bluedragon
Active member
My current set-up has battery isolators on the negative posts, not the positive terminals. It may not be conventional, but is there anything wrong with this? Is there in fact any good reason for it?
Battery isolation on -ve side?
- Thread starter bluedragon
- Start date
fireball
New member
Sounds french to me!!
Modern electronics almost all assume a negative earth so it would make sense to have an isolation switch on the positive ....
Modern electronics almost all assume a negative earth so it would make sense to have an isolation switch on the positive ....
sailorman
Well-known member
doesnt matter as long as the circuit is broken.
my boat was wired that way i have also added a change-over switch to the pos side to give more flexibility. the 2 battery banks were totally seperate exept when charging when a relay "pulls-in" the domestic bank.
by adding the change-over switch i can now access the engine battery as req. or infact more importantly use the domestic bank to start the engine.
the previous set-up req. a jump lead if the engine battery ever failed.
my boat was wired that way i have also added a change-over switch to the pos side to give more flexibility. the 2 battery banks were totally seperate exept when charging when a relay "pulls-in" the domestic bank.
by adding the change-over switch i can now access the engine battery as req. or infact more importantly use the domestic bank to start the engine.
the previous set-up req. a jump lead if the engine battery ever failed.
Phoenix of Hamble
Active member
Thats not strictly true..... if the positive remains connected, and an internal fault in a bit of kit causes the casing to go live, then you could become a human earth........ admittedly a lowish risk, but nonethless a risk.... breakers should really be in the +ve path....
steverow
New member
Yep quite right.
If the batteries get wet then this will set up a path to ground bringing the circuit at least some way back into play again.
Must be in the positive line IMHO.
Dunno how old your boat is, but it isnt positive ground is it,? or maybe has been
in an earlier engine incarnation. It would be quite correct to switch the negative in a Pos ground boat. A lot of engines were in the sixties and early seventies, especially thse marinised from road vehicles.
Steve.
If the batteries get wet then this will set up a path to ground bringing the circuit at least some way back into play again.
Must be in the positive line IMHO.
Dunno how old your boat is, but it isnt positive ground is it,? or maybe has been
in an earlier engine incarnation. It would be quite correct to switch the negative in a Pos ground boat. A lot of engines were in the sixties and early seventies, especially thse marinised from road vehicles.
Steve.
sailorman
Well-known member
i think were talking 12v here. not medium power ie 240 v.
you can still have neg ground to equipment but with a 12v system if the circuit is broken "its broken" by which ever wire
you can still have neg ground to equipment but with a 12v system if the circuit is broken "its broken" by which ever wire
Phoenix of Hamble
Active member
Yep.... but a few amps at 12V can still burn you pretty badly........ and is a serious fire risk.....
sailorman
Well-known member
cant "go live" with only one wire connected
Phoenix of Hamble
Active member
No, you are technically correct..... lets re-phrase that.... to be technically accurate, it will sit at a high potential difference to ground, and when something or someone closes the circuit, it will therefore create a current flow along that path......with voltage being developed across the earth path in proportion to the relative resistance of the earth path and the faulty +ve path.....
that however makes no difference.... good engineering practice does not allow this kind of risk when it is unneccessary........
that however makes no difference.... good engineering practice does not allow this kind of risk when it is unneccessary........
fireball
New member
12v dangers?
Have you ever held both terminals of a 12v battery?
I wouldn't deliberately short out a battery, but if the switch is off there is no connection - so shorting out post switch makes no difference, once the switch is turned on it will fizzle or bang and (hopefully) blow a fuse.... no difference whether the switch is on the pos or neg side.
It still sounds french to me though - could well be to do with the engine - as suggested...
Have you ever held both terminals of a 12v battery?
I wouldn't deliberately short out a battery, but if the switch is off there is no connection - so shorting out post switch makes no difference, once the switch is turned on it will fizzle or bang and (hopefully) blow a fuse.... no difference whether the switch is on the pos or neg side.
It still sounds french to me though - could well be to do with the engine - as suggested...
sailorman
Well-known member
so lets say that the Pos is disconnected then , the same thing could happen with the Neg still connected as most boats only switch the Pos. side.
pvb
Well-known member
Absolutely no problem at all...
If the isolator breaks the connection between the battery post and the rest of the boat's wiring, it isolates the battery. It therefore doesn't matter whether the negative connection is disconnected, or whether the positive connection is disconnected - the battery is isolated and can do no harm. It's conventional to isolate at the positive, but some boatbuilders chose to isolate at the negative terminal. Don't worry about it! It's not a fire risk. It won't result in current flowing if people touch things. It's OK.
If the isolator breaks the connection between the battery post and the rest of the boat's wiring, it isolates the battery. It therefore doesn't matter whether the negative connection is disconnected, or whether the positive connection is disconnected - the battery is isolated and can do no harm. It's conventional to isolate at the positive, but some boatbuilders chose to isolate at the negative terminal. Don't worry about it! It's not a fire risk. It won't result in current flowing if people touch things. It's OK.
Krusty
New member
See 'Boatowner's Mechanical and Electrical Manual' by Nigel Calder, (which should be required reading for any owner) --- Quote; Marine electrical systems should always use a grounded negative with no switches, fuses or breakers on the ground side. Unquote. The reason is that any stray current leakage from damp or defective equipment needs an unimpeded path back to the battery negative; otherwise it will find its way back by other routes, with the risk of :--Quote: rampant corrosion; Unquote. That could well include skin fittings, and sink your boat.
