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Old 12 November 2013, 15:06   #11
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Originally Posted by ashbypower View Post
Technically its not an Earth, it's the 12v -ve, not to be confused with a true earth, for shore power.
Thanks, I'm with you now. Confusion over
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Old 12 November 2013, 15:12   #12
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Yep that's about it. To think about it logically

You have black and red on a battery and the same on your electronics. You need to connect them together. If you only connect one side you haven't made the full circle.
If you need to do any wiring draw it out first and get it right in your head before you touch anything. Seriously though in your case I'd get someone else to do it. A loom burn out or a fire will really ruin your day
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Old 12 November 2013, 15:20   #13
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Tim at Mobile Centre is very helpful, prices can be a little high but all good quality stuff.
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Old 12 November 2013, 15:20   #14
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Originally Posted by biffer View Post
Yep that's about it. To think about it logically

You have black and red on a battery and the same on your electronics. You need to connect them together. If you only connect one side you haven't made the full circle.
If you need to do any wiring draw it out first and get it right in your head before you touch anything. Seriously though in your case I'd get someone else to do it. A loom burn out or a fire will really ruin your day
I get circuit continuity, what I didn't get was the use of the phrase Earth. An Earth is a way of bonding the potential of metalwork that may come into contact with an electrical circuit. Earths are used in higher potential systems (240V) to protect from live to metalwork short circuits, and also to tie the potential to that of the actual Earth or Ground. (Hence the term) also preventing casing to earth shocks. I couldn't work out why you would want to bond the metalwork on a boat to an Earth potential. What I didn't realise is that the term Earth and negative were being used in juxtaposition. ... Fizz
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Old 12 November 2013, 15:35   #15
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If you have a faulty earth/ neutral on a boat you will become what is best described as the earth rod or ground and you will get a good shock off it. This usually happens when everything is wet. You seem to know more than the basics you will do fine
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Old 12 November 2013, 15:46   #16
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Originally Posted by Fizzwizz View Post
I get circuit continuity, what I didn't get was the use of the phrase Earth. An Earth is a way of bonding the potential of metalwork that may come into contact with an electrical circuit. Earths are used in higher potential systems (240V) to protect from live to metalwork short circuits, and also to tie the potential to that of the actual Earth or Ground. (Hence the term) also preventing casing to earth shocks. I couldn't work out why you would want to bond the metalwork on a boat to an Earth potential. What I didn't realise is that the term Earth and negative were being used in juxtaposition. ... Fizz
Its fairly common to bring a bunch of small negative wires back to a common negavtive post or buss bar. That point is then wired to the battery with a single larger cable capable of carrying all those individual currents. The negative post or buss may also be wired to something like the engine block to keep everything at earth potential. The use of single "ground" wires from the negative buss (or post) to the engine and another to the battery avoids multiple wires and circuitous routes so that no current can flow between difference bits of metal which might want to be at different potentials. E.g. the engine block vs the stainless prop shaft. The whole idea being that all currents are carried by wires, not by any metal parts at all.
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Old 12 November 2013, 15:55   #17
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As clearly stated by others, the term 'earth' is being used generically to mean the negative or 'ground' side of the battery. The reason you can combine the negative wires to a single point or bus is mainly for convenience but also because it is normally the positive side that is fused.

You will sometimes find a genuine earth on a boat which normally comprises of a copper plate on the hull under the waterline. This can be for lightning protection and to improve radio reception on SSB and the like.
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Old 12 November 2013, 16:12   #18
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Actually the OP sounds like he is putting the threaded bolt in the GRP - that does sound like the bolt was going through the skin or something... I can see why Fizz was confused.

As he only had about 3 bits of electrical kit I'm not sure why he feels the need to have a bus-bar in addition to whats already on the fuse...
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Old 12 November 2013, 16:16   #19
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The whole idea being that all currents are carried by wires, not by any metal parts at all.
Many thanks for the info. It does strike me though, that the best way to prevent these erroneous currents through metalwork is to keep your 12V circuitry away from it. i.e don't tie your negative to your engine (ground). The minute you do this you have the potential for aberrant circuits running through props/shafts etc. as you've created a path back to negative through the metalwork. Despite peoples belief, current cannot flow from the positive terminal of your battery to the sea. (This only happens with powerstation generated electricity as the powerstation bonds neutral to the ground creating a path back for current through the soil)
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Old 12 November 2013, 16:29   #20
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Many thanks for the info. It does strike me though, that the best way to prevent these erroneous currents through metalwork is to keep your 12V circuitry away from it. i.e don't tie your negative to your engine (ground).
You don't have a lot of choice about that unfortunately. Battery negatives bolt directly to the powerhead. There's no avoiding it as without the negative bolted to the powerhead, you're not going to be able to generate an HT spark.

It's a good reason to keep an eye on your anodes
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