Battery isolation on -ve side?

[pvb]

And nobody mentioned fuses!...

If we'd got on to high-current fuses we could have had some real disagreements! /forums/images/graemlins/wink.gif

 

[oldharry]

Just in case....

Thanks PVB - it all as clear as it was when I first learnt it 50 years ago.... by the way - any idea how to re-set a lost car radio code??? /forums/images/graemlins/smirk.gif /forums/images/graemlins/smirk.gif /forums/images/graemlins/grin.gif

However in the real world - particularly that of older boats - some very strange things happen with electrical circuitry and in the last 40 years I have seen some effects which appear to defy the 'normal' rules. Not least the example I mentioned of a 2 volt reading off the plastic case of a battery.... there was enough power available to light a small torch bulb - off the insulation!

So I prefer to be safe and not have electrical potentials around until I need them.

In my book it is the same safe practice as turning off the gas at the gas bottle, as well as at the cooker. OK the consequences are not so spectacular as a gas leak, but electrical failure from corroded contacts is enough to escalate to a serious - even fatal - mishap at sea. (Non starting engine, failure of nav gear/GPS etc, Nav lights not working etc etc...)

You only turn off the gas supply at home in an emergency, so why do we always do it in the boat even when we know the pipework is OK?

Just in case.....

 

[Benbow]

Re: Just in case....

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Leaving the +ve connected means that a large part of the circuitry is at 12 volt potential, 'seeking' by any means to complete the circuit, damp being the ideal means of doing so.

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So I prefer to be safe and not have electrical potentials around until I need them.

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This is seriously misleading. There is nothing inherently more 'live' in the positive side than the negative side of a circuit. It is completely arbitrary that we call the side the elecrons leave from the 'negative' and the side they return to the 'positive'.

The consequences discussed above arise from arbitrary conventions about grounding engines, and in which line you place fuses. But in NO sense is the 'live' side of a circuit 'seeking the means to make a circuit' any more than the negative side is.

 

[bluedragon]

Re: Just to Recap then...

Been down on the boat planning it out this morning. I'm fitting new single pole isolators adjacent to each battery +ve (with switch access through a bulkhead into the cabin), then keeping the removable -ve terminals I've already got in the battery locker. So if needs be I can isolate the batteries from down below (+ve) or on deck in the locker (-ve) or both. I'm wiring-in one of KDD Powercentre's VSRs at the same time to update my charging system. Most of what I've got was done in 1972 apparently, so time for an update I think! Thanks to all. Great discussion...as an amateur engineer and experimenter I loved it! In fact I'm going to dig out some of old college books on electro-chemistry...I'm fascinated by all this stuff now.

 

[oldharry]

Boat electrics have rules of their own!

I am sorry that you find what I say misleading Benbow, but I have to disagree with you.

In the vast majority of small boats I have dealt with, both for my own use, and professionally, the 'negative' side of the battery has been wired according to automotive industry practice of having the engine frame or chassis connected to the negative battery terminal.

My point is quite simply that it is highly undesireable to have conductors at a different potential until the circuit is required to operate, because of the effects of damp.

40 years experience of dealing with the effects of damp in a range of boats both for personal and professional use, leaves me in no doubt that a conductor carrying an electrical charge of whatever voltage will succumb to the effects of the marine environment sooner or later and a circuit will be created - to the rapid detriment of any electrical equipment.

I am aware of the desireability of having both sides of an electrical supply 'floating', but have yet to see an installtion where this genuinely applies to the extent that it does not matter which terminal is used to isolate the source.

And from many hours of frustration I am only too well aware of what happens to electrical installations where damp has been able to complete the connection. Tracking down the damage, rectifying it, and trying to prevent it from happening again is - well, nothing short of fiendish!

As I become older I become increasongly convinced that while boat electrics clearly must follow the laws of physics, they also have an entirely independent set of rules of their own!