Reverse polarity points

[spospe]

I thought that Forum members might be interested in a couple of points concerning the much discussed 'Reversed Polarity' and which I do not remember being mentioned previously.

 

1) Our Duetto has an Eberspächer diesel heater and this will not work at all on mains, if the mains is reversed (Live and Neutral swapped). It will still work perfectly well on diesel, but not mains. This is true even though the mains is in effect providing the power via the mains to 12 volt system.

 

2) UK equipment switches just one leg of a mains circuit and not both as do continental systems and this is usually held as an example of cheapskate British engineering. It is in fact a good example of safe practice and overcomes the problem of failing switchgear just switching one leg and leaving the other connected.

 

I have had the experience of a double pole switch failing and switching just the neutral, this has of course left the circuit live, even though it was 'switched off' and not working (it was a Sony television).

 

Using the British system (given that the circuits are properly wired) then there is no problem, a switch either works or it does not and no one is left in any doubt as to whether a circuit is live or not.

 

The British system depends critically on mains circuits being wired correctly with Live and Neutral the right way round, but given that this is what we want, there is no problem at all surely?. it is not a cheapskate system and it avoids shoddy workmanship like you meet all the time abroad where even though sockets are marked, electricians wire them up any old how.

 

Naturally a British van encountering Reversed Polarity, is at a disadvantage and this does explain why we need to take care; but please remember, it is us protecting ourselves from poor continental practice and not us having poor equipment.

 

 

 

 

 

[Brian Kirby]

I don't think either system is intrinsically better or worse than the other, albeit they are different. Properly engineered and installed, both are, IMO, equally safe. Bringing continental, DP switched, appliances to UK is safer than taking ours abroad, but only because it is the fail safe option, not because the standard is intrinsically superior.

 

What interests me is the claim that your Eberspacher unit won't run if mains polarity is reversed.

 

Is it the fan that won't run, or the heater element that won't heat? Being a German company, I would expect Eberspacher installations to be "polarity blind". It seems very odd.

[spospe]

Brian Kirby - 2013-12-02 6:39 PM

 

I don't think either system is intrinsically better or worse than the other, albeit they are different. Properly engineered and installed, both are, IMO, equally safe. Bringing continental, DP switched, appliances to UK is safer than taking ours abroad, but only because it is the fail safe option, not because the standard is intrinsically superior.

 

What interests me is the claim that your Eberspacher unit won't run if mains polarity is reversed.

 

Is it the fan that won't run, or the heater element that won't heat? Being a German company, I would expect Eberspacher installations to be "polarity blind". It seems very odd.

 

1) Properly engineered and installed. Well Sony is generally regarded as a 'proper' company, yet they fitted a double pole switch to my TV which failed in the neutral leg and when switched off left the TV chassis live.

 

I agree that really both systems are OK, but I am fed-up with some whingers 'knocking the British system and thought to redress the balance somewhat.

 

2) The heater was turned on at the thermostat / control unit with the (reversed) mains connected and it started-up on diesel. In the morning (it was a wet night and I did not want to go out investigating), I checked the polarity, found that it was reversed and used my home-made reversed connecting lead to re-reverse the polarity and the heater worked as it should.

[Dave225]

I do not think anyone is knocking the British system, it works fine for us. However, the European nations decided to approach the issue in a slightly different fashion as they felt that having a system where the customer had to fit the correct fuse was very risky. In far too many cases a 13 amp fuse was fitted regardless of the wattage of the device, and yes, it all worked until....... Yes ,we now do not do that but historically we have stuck with the same approach. The Europeans decided the have a central fuse system with trip fuses so allowing individual devices not to require them.

 

It is interesting to note that if a EU outfit is sold in the UK it is fitted with 3 pin sockets as per our system, however these are merely swops for the existing sockets and do not involve any other changes to the electrical system.

[Derek Uzzell]

Spospe has mentioned on the Caravan Chat forum his Duetto’s Eberspacher heating and the effect on it of a reversed-polarity mains power-supply

 

http://www.outandaboutlive.co.uk/forums/Caravans/Caravan-Chat/Reverse-polarity-extension-leads-help-please/32476/#M400471

 

I believe the air-heating system Auto-Sleepers will have installed in his Duetto combines an Eberspacher diesel-fuelled “Airtronic”” D2 unit with a separate Eberspacher “Airlectric” mains unit that’s fitted in the air-trunking ‘downstream’ of the D2. “Airlectric” is described here:

 

http://www.pfjones.co.uk/eberspacher-mains-heating-system-for-airtronic-d2-d4.html

 

This link

 

http://www.auto-sleepers.co.uk/uploads/handbooks/2009/Symbol%202009%20handbook.pdf

 

retrieves the handbook for a 2009 Auto-Sleepers Symbol and includes information on the Airtronic and Airlectric units on Pages 9.12-9.19 (I would have thought it likely that a contemporary Duetto would have the same system.)

