Mains polarity - what is this?

[Gilitygirl]

I plugged my van into the mains last night ready to cool the fridge for a weekend away.

I only noticed the mains polarity light after I found the plugs/ fridge not working - about eight hours later - d'oh!

Can anyone explain to a complete technophobe what mains polarity is? And has it done my electrics any damage by leaving it like that dfor so long?

Also why did it rectify itself just be removing the lead and shoving it back in again?

Thanks

 

[rob_jax]

Beej,

If it sorted itself out just by disconnecting and plugging in, then that's a symptom of a dirty or corroded mains connection (when the caravan is disconnected clean the EHU plug and socket) because the first connection spiked that electronics module - or worse but let's just see if a clean will solve the issue.

I take it you're familiar with TV remote controls or clocks not working if you insert the battery upside down - well that's a true case of polarity reversal. Batteries are a source of DC power - and the appliance won't work if the polarity is reversed.

Mains power is AC - and this means that the polarity is naturally reverses 50 times a second. Indeed with AC power the appliance doesn't care how the live and neutral are connected - If you have a mains powered razor - you notice it works no matter which way round you plug it in.

And so polarity reversal is a misnomer for AC power - but I guess someone thought it would be "understandable" if that term was used (as if mains AC power was a battery) - and all it's done is confuse people as to where the problem actually is...

"Polarity Reversal" on AC mains supplies actually refers to which wire carries the power and which wire returns to the power station. Something that you may know as "Live" and "Neutral"

In the past, UK caravans (and houses) were wired with single pole switches on the live wire in the distribution board - European caravans (and houses) have always? been supplied with switches that cut both the live and neutral wires.

So no one in Europe worries about this - if the switch is off - power is cut to both wires and their system is safe. And in the UK - if the switch is off - power is cut but only to the "Live" wire - and this means that it's not foolproof.

A possible problem arises if you take your caravan to Europe - because their sockets are randomly wired for live and neutral - this doesn't matter with dual switching.

But if your caravan only has single pole switching - then there is a 50% chance that (in Europe or if someone didn't wire you house up correctly) the live and neutral will be swapped - and then your UK switch will cut the power on the way out of the appliance and not before it enters.

This has no practical consequence - other than if the appliance develops a fault, you switch it off at the mains, but you fail to up-plug it, and then you start fiddling around inside (never a good idea anyway) - and then you could find out the hard way that it's still live internally...

Your caravan appears to have a "polarity reversal" protection circuit - and the money spent on that would have been better spent on dual wire circuit breakers - rather than a piece of electronics that can easily get spiked and cut the power...

Instead of "polarity reversal" let's call it "cheapskate single pole switching that's not intrinsically safe"

Robert

 

[Rubix]

Beej,

When you put a battery in a piece of equipment, you can put it one of two ways. In your domestic mains wiring you have three wires, live, neutral and earth. Think of the live and neutral as being like the + and - of a battery.

So the live (brown wires usually) and neutral (blue normally) should be the correct way around (even though the mains is 230v ac - alternating current). It would appear that you put the lead in the wrong way around, but if it were the blue plugs usually used on caravans clearly this is impossible as there is only one way around.

The polarity light coming on should have detected that the live and nuetral had been reversed. How it corrected itself is unclear, but this is what it indicated.

I am not an electrician so am not entirely sure the reason for the importance of polarity.

Hope this helps

 

[Lutz]

On the Continent, only the MCB's are dual pole. In general, switches for electrical equipment are single pole, same as in the UK. Most relatively low current equipment, such as TV's, stereos, computers, lighting, etc., are not earthed, either but only have flat 'Euro' two pin plugs which can be inserted either way into the socket.

 

[BurtonRob]

The advice given is good stuff. Follow Roberts advice and you should be fine. If you don't travel to the continent this will probably never you trouble again. If you are intending to caravan abroad you may need a simple home made set up with the polarity switched. If you are unsure get some qualified help. To check polarity abroad you will need a plug in tester. The reason I'm posting this is to say they are much cheaper at Maplins than at most caravan shops.

Rob

 

[trucker]

So guys, I go to France with my polarity tester plug in and find there is reversed polarity on the site im on what next??

 

[ProfJohnL]

Hello Trucker,

as Rob says, all it requires is a short lead to be made up that plugs between the electric hook up wire and the caravan. this is wired so the live and neutral are crossed over, and that the presents the correct polarity to the caravan.

As Lutz points out, the continental practice overcomes the inherent dangers of reversed polarity by using double pole switches and safety devices. These disconnect both live and neutral simultaneously and thus fully isolates the appliance.

All new caravans are wired with dual pole isolators, so there is no real danger in the caravan, but for added peace of mind a polarity reversing lead can eb a simple but effective answer.

 

[gary]

Picking up on why it's called 'reversed polarity', the people that supply the power refer to 'live' as 'phase' and 'neutral' as 'earth' which in fact it is.

That fact makes the above references to batteries so much more akin to the truth!

 

[Gilitygirl]

Thanks for all your replies folks. Still a bit bamboozled by the techology (I'm a complete girl when it comes to science!!!)

Still bemused by the fact that it was at home in the UK that it happened - and that just be removing the electrics and shoving back in fixed the problem.

Probably a bit like computers where turning off and switching on again fixes a lot of things, eh!

Thanksfully I've just had a wonderful weekend away, the battery had been charged enough and nothing went 'bang' - so fingers crossed no permanent damage!!

 

[rob_jax]

Beej,

Most caravans that I'm familiar with don't have the built-in "polarity" device that your caravan has.

And whether the "polarity" tester is built in or a plug-in device - it's going to need some (very simple) electronics to compare "live" with "neutral" and "earth".

And this type of system can be false triggered - usually by corrosion on a connector causing a temporary high resistance or a "spark" - and that'll set or trip the warning device.

Plugging back in - when the first insertion has already moved the corrosion or dirt - and there's no problem...

Hence the suggestion to make sure all the connectors are clean and corrosion free...

Robert