Understanding “Reverse Polarity”


Reverse Polarity is nothing to do with your compass pointing in the wrong direction! It is something that can happen when you plug into the Electrical Hook Up (EHU) on a caravan site. It is unusual for it to happen in the UK, it is more likely to happen when touring on the continent. So what is it and how does it affect you?

Reverse polarity is quite easy to understand and once you know about it, it’s fairly easy to correct. Simply, it is the live and neutral wires are the wrong way round on the EHU bollard.

Wow… that was some mistake by the electrician that wired it up wasn’t it?” Well, not quite. To understand how the situation comes about we need to go back in history a bit. When electricity was first started to be distributed and sold on a commercial-scale, all the switches had two poles… a live and a neutral, so when you switched something on, both live and neutral were connected at the same time. Conversely, when you switched something off, both live and neutral were disconnected. Quite simple really and quite safe. So we and the rest of Europe carried on switching both wires. At some point, some bright spark (excuse the pun) said… “Hey, we can save some money here on switches. Lets make them so they only have to switch the live on and off… leave the neutral connected” To make sure we got it right, we coloured our wires Red and Black so everyone knew which was live and which was neutral. Meanwhile in Europe… they said “…Pfft….” and carried on using ‘two pole” switches and their cables stayed black and grey or white and grey or black and white.

We became obsessed slightly and said “what if someone wires it wrong?… we better put another cable in there” so we suddenly gained an ‘earth” cable… coloured green for our protection. The electricians in Europe liked the idea of an earth cable so they had a yellow one… or green depending in which country you were in. Now, the cable manufacturers were up in arms… “can we please agree on a colour for this earth“… no agreement was made, so the manufacturers got smart and made the earth cable stripey… green and yellow stripey to be exact. We saw the new stripey cable and thought “this is good” so we went stripey too, and not to be out done we decided that Brown was the new Red and Blue was the new Black!

Now I may have just re-written history slightly there… but I think you can grasp what went on. So here we are… Brown, Blue and Green & Yellow cables and switches that only operate on the live (Brown) cable, While our continental cousins still use White and Grey and Black… with a Green & Yellow striped earth but importantly switch both the live and the neutral.

So what goes on when you plug your caravan into a bollard?. Well, the current flows down the brown wire to your circuit breaker, then on to your socket and in to your appliance… lets say a toaster. It returns up the Blue wire and back to the EHU bollard. When you switch off your toaster, it turns off the Brown wire and stops the current flowing. If you turn it off at the socket on the wall, it still stops the current flowing down the Brown wire. If you turn it off by flipping the little switch on the circuit breaker, it still stops the current flowing down the brown wire to all the sockets.

Now, if we plug into a continental bollard, that has the live and neutral the wrong way round and we look again at our toaster we see there is a problem. We turn the toaster off and this time the switch is on the neutral or return path, so when we switch it off, the element ‘may’ still have a voltage present on it. If we turn it off at the wall socket, again we switch off the neutral side of the circuit, so the toaster still might have 240 volts going to it. Even if we switch it off at the circuit breaker… it will still might have 240 volts going to it.

Now 99% of the time, this is not a problem…. until that is, someone decides to put their fingers into the toaster for what ever reason…. they can touch a live element. So the toaster doesn’t even need to have a fault with it. It can still be potentially dangerous.

Keep Calm and Carry On

Don’t get too panicky though! Modern caravans are fitted with and RCD (Residual Current Device)… It’s the job of this little device to check the current flowing in on one wire and out on the other. If it detects that more current is flowing in than out because it’s leaking out somewhere – it switches off. The RCD doesn’t really care what colour the wire is that the current flows in and out on, it just wants to see the same on each wire.

Martindale CP201 250V Industrial Check Plug

Now how can we correct it? Well, checking for reversed polarity is easy and simple. All you need is to buy a “socket tester”.  You can get them from various outlets, I have listed a few links at the bottom of the page. They are simple to use and will usually have three lights on the front. Just look at the lights that are lit and they will have a label on the front telling you what each light means. If it indicates anything that is not “normal”, switch off your van’s master switch and unplug from the bollard. If necessary, consult the site warden before you do anything else. I prefer to use a Martindale CP201 tester, so I can check the bollard before plugging my van in. If on the continent and you use one of the continental adaptor leads, you can check that before you plug your van in. I’ll assume though you are using one that plugs into the normal 13 amp household socket in your van.

Now, if it indicates reverse polarity, you must switch off and unplug from the bollard. Now we can correct a reverse polarity bollard. The best way is to have a short lead, like your 16 Amp EHU lead with the blue plug and socket. This one only needs to be about a foot (30cm) long. The plug is wired up in the correct way… brown to live, blue to neutral and green/yellow to earth. The socket at the other end however needs wiring up the wrong way… Brown to Neutral, Blue to Live and Green/Yellow to Earth. If you are unsure about this, please go to your local qualified electrician, they will be able to make it for you for a small charge. It’s better to pay to have it done correctly than attempt it yourself and get it wrong. It is vitally important that you mark the lead “Reverse Polarity Correction Lead” clearly and make sure it stands out.

A note of caution. There are some of these leads available via on-line sellers at a well-known trading site. The ones I have seen are made with 1.5mm cable and are not really safe to use at 16 Amps. They won’t catch fire or melt, but they might warm up a bit at the full 16 amps.  Any EHU lead that is supplied with a new caravan must be made of 2.5mm cable. I don’t see any reason reverse polarity leads are any different. It is actually illegal to sell these “reverse polarity correction leads” in the UK. It is however, not illegal to make one for your own use.

