Where should the battery isolator switch(es) be installed

We've been asked this and related questions so many times we can answer them in our sleep.

In reality the answer to the question is not even open to debate.

1. Should they be in the negative or positive battery leads ?

Positive. Every time. The one and only exception to this is in the case of a vessel using a grounded positive DC system (very very rare these days). And even then, you have to remember that a positive grounded system is VERY bad for stray current corrosion in a marine environment. It should be avoided at all costs.

2. But we often see them in the negative leads. Why is this ?

The only answers we ever hear are "we've always done it this way" or "it prevents sparking when connecting the batteries if a spanner touches the hull" or "we only need to use one isolator switch if we put it in the negative"

Let's answer them one by one........

"we've always done it this way" .... er... so what? What's wrong with change?

"it prevents sparking when connecting the batteries if a spanner touches the hull".... then don't let the spanner touch the hull.

"we only need to use one isolator switch if we put it in the negative".... hmmmmmm hardly a major expense is it?

3. Ok so what are the arguments against fitting the isolator in the negative side ?

How many do you want ?

Try these for starters....

When you isolate the negative side of the batteries the 12 volt feed is still there round the boat. It simply has no way to get back to the battery negative. Well..... not that you know of...... believe me.......... in a damp marine environment it will find one. Probably through a mixture of dampness, dirt and the hull. A perfect recipe for stray current corrosion. Do you really want to add another cause of hull corrosion ?

Here's a more short term effect. Many boats are now fitted with some (comparatively) elaborate electronic equipment. One that springs to mind is a central heating system controlled by an electronic timer. Obviously the clock has to have a permanent battery feed in order to keep the clock running (not all central heating timers are powered by internal batteries). The timer/clock is therefore wired up, bypassing the isolator switch. If the timer attempts to fire up the boiler with the isolator switched off, the boiler will attempt to draw it's initial start up current (some 20 to 30 amps with a diesel boiler) down the small ground connection between the timer and the boiler. At best this will burn out the cabling between the two (remember, fuses are not normally [and only very rarely should they be] put in ground connections) or at worst it will destroy the timer and the boiler control panel.

This same problem can ocurr with some clever inverter/control panel combinations. In particular, combined

inverter/charger control panels and battery monitors. In this case we have actually seen it do over 1000 worth of damage. All because the isolator was put in the battery negative cable. All because "we've always done it this way" or "well it saved the cost of another isolator switch"

Anyone called in to trouble shoot the electrical system will expect the isolator to be in the positive side. Why confuse the issue ?

Separate isolator switches allows each battery bank to be treated (and therefore worked on) independantly. This is a great bonus in many circumstances.

There are many, many other arguments AGAINST negative isolators and FOR positive isolators but I hope the above is enough to convince you !

If not, then draw yourself a diagram of an electrical installation on a boat, then put various electrical "leaks" (damp, dirt, moisture etc) between various parts of the electrical system and the hull and then, remembering that the more positive electrical point will be eroded, work out what will happen with each and every electrical "leak".

You will soon realise that stray current corrosion is FAR more serious with positive grounded systems or with the isolator in the negative lead.


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Page last updated 02/04/2008.
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