Battery fuse/isolator position

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Hi, i'm changing my battery wiring and switching; removing 1-B-2 switch and opting for an isolator switch on both batteries. I'm adding main battery fuses to both house and engine start; should I put the fuses before or after the switches?
 
They'll be fixed to the bulkhead very close, about 6" away, but I was considering should the fuse also protect the switch or should the switch be first so the fuse can be changed when the circuit is isolated.
 
Fuse

Fuse is there to protect the wiring from getting too short in a short circuit situation. It is not a problem to replacee a fusee with power still on it. If you turn the switch off then fopefully there will be no load or current flow when you refit the fuse. good luck olewill
 
Battery Fuses

The fuses should be as close to the battery as possible, as they are there to prevent short circuits causing a possible fire. You can get fuses which actually fit to the battery terminal posts ........ I don't know where from, but Merlin would be a good place to start searching. They look like a square (1 cm X 1cm) of whitish insulation material.

Alan.
 
BradC said:
They'll be fixed to the bulkhead very close, about 6" away, but I was considering should the fuse also protect the switch or should the switch be first so the fuse can be changed when the circuit is isolated.
Click to expand...

I have separate isolator switches for the domestic and starter batteries, with a 3rd switch linking to 2 circuits to allow me to start the engine from the domestic batteries if the starter battery is dead. The domestic circuits are protected by a 100A fuse imediately after the isolator.

My reason for this configuration is that the engine starting current could exceed the rating of the main domestic fuse, and so if I were to use the domestic batteries in an emergencu to start the engine I could end up with no engine and no instruments.

John
 
I have separate isolator switches for the domestic and starter batteries, with a 3rd switch linking to 2 circuits to allow me to start the engine from the domestic batteries if the starter battery is dead. The domestic circuits are protected by a 100A fuse imediately after the isolator.
Click to expand...
I'd been wondering what to say on this topic but I like that arrangement. Seems very sensible.
A suitably rated fuse could also be included in the feed to the starter, immediately after the switches, if that is desired.
 
John's answer is the best I've heard so far. Before touching anything, always disconnect the earth (NEG) unless neither leg is earthed (ie 24v floating systems, in which case take your pick). The reason for this, is that attempting to remove the positive terminal in a confined space, can lead to the situation of spanner slipping, grounding itself, many 100's amps flow via spanner, could cause burns or a fire. By disconnecting the neg earth first is far safer.

The starter circuit, is of a very low impedance when cold cranking the initial surge would blow any fuse. Fit your isolators as John suggested. One for each battery, common link switch across the output side of these. Then via a fusable link to the distribution system fuse panel for all your auxilliary circuits. fuse rating of auxilliary circuits should be of 1 1/2 - 2x the expected load.

Good luck. Hope this is of some help. (Fernhurst Books have a range of subjects covered. Maybe one is available for boat electris.
 
"The 12 volt Bible for Boats" by Miner Brotherton is a book very commonly recommended

Another which finds favour with some is Practical Boat Owners "Electrics Afloat" by A.E Garrod

Yet another is "The 12 Volt Doctors Practical Handbook " by E Beyn which you will also find on line HERE

Much useful information especially for the novice HERE
 
Carribean Blue said:
John's answer is the best I've heard so far. Before touching anything, always disconnect the earth (NEG) unless neither leg is earthed (ie 24v floating systems, in which case take your pick). The reason for this, is that attempting to remove the positive terminal in a confined space, can lead to the situation of spanner slipping, grounding itself, many 100's amps flow via spanner, could cause burns or a fire. By disconnecting the neg earth first is far safer.
Click to expand...

Fully agree. My battery clamps are all metric, 13 mm AF, so I have one dedicated 13mm spanner with the handle wrapped in PVC tape. It costs nothing and may save my day.

Theo.
 
Carribean Blue said:
John's answer is the best I've heard so far. Before touching anything, always disconnect the earth (NEG) unless neither leg is earthed (ie 24v floating systems, in which case take your pick). The reason for this, is that attempting to remove the positive terminal in a confined space, can lead to the situation of spanner slipping, grounding itself, many 100's amps flow via spanner, could cause burns or a fire. By disconnecting the neg earth first is far safer.

The starter circuit, is of a very low impedance when cold cranking the initial surge would blow any fuse. Fit your isolators as John suggested. One for each battery, common link switch across the output side of these. Then via a fusable link to the distribution system fuse panel for all your auxilliary circuits. fuse rating of auxilliary circuits should be of 1 1/2 - 2x the expected load.

Good luck. Hope this is of some help. (Fernhurst Books have a range of subjects covered. Maybe one is available for boat electris.
Click to expand...

This is all wrong - with the greatest of respect.

First of all the advice about always removing the negative terminal connection applies to cars. In a boat where there isn't a metal chassis which any spanner on the positive can hit at any time, the advice isn't relevant.

Secondly, lots of boats have fuses in their starter circuits and the cold cranking current DOESN'T blow them every time as suggested. Our last boat had a fuse in the starter circuit and it was fine. Cold cranking currents might be one or two hundred amps, but you can get fuses that size without problem.

The fuse rating should be more for the wire it is protecting - not for the expected load.

A switch as a link for emergency starting is the only thing I will agree with.
 
john_morris_uk said:
This is all wrong - with the greatest of respect.

First of all the advice about always removing the negative terminal connection applies to cars. In a boat where there isn't a metal chassis which any spanner on the positive can hit at any time, the advice isn't relevant.
Click to expand...