If you are unsure about any aspect of reverse polarity and using a “reverse polarity correction lead” Ask a qualified electrician. If you have any doubts about your ability to make one… don’t… get your local qualified electrician to make it.

Testers can be bought from these and other places: Maplin  |  Amazon  |  PASS  |

 
Post updated June 2012

Copyright © 2011 – 2015 Simon P Barlow – All rights reserved

13 thoughts on “Understanding “Reverse Polarity””

  1. Very good informative post Simon, most interesting, I would think many would not give this a thought when plugging in abroad untill now!

    Kind regards

    Colin

    • Hi Simon,
      I live abroad and my caravan has 2-pin continental earthed sockets internally and an external CEE plug for the mains supply. Is reverse polarity an issue for me?

      The caravan is equipped with a two pole 13 A circuit breaker. Is an earth leakage circuit breaker a desirable addition to the mains electrical system?

      • Hi
        If your caravan is fitted with 2-pin ‘continental’ sockets and two pole circuit breakers, then it’s probably wired to European standards. Caravan’s and motorhomes manufactured in Europe are wired the same way and reverse polarity is not an issue for them as both conductors are switched, only UK manufactured caravan and motorhomes switch one conductor – live, therefore reverse polarity poses more of an issue.

        An ELCB would give an additional layer of protection.

        S

  2. Thanks Simon,
    I can stop worrying about reversed polarity. I’ll get an ELCB fitted too.

    We are planning a caravan holiday in the UK next year. Is there anything I might need? I was thinking about electrical adaptors. Do UK sites use the CEE socket on the electrical posts or the UK 13 Amp socket?

    My wife was wondering if there is a site handbook avilable, either in book form or on the internet?

    Lots of questions there. Thanks for the wiring diagram for 13 pin towing plugs We have just returned from a 2 week holiday touring in Southern Norway. The towing electrics worked like a dream, and there was no worry about the caravan fridge draining the car battery when the combo was parked.

    • Hi Andrew
      The CEE 16A plug is the only socket used for caravan hook up in the UK, so you won’t need any adaptors.

      To research sites, the best way would be via the two main UK clubs….
      http://www.caravanclub.co.uk/
      http://www.campingandcaravanningclub.co.uk/

      You can find all the details about the sites and find things to do around the local areas. SOme of the sites listed in the two links require you to be a member of either club, the mains sites are open to non members. Each individual site web page will carry this info.

      Glad the wiring worked out 🙂

      S

  3. hi
    we will be using our motorhome for 10 nights off grid and planned to hook into a friends new Honda generator that they power their caravan with but when we tried it using our normal hook up lead our control panel said “mains polarity reversed” any ideas? daren’t use it now.

    • Hi Carol
      Generators don’t have the earth tied to the neutral line, as it is in your home or on a caravan site EHU, this has the effect of causing the reversed polarity light to sometimes come on on some caravans and motorhomes. If the generator is fairly new and been serviced regularly there should be no problems in using it with your motorhome.

      Simon

  4. We’re about to take delivery of a Challenger motorhome here in Spain, do I have to worry about reverse polarity when using our motorhome in the UK or Europe?

    • Hi Glenn
      Honestly I don’t actually worry too much about reverse polarity. The UK wiring standard only requires switching on the live circuit at sockets, where as in Europe they switch both live and neutral. As long as you understand this, then reverse polarity becomes less of a worry. As the motorhome is protected by a ELCB that will switch off both live and neutral in the event of an earth fault you are protected. The owners of continental motorhomes and caravans don’t seem to consider reverse polarity as an issue. For piece of mind, it is fairly easy to make up a reverse polarity lead.
      S

  5. A few people say that some sites in Spain are not correctly earthed, or don’t have an earth. I don’t know if this is true or not. So what happens then if the supply doesn’t have an earth?

    If there’s a fault an rcd will trip even without an earth wire back to the bollard, if there is another route to earth, but if a motorhome or caravan isn’t grounded, because of the insulating tyres, and/or steadies on plastic blocks, then if you touch a “live” appliance or piece of van metalwork that’s live then you’ll become live too, but no harm will be done unless you touch ground and become earthed, in which case the red should trip. But not a very satisfactory state of affairs.

    • Hi
      Most modern caravans and motorhomes are protected by RCD’s. RCD’s work, by detecting the current being supplied on the live feed and comparing it to the current leaving on the neutral cable. If you touch a live component and earth it, there will be a slight imbalance between the current on the live side and the current on the neutral side (some of it is leaking through you to earth) then the RCD will switch off. So in reality you don’t actually need an earth cable as long as the supply to your caravan or motorhome is earth bonded at the substation to provide an earth return path. By having an earth to your caravan or motorhome gives you an extra level of protection.

      All that said, I do check for a functional earth with my plug in tester each time we hook up to a bollard and I would always think carefully about whether I really needed power using an un-earthed supply, and I’d always report it as sometimes the site might not be aware of it.

      I think the standard figures for RCD’s are around 10 mA difference in current between the two conductors and a trip time of 60 mS – but don’t quote me on this I’m doing it from memory.

  6. Cheers, Simon.

  7. GOLDEN!

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