Polarity Reversal; why is this an issue?

[JohnI.Stephen]

I apologie for exposingmy doubtless ignornace but I have always understood that, with alternating current (AC), the polarity swaps from positive to negative approx 60 times per second. Therefore, it makes no difference whether you swap the positive and negative wires as they will alternate - and +.

However with DC (Direct Cirrent) you MUST ensre that the polarity is correct.

Please explain / advise.

 

[rob_jax]

Hi John,

Easy one - you need to think of a harder question.

Polarity doesn't matter for the correct operation of an y mains powered appliance.

Where it does matter is because European appliances, distribution boxes and everything to do with mains electricity switch both the live and the neutral - so when you flip the switch both lines are cut - and consequently the sockets are randomly wired - it just doesn't matter.

The UK electrical system only cuts the live wire, so with the power off, the neutral wire in your kettle is still connected to the power station.

If you use a UK appliance in Europe, then 50% of the time the neutral wire will be live. Then when you flip the switch on the kettle, live power still stills runs into the kettle until it meets the open switch on it's way out.

Still not a problem, until you either have a problem with the insulation or decide to have a poke around inside with your fingers.

We have an Eriba (all power lines double switched) with European appliances - so we don't care.

And generally speaking, even if you don't correct for the polarity, it doesn't matter too much with a UK van in France - as long as you always unplug after use.

It would be so much better if the UK used double switching switches - so very much safer - and they only cost a few pence extra. Do you think every socket in every UK house is wired correctly?

And don't get me started on (the totally cheap-skate reason) why the UK has a fuse in the plug.

Robert

 

[JohnI.Stephen]

Robert; I suspected you would rspond first (being permanently online (or so it seems !!))

So, in a nutshell; all this makes no odds unless the appliance is faulty or you decide to 'fiddle about'. If I simply plug in my mobile phone adapter or that to my PC (my only real use of AC power on site), it seems a waste of time and money to worry about polarity......

Have I understood correctly?

John

 

[rob_jax]

Hi John,

You also need to consider that UK caravan circuit breakers only switch the live wire - whereas European caravans switch both.

So if the wiring in the van is faulty - or you turn the MCB off and expect the wiring to be isolated - then reversed polarity could well be a problem.

However your PC power supply and phone charger are pan-European and were designed to work safely on random polarity.

Robert

 

[JohnI.Stephen]

Hi John,

You also need to consider that UK caravan circuit breakers only switch the live wire - whereas European caravans switch both.

So if the wiring in the van is faulty - or you turn the MCB off and expect the wiring to be isolated - then reversed polarity could well be a problem.

However your PC power supply and phone charger are pan-European and were designed to work safely on random polarity.

Robert

Fair enough Robert; I saw the articles in this month's PC so may proceed if I have

 

[Lutz]

Robert, not quite. Distribution boxes in European caravans do have twin circuit breakers on both live and neutral but most European appliances are generally only switched on one of the two sides, just as in the UK. Few are double-switched. I haven't found any small appliances around my house that are.

 

[rob_jax]

Hi Lutz,

Thanks for the update - the caravan appliances we bought in Holland - Coffee machine (1 kW) is dual switched - but the Remoska (500 w) is single switched.

Robert

 

[vernon]

My new Avondale blew the site trip, due to the neutral (blue) wire being loose in the external socket (the caravan had an electricians test cert!), with reversed polarity this loose wire was live as soon as I plugged into the site supply.

Why do we have to have to put up with such bad (and dangerous) workmanship, doesn't the Btitish Caravan Industry ever learn? The Motor Industry didn't and look what happened.

Avondale took no notice of my email about this fault, but I made a point of checking all the other sockets, and made a Reverse Polarity Adaptor Lead.

 

[GARY_5129034]

a qualified electrician in formed me whilst in France that if a UK caravan is left hooked up with incorrect polarity, that the diodes in the charging unit would fry within days. I don't know if he is right or wrong.

 

[rob_jax]

Hi Gary,

Your electrician friend is wrong, nothing to do with his electrical qualification - electronics is just a different area of expertise.

The battery charging unit has 4 diodes in the part that converts the stepped down AC into 12 Volt DC. 50 times a second, the AC current flows one way, 2 of the diodes "block" and 2 allow flow. Then as the AC flow reverses, the first 2 diodes allow flow and the second pair "block". This lets the battery charger extract power on both cycles and is called full wave rectification.

So no - some of the diodes are always the "wrong way round" 50 times a second - and they should last for many tens of years.

Robert

 

[rob_jax]

Hi Vernon,

The test certificate probably proved that the wires were all correctly configured, and that there were no earthing problems, etc. The test probably wasn't intended to confirm that all the wires were tightly held in place - and maybe it should.

If by external socket you mean the ISO power input - where you plug the hook up lead into the caravan - then actually with reverse polarity this loose wire would be safe. The socket would be live, but when the wire disconnected itself it would be OK.

Perversely, if you were connected with normal polarity - then the end of the neutral wire would then be live.

Nothing to get happy about, if it had been the live wire that was loose, then the situation would be reversed as well - Dangerous loose live wire with reversed polarity and safe wire with normal polarity.

Robert