It is still good practice to remove the negative first no matter what.
There are many boats out there with metal battery boxes and metal hulls, if you get into the habit of going for the positive first then one day it will come back and bite you.

As for the battery post fuses I used to fit them but then the thought came to me what if they blew whilst the battery was gassing, could be a large bang.
They were originally designed for forklifts where the system is unplugged whilst charging so no danger of something in use in the system causing them to blow
 
Carribean Blue said:
John's answer is the best I've heard so far. Before touching anything, always disconnect the earth (NEG) unless neither leg is earthed (ie 24v floating systems, in which case take your pick). The reason for this, is that attempting to remove the positive terminal in a confined space, can lead to the situation of spanner slipping, grounding itself, many 100's amps flow via spanner, could cause burns or a fire. By disconnecting the neg earth first is far safer.

The starter circuit, is of a very low impedance when cold cranking the initial surge would blow any fuse. Fit your isolators as John suggested. One for each battery, common link switch across the output side of these. Then via a fusable link to the distribution system fuse panel for all your auxilliary circuits. fuse rating of auxilliary circuits should be of 1 1/2 - 2x the expected load.

Good luck. Hope this is of some help. (Fernhurst Books have a range of subjects covered. Maybe one is available for boat electris.
Click to expand...
I answered this post yesterday - but my post seems to have disappeared. (I didn't think that I was that rude!)

I will try again:

With the greatest of respects I disagree with nearly everything that Carib-Blue says.

1. The suggestion to always disconnect the negative is good practice in car electrics but doesn't apply to most boats. In a car, there is always a chassis nearby that a spanner on a positive terminal might short to - hence disconnect the negative from the battery/chassis first. On a boat it is irrelevant.

2. Fuses are readily available that will carry the cold cranking current of a diesel starter. I fitted such fuses in our last boat.

3. The fuse rating is chosen to protect the wiring and not just the load that the wire is feeding. You might argue that its safer to choose a fuse in the way that Carib-Blue suggests, but its not the whole story.

4. Fitting a switch/breaker to connect the domestics to the engine battery for emergency starting the one good idea I agree with.

I believe that the books he mentions are more for car electrics. Try reading Nigel Calder on the subject of marine electrics or some of the other good marine maintenance books available?
 
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Cervinia said:
It is still good practice to remove the negative first no matter what.
There are many boats out there with metal battery boxes and metal hulls, if you get into the habit of going for the positive first then one day it will come back and bite you.

As for the battery post fuses I used to fit them but then the thought came to me what if they blew whilst the battery was gassing, could be a large bang.
They were originally designed for forklifts where the system is unplugged whilst charging so no danger of something in use in the system causing them to blow
Click to expand...

Hmm- I will let the readers decide how many boat builders are stupid anough to fit metal battery boxes and then connect them to the negative terminal. There are a few metal boats about - but most of the steel and aluminium boats that I have sailed have made strenuous efforts to isolate the boats batteries and wiring from the hull. As perhaps 99% of leisure boats are GRP or wood, then I would politely maintain that the suggestion that it is best practice to disconnect the negative terminal first is a waste of time and a misunderstanding of boat electrics.

Furthermore, I don't seem to remember mentioning battery post fuses. The fuses that I fitted were outside the battery box and are well away from gases caused by battery charging. Besides, best practice is that battery boxes are well vented and so there should be no risk of explosion. The fuses I suggest that are used are readily available and come in a variety of sizes up to hundreds of amps. As I said, I have no idea where you got the idea that I was suggesting post fuses from fork lift trucks...

Very sorry that I won't be able to continue this interesting discussion - I am going sailing for a couple of weeks...
 
Many thousands of boats have steel hulls and steel battery boxes.
Just because you havent seen one doent mean they dont exist!
Many boats are built that way, having said that they are mainly inland.

The comment on fuses was aimed at an earlier post by Alan and they were developed first for forklifts!
 
Cervinia said:
Many thousands of boats have steel hulls and steel battery boxes.
Just because you havent seen one doent mean they dont exist!
Many boats are built that way, having said that they are mainly inland.

The comment on fuses was aimed at an earlier post by Alan and they were developed first for forklifts!
Click to expand...
Many thousands - are you sure? I have been sailing and involved with boats for a few years, and I don't remember seeing one yet that had the negative battery terminal connected to the hull.

If Carib-Blue wishes to question a post by Alan on fuses, then he should make it clear that he is not questioning my comments... I commented on his assertion that you can't possibly fuse a starter cable (which you patently can) and get some drivel about battery post fusing and their dangers.

Let the reader decide who is making sense here...
 
I fitted two Blue Seas Mega fuse blocks with 200amp fuses, one for the engine start battery, and one for the domestic bank. These then feed the selector switch and then onto various circuit breakers for the large loads, i.e. windlass, inverter and domestic distribution panel. Each time the wire size drops there is a fuse or breaker.

I've not had any problems starting the engine on a fused circuit, I think the blue seas mega fuses will actally pass a much higher current for a short period.
 
john_morris_uk said:
Many thousands - are you sure? I have been sailing and involved with boats for a few years, and I don't remember seeing one yet that had the negative battery terminal connected to the hull.
Click to expand...

Yes THOUSANDS
You may not class them as boats however as they tend to ditch crawl.
and are called Narrow Boats, as well as most barges that are made of steel.

BTW I seem to remember that its part of the regulations that the -ve should be connected to the hull in a steel boat as should the earth on any 240v system.


However I bow to your superior knowledge as you are obviously a professional in this area and I just work with electrics for a living.
 
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