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Electrical questions (from a plumber, please be gentle...)


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Its the watts that in effect do the heating so as W = amps X volts any given current at 12V will be half the wattage of the same current at 24 volts.

But how will the switch passing 25 amps in the positive conductor 'know' what voltage that conductor is relative to the negative? The heating effect in the switch will be the current times the voltage drop across the contacts, and nothing to do with the system voltage.

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How does 250A at 24V differ from 250A at any other voltage?

Let's take it to an extreme. If it was operating at 4kV what do you think would happen if you tried to disconnect while a large current was flowing? A big fat arc would occur across the contacts, no?

 

So the spec is stating that the switch will be happy switching up to 250A at 24V. It probably wouldn't be happy with the same current at 48V. We don't know what current it would be happy with at 48V because they don't tell us (quite probably because they've not tested it).

 

I still don't know why you quoted my post which pointed out their two typos.

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Thanks for all the replies folks. I think the ones I've just bought could well be going back for a refund.

 

I like the look of Tony's Lucas ones very much but, for where mine are, I really need a removable key

so I think the SH24 that Andy B suggested looks promising.

 

I'll keep you posted.

 

Thanks again.

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After further searching, these look just like the SH24 but at half the price:

 

https://www.amazon.co.uk/Battery-isolator-switch-MarkUK-removeable/dp/B01CMK5PYG/ref=sr_1_10?ie=UTF8&qid=1487535846&sr=8-10&keywords=battery+isolator

 

Being 'fiscally aware', has anyone tried them please?

 

Copies of the Lucas ones are also on sale, and they look so much like the Lucas ones that I wonder if the Lucas ones are in fact simply rebranded

cheapo ones that will make a fat profit for the seller. I'm getting muddled now....

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But how will the switch passing 25 amps in the positive conductor 'know' what voltage that conductor is relative to the negative? The heating effect in the switch will be the current times the voltage drop across the contacts, and nothing to do with the system voltage.

 

 

They quote it in amps and volts because that is what more people sort of understand. They really are using a more easily understood longhand for watts. If you had 6v of voltdrop anywhere in a nominal 12V circuit then the watts would be half that if there were no voltdrop so would be well within the switch rating.

 

You are ignoring any voltdrop through the parts of the switch that do not actually make contact like the typical "bar" that moves to join the two contact stud ends or even the studs themselves. From my observations of stripped switches that have failed those bars are much smaller than one would expect so they heat up - especially where the cross sectional area has been reduced by drilling a hole through the middle to accept what is often a thermoplastic insulating bush for the operating mechanism. Sometimes the CSA has been reduces by over half.

After further searching, these look just like the SH24 but at half the price:

 

https://www.amazon.co.uk/Battery-isolator-switch-MarkUK-removeable/dp/B01CMK5PYG/ref=sr_1_10?ie=UTF8&qid=1487535846&sr=8-10&keywords=battery+isolator

 

Being 'fiscally aware', has anyone tried them please?

 

Copies of the Lucas ones are also on sale, and they look so much like the Lucas ones that I wonder if the Lucas ones are in fact simply rebranded

cheapo ones that will make a fat profit for the seller. I'm getting muddled now....

 

That is my fear but without stripping one and testing the components we have no way of knowing. I would find it hard on an assembles switch to differentiate between thermoplastic and thermosetting plastic parts. It is much faster/easier to make thermoplastic parts I would have thought but they will soften with heat. II also wonder if similar switches by Durite are any different.

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I looked on http://www.rjsmarine.co.uk/They have an excellent range of Blue Sea switches and breakers. The specs are detailed and show load capacity and fault capacity etc but all there circuit breakers are 'out of stock'. I emailed them to query this. Either they are not selling them or their website has thrown a wobbly. They have battery isolators which can handle 750 amps for one minute and can be switched 'on load' at 300 amps. This suggests a very robust construction.

 

http://www.rjsmarine.co.uk/6005200-m-series-mini-on-off-battery-switch-with-key-blackis well specced and spare keys are available so you could leave one with a marina.

 

The owner, Richard, replied very quickly. The website is new and he hasn't entered all the stock yet. Just email or phone if you need anything.

 

I don't know this company but their name came up when I searched for Blue Sea. They are authorised UK retailers.

Edited by mross
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It is much faster/easier to make thermoplastic parts I would have thought but they will soften with heat. II also wonder if similar switches by Durite are any different.

Soldering iron will tell you that - 'try on an inconspicuous area etc'... ;-)

 

IF it is then I'd just make sure the cables are well supported and not pulling on the terminals.

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After further searching, these look just like the SH24 but at half the price:

 

https://www.amazon.co.uk/Battery-isolator-switch-MarkUK-removeable/dp/B01CMK5PYG/ref=sr_1_10?ie=UTF8&qid=1487535846&sr=8-10&keywords=battery+isolator

 

Being 'fiscally aware', has anyone tried them please?

 

Copies of the Lucas ones are also on sale, and they look so much like the Lucas ones that I wonder if the Lucas ones are in fact simply rebranded

cheapo ones that will make a fat profit for the seller. I'm getting muddled now....

Look decent enough to me, ISTR that type got some good feedback on one of the off-road forums.

 

The red key ones are rather poor by design as they rely on fairly weak contact pressure.

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Thanks as always folks. I've now fitted the two I bought last week just to get us going again but I'll have a good look at

the alternatives for next time.

 

I drilled out the rivets on our faulty ones last night and found the engine battery one wasn't too bad, but the

contacts were tarnished so not making good contact. The domestic one was really badly tarnished and the

contacts were well pitted too.

 

No wonder the electrics were playing up, and a repair of either would be impossible. They're certainly crap!

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What is your budget , per switch? I'll do a search. I like to pay about half of the most expensive ones, as a general rule. £20 TO £30, that sort of range. And ones specced with normal load and short term load, especially for the starter battery.

Edited by mross
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Thanks as always folks. I've now fitted the two I bought last week just to get us going again but I'll have a good look at

the alternatives for next time.

 

I drilled out the rivets on our faulty ones last night and found the engine battery one wasn't too bad, but the

contacts were tarnished so not making good contact. The domestic one was really badly tarnished and the

contacts were well pitted too.

 

No wonder the electrics were playing up, and a repair of either would be impossible. They're certainly crap!

Have used those Metal ones with the removable Key,as you say,the Contacts do get Tarnished

 

Drilled out the rivets and cleaned the Contacts up but the problem returned after a few months.Proper D.C. switches should be designed so that the Contacts are Swiped each time they are actuated

 

Came back to using a Rotary 3 position Switch so that it will self clean to some degree as the moving contact will remove Tarnish in use

 

CT

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