Isolator switch

[Rojo]

Can I ask which Battery Isolator switch is recommended please. I currently have one that the key turns 90 degrees and is removable when in the off position. It is connected to Negative! I have read comments that advise that it is a better option to Isolate the positive side. Looking for guidance on this please!

[Tony Brooks]

21 minutes ago, Rojo said:

Can I ask which Battery Isolator switch is recommended please. I currently have one that the key turns 90 degrees and is removable when in the off position. It is connected to Negative! I have read comments that advise that it is a better option to Isolate the positive side. Looking for guidance on this please!

 

Using just one isolator in the negative to side to isolate both batteries banks use to be fairly common, but under some very rare and particular circumstances it can lead to damage to equipment and wiring cause by high currents from isolated (-) equipment finding its way back to the battery by the negatives of an item that is allowed to bypass the isolator. Hence, now putting a pair in the positives. If yours has simple electrics and is working well, I see pressing need to change until a problem occurs, but be aware that some young professionals may not be aware of history, so it might confuse them.

 

FWIW Hella makes master switches that fit your description but the key is metal, not plastic. It is the ones with plastic keys that commonly give trouble.

Almost any master switch from one of the main UK marine electrical suppliers will be more reliable than what you have, two brands that come to mind are BEP and Blue Sea.

 

Edited to add: When buying, pay very close attention to the currant ratings. The online boys seem very fond of quoting the maximum surge currant rating rather than the continuouse rating. The contiuouse rating may well be only 20% of the surge  rating.

Edited July 6, 2023 by Tony Brooks

[Rojo]

Thank you Tony, I do have an issue in that when the switch is off I am getting a 9 volt reading from my start battery, though it shows 12.6 volts with the switch on! ( The leisure batteries give no reading when the switch is off) Thought there must be a switch fault.

[mrsmelly]

BEP are good, never had a problem.

[Tony Brooks]

40 minutes ago, Rojo said:

Thank you Tony, I do have an issue in that when the switch is off I am getting a 9 volt reading from my start battery, though it shows 12.6 volts with the switch on! ( The leisure batteries give no reading when the switch is off) Thought there must be a switch fault.

 

If you are asking for an opinion of what might be wrong, I need to know exactly where you are connecting your meter to get that reading, any other equipment that might be turned on and drawing current, and what equipment is bypassing the isolator switch, like a battery charger, radio memory, or automatic bilge pump.

[Rojo]

Thank you, I made the connection to the load side of the switch and directly onto the positive on the start battery.

Using the same load side of switch connection and positive onto the leisure batteries shows 0 volts with the switch off, as I would expect. 

The automatic bilge pump is wired to bypass the switch.

[ditchcrawler]

So its possible to have a negative to earth fault on the battery side of the isolator giving a second return to the batteries that doesnt go through the isolator, one of the dangers of switching the negative, get two earth fault and you could try to start the engine with all the current flowwing through the 2.5mm negative cable from the bilge pump 

[Tony Brooks]

59 minutes ago, Rojo said:

Thank you, I made the connection to the load side of the switch and directly onto the positive on the start battery.

Using the same load side of switch connection and positive onto the leisure batteries shows 0 volts with the switch off, as I would expect. 

The automatic bilge pump is wired to bypass the switch.

 

This suggests that something on the engine circuits is trying to run but can't because the path back to the engine battery is open because of the switch. In this case, the load side of the switch is likely to be at an unknown voltage so you could easily get a voltage reading. I suspect it might be a negative reading rather than positive.

 

Try measuring between the load side of the switch and engine battery NEGATIVE with the switch off. If you find voltage, it will indicate that something is drawing current and trying to return it via the switch that is off, so the whole negative circuit has a positive voltage on it until you turn the switch on, so it can get back to the battery. Then I would expect it to drop to zero unless there is resistance in the switch.

 

Going to the positive gives a path to negative via other equipment like the bilgepump.

Edited July 6, 2023 by Tony Brooks

[Rojo]

Thanks for that advice, I will give that a try at the weekend 

[Rojo]

Tony, you are correct in your assumption that there would be voltage between the load side of the switch and the negative battery terminal with the switch off. Also that the load side is positive until the switch is on. I have disconnected all cables from the load side and have a positive on each individual one. I have yet to discover how the bilge pump is wired as it is automatic with a switched override. More investigations needed!

