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Polarity Testing and Change-over Cables Author = Patrick_Phillips

What is polarity and why does it matter?
Direct Current (DC) is electricity travelling from the positive terminal to the negative.
In Alternating Current (AC) the polarity is changed rapidly and continuously. In Europe this happens 50 times per second. In the US it is a little faster at 60 cycles.
On the face of it, if the polarity is changing, it does not matter which way round AC is connected. However, somewhere between the power station and your supply (usually at the nearest sub-station) the Neutral is taken to ground (earthed).

Given this fact, touching a live wire and having any connection to the ground will result in a potentially fatal shock. However, this will not be true of the neutral (earthed) wire.

For this reason, amongst other less drastic reasons, it is important to ensure that when you switch a circuit off, it is the live side (normally the brown wire) that is switched, not the neutral (normally the blue wire). Reversed polarity will make your van neutral wire the un-switched live circuit. The result of this is that when you switch off a circuit it, the wiring to the equipment will be live all the way back to the switch!
Most domestic switches only cut the connection in one wire.
There are a few pieces of equipment that are adversely affected by reverse polarity - the most common, nowadays, being audio equipment.

Why does this matter so much to motor-homers?
In a building, the polarity of the supply will have been correctly set by the installer and generally will not be changed other than by the miss-wiring of a plug top.
Connecting to a campsite hook-up is much more hit-and-miss. This is especially true outside the UK. In some countries, like France, the approved procedure is to use switches that disconnect both live and neutral wires (double pole) and this allows them to use reversible plugs. Having said that, the constant drive to cut costs has lead to the use of cheaper single wire switches (single pole).
In the UK and many other parts of the world, outdoor connections are made using 16-amp industrial round-pin connectors that are "keyed" by a notch and slot system that prevents them from being wrongly connected.

Photographs from
However, many other countries, especially in Europe still use the reversible "continental" plugs and sockets.

This can make an already uncertain polarity completely random!

How do you test for polarity?
The best method is to carry a polarity tester. These are inexpensive and easily sourced.
Even cheap ones, from £3 to £10, will tell you more than just whether the wires are crossed. They will tell you if there is an earth connection but not whether it is good enough. It will also tell you if the earth wire is live (and it can happen)!

The simplest way of using them is to plug them in within your motor-home and look at it whenever you make a hook-up. Some would advise keeping it permanently plugged in as some campsites can change polarity after a power cut! Really. For this reason, it is a good idea to have your polarity tester permanently plugged in and check it after any power cut has been restored - especially in Spain...
The best practice is to test the supply before you connect to the hook-up as this means you know what you are dealing with before you connect it to your van at all.
Carry a short lead with a UK rubberised socket and a 16-amp plug on the other. Connect your normal hook-up cable to this before connecting the cable to the van.

How to change polarity?
If you make up a short cable with a 16-amp socket on one end and a 16-amp plug on the other but with the brown and blue wires crossed between the ends, you will have a simple means to change the polarity any time you find it incorrect. You won't be able to buy a ready made one off-the-shelf as this would mean a retailer selling an incorrectly wired plug. Make sure you lable it clearly so that anyone picking it up will know what it is.

Note that although the convention of brown = live and blue = neutral has been in force for many years, you may well find the older red = live and black = neutral still being used. In fact, the proper colour coding should not be taken for granted ever. US manufacturers can be very lax about it and It has been known for new modern equipment using green wiring for live!
Of course, if the campsite hook-up socket forces you to use a continental plug, just plug it in the other way round!

What about RCDs?
An RCD is a useful safety device to use in any situation where you are using electrics outdoors but the do not solve the problem of reverse polarity.
An RCD measures the current at the live wire and again at the neutral one. If it sees a difference of more than about 30mA, it assumes that there is a fault in the circuit and switches the...
Well, some RCDs have two switches and will disconnect the live and neutral wires but others only switch the live wire. If you have a reversed polarity, then it will leave the circuit live by just disconnecting the neutral - actually making the risk of serious shock greater!
An older form of "RCD" is the "Earth Leakage Trip". This device looks for any voltage between the live wire and the local earth wire. These devices are very effective at preventing major shocks but are not usually found in outdoor domestic situations. Yet again, if the live wire is actually the neutral, such a trip will not work. Actually , they would trip at power up.

Many campsites suffer from "brown-outs". These are where you do not loose power altogether but the voltage dips well below the nominal 220v you expect. Some sites may often supply as little as 170v. This won't affect the lights but won't heat the water much. A voltage meter is another very useful thing to keep in a motorhome and may well help you understand why some piece of equipment like the water heater or fridge seems to be acting below par. Whether you can get the operators of the site to do anything about it is quite another matter...

Patrick Phillips
07 March 2009
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