS Thanks. I'm wiring in a three pin deck plug. Originally hard wired to connectors below deck. I disconnected them and marked them with coloured tape but then I was taken ill and now six months later I have lost the 'code'. It all worked fine before so although a possibility a short is unlikely. I assumed that if I were to wire one of the positives to the return and one to the supply that would blow a fuse but as you can tell my knowledge of electrics is rudimentry
 
fortynine said:
PRV/ VicS Thanks. I'm wiring in a three pin deck plug. Originally hard wired to connectors below deck. I disconnected them and marked them with coloured tape but then I was taken ill and now six months later I have lost the 'code'.
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Ah hah! And presumably the below-deck cores are different colours to the on-deck cores? Now I understand your problem, though I can't imagine how you expected anyone to guess it.

I would start by identifying the cores on the deck cable. Put the steaming light (only) on, and test with a multimeter until you find the two which have 12v across them. If your meter is saying plus12v, then the one under the black probe is the common negative, and the red probe is the steaming light positive. Confirm by switching the deck light (only) on, and testing for 12v between the negative and the other positive. There should be no significant voltage between the two positives. Now label these cores.

Next you need to identify the wires to the lights. You'll need a source of 12v. The previously-identified wires will do, though I have a pair of croc clips on a long lead from a lighter plug, for this kind of thing. Apply the 12v to different pairs of wires until you find which combination lights up which light. I can't see how this would cause any fuses to blow.

Pete
 
Graham_Wright said:
If the fuse blew, there must be a short. It is possible that the light is earthed to the mast so applying volts to what turns out to be the common return would blow a fuse.

You need a multi meter - cheap as chips for a minimum one.

Another trick would be to connect volts via another (12v) lamp. If it glows brightly, you have found a short (and saved a fuse).
Click to expand...
Very unlikely to have current flowing through the mast in the same way as cars are wired as this would encourage rapid corrosion of the aluminium alloy.
American aircraft during WW2 were wired "negative earth" like this and all the ones supplied on Lend-Lease to the RAF had to be rewired with a wiring for the negative side.
 
PRV I only asked if anyone knew the red, green, black colour code. I didn't think anyone would be interested in where the connection was to be made, sorry.
I had identified the wires at the deck plug and connected to the mast wires as you suggest which is when the fuse blew. I will have to use trial and error as you advise. Thanks again.
 
Using your Ohmeter, check between the 3 cores.

If you get a high resistance reading from any two, you have found your positive, and lower resistance between one of the high resistance reading pair and the third wire, and again between the other high resistance reading pair and the third wire, you will have found the negative.

For instance - using red, green, and black (yours may well be a different combination). If you get a high resistance reading between red and green, you are reading the resistance through both bulbs. Now you know the positive wires.

Next, find the resistance between red and black, and green and black. These should show a lower resistance to your first reading. One of these readings could be lower than the other. This could indicate the steaming light and deck light. These could have different wattage bulbs. Maybe the deck light is of a higher wattage than the steaming light. If both resistance readings are the same, you will only know which is which by connecting and switching on. 50/50 chance of getting it right the first time.

I had this problem early this year when fitting new plug/socket at deck level. I forgot to check which was which when I disconnected them - and me an electrician!! My check was the mast-head and all round white. The mast-head was 25watt and the all round white was 10w. The 25w gave a (higher - wrong, see VicS post) lower resistance than the 10w.

I hope I have explained it simply enough.
 
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