 

[blackrose]

I'm not an electrical expert but I've been told that the problem with cheap, underrated battery isolators, is that with use eventually the contacts can fuse together, so that you may think you've isolated the batteries but in fact you haven't. A potentially dangerous situation.

[Tracy D'arth]

9 minutes ago, blackrose said:

I'm not an electrical expert but I've been told that the problem with cheap, underrated battery isolators, is that with use eventually the contacts can fuse together, so that you may think you've isolated the batteries but in fact you haven't. A potentially dangerous situation.

It is far and away more often that they go open circuit if they are the red loose key type. The Lucas ones with the steel bar handle are much better as are the ones wit a lozenge shaped knob.

[Tony Brooks]

27 minutes ago, Rojo said:

Tony, you are correct in your assumption that there would be voltage between the load side of the switch and the negative battery terminal with the switch off. Also that the load side is positive until the switch is on. I have disconnected all cables from the load side and have a positive on each individual one. I have yet to discover how the bilge pump is wired as it is automatic with a switched override. More investigations needed!

 

 

The "problem" you have is very common when negative cable go open circuit. Suddenly, every negative cable joined on the load side of the fault seems to become live (12V+) and it confuses the hell out of many people.

 

When you turn the master switch off, you effectively open circuit ALL the negatives for all the equipment on the boat. As long as a voltmeter connected battery neg to load side neg drops to zero when you turn the switch on I THINK you are OK to carry on using it as it is.

 

I don't fully understand why, when you were measuring between each battery pos and the load side of the switch turned off you only got a voltage on the engine battery test, not the domestics. That suggests to me that something connected to the engine battery is drawing current (turned on). Does the bilge pump have a fuse or an OFF position on the switch? If so, either will allow you to isolate it and see if the voltage with the master switch open goes away.

 

If it is not that then all you can do is leave the voltmeter in place bat neg to load side, switch off, and pull fuse or switch things off until it goes away.

 

PS, I don't think it is anything to do with a resistive master switch.

 

Many thanks for comming back.

[Tony Brooks]

Further thoughts.

 

If you know how to do then if you set your meter for amp (10 or 20 amps DC - depending upon the meter) and connect it between bat neg and the load side of the switch with the switch turned off I expect you will find that there will only be a very few 0.1 amp flowing, but if there are whole amps it might give you a clue as to what is "leaking" current.  personally, I suspect you will never get to zero amps and will always get some voltage with the switch off and the volt meter connected. When it is all connected up, turning the switch off will prevent any current flow.

Edited July 9, 2023 by Tony Brooks

[blackrose]

2 hours ago, Tracy D'arth said:

It is far and away more often that they go open circuit if they are the red loose key type. 

 

Well if they fail open circuit that's inconvenient but not dangerous. I was just pointing out the potential danger of closed circuit failures even if they may not happen that often.

[Rojo]

I have yet to solve this issue. I posted yesterday that having carried out Tony's advice by checking for any voltage between the load side of the switch and battery negative (start and domestic's are connected) that i did find there was a voltage. I consequently removed all the cables from the load side of the switch in order to check whether any were showing as positive, thinking that if one was positive and the rest negative that would pinpoint where the voltage was coming from. As it turned out all of the cables checked individually showed positive! I refitted the cables and checked for a voltage between the load side of the switch and batteries negative,no change. I have then isolated the bilge pump completely and checked for voltage again, no change! I got no reading of amps on the multi-meter test that Tony suggested. On carrying out these tests I had all switches on the control panel switched off. It doesn't take much, but I'm confused!  

[jonathanA]

what is the actual issue you are trying to resolve ?  

 

Personally I would remove the isolator from the negative lines and fit one each in the positives for the starter and domestics.  (negatives connected together and connected to the hull bonding point). I appreciate that might not be so easy for you if you don't have the right heavy duty leads available or the capability to make them. 

 

I just think you maybe overcomplicating things. (and/or I'm misunderstanding the issue)  what are the voltages on the battery terminals (i.e directly across the +ve and  -ve) with the isolator on and off. (keep it simple) ?

[Tony Brooks]

If you got no amps flow with the ammeter between battery negative and the load side of the switch that is turned off, then I think it is a fairly normal situation and your tests between load side of the switch and battery pos, with the switch turned off was, in my view, an invalid test and is just confusing things.

 

With no current flow I think it is fine