Phoenix of Hamble
Active member
Re: 12v dangers?
"Have you ever held both terminals of a 12v battery?"
A fair few times.... I have a masters degree in electrical engineering.....
have you ever tried holding one terminal, and brushing the other with the smallest fraction of a finger tip... it can spark... and when it does, it hurts.... even better, lets get realistic... try using your tongue... wet like a damp crew member could be..... now I can assure you (sadly, from personal experience!) that REALLY hurts.....
"Have you ever held both terminals of a 12v battery?"
A fair few times.... I have a masters degree in electrical engineering.....
have you ever tried holding one terminal, and brushing the other with the smallest fraction of a finger tip... it can spark... and when it does, it hurts.... even better, lets get realistic... try using your tongue... wet like a damp crew member could be..... now I can assure you (sadly, from personal experience!) that REALLY hurts.....
Phoenix of Hamble
Active member
Re: Absolutely no problem at all...
Paul my main concern would be that in reality there is very often a leak on the ground side... for example, a bilge pump wired in around the breaker, or a radio wired in around the breaker (perhaps to avoid tuning memory being lost)... all of which could create a route....
Paul my main concern would be that in reality there is very often a leak on the ground side... for example, a bilge pump wired in around the breaker, or a radio wired in around the breaker (perhaps to avoid tuning memory being lost)... all of which could create a route....
pvb
Well-known member
Do you believe everything you read in the papers, too?
Nigel Calder is fond of making earnest statements, all of which are probably well-intentioned, and many of which are true. But he's not always right.
Nigel Calder is fond of making earnest statements, all of which are probably well-intentioned, and many of which are true. But he's not always right.
fireball
New member
Re: 12v dangers?
9v battery on your tounge is one thing ... 12v on your tounge?? Now thats being daft ... sorry, no masters in electrical engineering needed to work that one out! /forums/images/graemlins/tongue.gif
Done the 12v through the fingers whilst standing in salt water in the bilge - gave a shock yes ... but if the battery negative was disconnected - and assuming it wasn't always connected to thru-hull connections - I wouldn't've got that shock... cos there is no route back...
Any spark is a fire risk if close to flamable material.
9v battery on your tounge is one thing ... 12v on your tounge?? Now thats being daft ... sorry, no masters in electrical engineering needed to work that one out! /forums/images/graemlins/tongue.gif
Done the 12v through the fingers whilst standing in salt water in the bilge - gave a shock yes ... but if the battery negative was disconnected - and assuming it wasn't always connected to thru-hull connections - I wouldn't've got that shock... cos there is no route back...
Any spark is a fire risk if close to flamable material.
steverow
New member
Re: 12v dangers?
Conventional current flow dictates that the switch should be placed as close as possible to the "Exit" of current from the battery, in a negative gound circuit that is the positive terminal of the battery.
The fact that electrons dont really flow this way at all, is irrelevant in this situation, because the load is placed essentially to make conventional current flow apply.
Check your basic physics.
Steve.
Conventional current flow dictates that the switch should be placed as close as possible to the "Exit" of current from the battery, in a negative gound circuit that is the positive terminal of the battery.
The fact that electrons dont really flow this way at all, is irrelevant in this situation, because the load is placed essentially to make conventional current flow apply.
Check your basic physics.
Steve.
steverow
New member
Agree totally.
Normal safe electrical practice is to have SW in Positive Feed as close as possible electrically to battery terminal.
Steve.
Normal safe electrical practice is to have SW in Positive Feed as close as possible electrically to battery terminal.
Steve.
Ships_Cat
New member
Re: Do you believe everything you read in the papers, too?
Nigel Calder is fond of making earnest statements, all of which are probably well-intentioned, and many of which are true. But he's not always right.
Agreed. While he says the negative should always be grounded (and the outdated ABYC says the same) many boats are not and on small (ie not ships) metal boats it should never be. My opinion is that the negative should not be grounded and the builders I work with prefer the same.
In the case the negative is not grounded it doesn't matter one jot which side the breaker is on.
As an aside, while I believe that Calder gives much really good advice and his books are very good, a boatbuilder recently said to me that one of the greatest bains of his life was Calder. Boat owners insisting that their interpretation of Calder be followed - usually they know so little about the subject that they have totally screwed up what Calder is saying, or what Calder is saying does not apply to their boat (eg the grounded negative bit), or else Calder has slipped up.
John
Nigel Calder is fond of making earnest statements, all of which are probably well-intentioned, and many of which are true. But he's not always right.
Agreed. While he says the negative should always be grounded (and the outdated ABYC says the same) many boats are not and on small (ie not ships) metal boats it should never be. My opinion is that the negative should not be grounded and the builders I work with prefer the same.
In the case the negative is not grounded it doesn't matter one jot which side the breaker is on.
As an aside, while I believe that Calder gives much really good advice and his books are very good, a boatbuilder recently said to me that one of the greatest bains of his life was Calder. Boat owners insisting that their interpretation of Calder be followed - usually they know so little about the subject that they have totally screwed up what Calder is saying, or what Calder is saying does not apply to their boat (eg the grounded negative bit), or else Calder has slipped up.
John