 

I haven’t been able to find any indication (in the Auto-Sleepers Symbol handbook or elsewhere) that an Airtronic/Airlectric system might be 230V polarity sensitive, though I note that the Sargent PSU senses polarity and warns if it is reversed (Page 7-19)

 

Although there are 10 references to 230V polarity in the A-S handbook (eg. “OVERSEAS CONNECTION” on Page 7-11), none of these refer to the possibility of reverse-polarity affecting the Eberspacher system.

 

As Brian says (and as I said in the Caravan Chat thread) it would be logical to expect an Eberspacher system to be polarity insensitive, but the air-heating system in spospe’s Duetto is evidently not.

 

Gut-feeling leads me to suspect that the reason for spospe’s heater acting as it does lies in the installation (and is possibly linked to the Sargent system) and is not a normal limitation for an Eberspacher Airtronic/Airlectric combination.

 

There's also the possibility that the way spospe's Eberspacher system behaves is unique to his Duetto. It would be worth knowing if he has explored this with Auto-Sleepers and/or owners of other A-S motorhomes having this Eberspacher arrangement.

[Brambles]

Maybe the heater went into some form of lock out and disconnecting power and reconnecting reset the lock out, and not the reversing of the power conductors.

[Brian Kirby]

spospe - 2013-12-02 7:19 PM..................2) The heater was turned on at the thermostat / control unit with the (reversed) mains connected and it started-up on diesel. In the morning (it was a wet night and I did not want to go out investigating), I checked the polarity, found that it was reversed and used my home-made reversed connecting lead to re-reverse the polarity and the heater worked as it should.

But, was it only this once, Michael? If you have a polartty reversing lead, have you tried firing-up the heater with polarity deliberately reversed, to see if it makes a difference? Jon's suggestion, that for some reason the heater locked out, seems a more feasible explanation, but equally may indicate an as yet unidentified underlying fault.

 

I'm intrigued that you said the heater initially fired up OK, but had turned off by morning, so presumably you'd left it running overnight? If RP were the cause of the failure, it seem odd it would at first run. Might something (return air intake obstructed by bedding, for example) have caused it to overheat and turn it off? Would disconnecting/reconnecting then re-set an over-temperature cut out?

 

Re that switch, how did you discover the chassis was live? :-)

[rupert123]

spospe - 2013-12-02 5:39 PM

 

I thought that Forum members might be interested in a couple of points concerning the much discussed 'Reversed Polarity' and which I do not remember being mentioned previously.

 

1) Our Duetto has an Eberspächer diesel heater and this will not work at all on mains, if the mains is reversed (Live and Neutral swapped). It will still work perfectly well on diesel, but not mains. This is true even though the mains is in effect providing the power via the mains to 12 volt system.

 

2) UK equipment switches just one leg of a mains circuit and not both as do continental systems and this is usually held as an example of cheapskate British engineering. It is in fact a good example of safe practice and overcomes the problem of failing switchgear just switching one leg and leaving the other connected.

 

I have had the experience of a double pole switch failing and switching just the neutral, this has of course left the circuit live, even though it was 'switched off' and not working (it was a Sony television).

 

Using the British system (given that the circuits are properly wired) then there is no problem, a switch either works or it does not and no one is left in any doubt as to whether a circuit is live or not.

 

The British system depends critically on mains circuits being wired correctly with Live and Neutral the right way round, but given that this is what we want, there is no problem at all surely?. it is not a cheapskate system and it avoids shoddy workmanship like you meet all the time abroad where even though sockets are marked, electricians wire them up any old how.

 

Naturally a British van encountering Reversed Polarity, is at a disadvantage and this does explain why we need to take care; but please remember, it is us protecting ourselves from poor continental practice and not us having poor equipment.

 

 

 

 

Mine works perfectly well on reverse polarity so see no reason why others would not. Why is a British van at a disadvantage with reverse polarity, unless you are in the habit of taking things apart without un-plugging them or disconnecting from the mains it makes no difference. I gave up bothering to check years ago and see no reason to start now. Why does this keep on coming up, reverse polarity is not a problem, never has been, never will be.

[spospe]

Derek Uzzell - 2013-12-03 8:11 AM

 

I believe the air-heating system Auto-Sleepers will have installed in his Duetto combines an Eberspacher diesel-fuelled “Airtronic”” D2 unit with a separate Eberspacher “Airlectric” mains unit that’s fitted in the air-trunking ‘downstream’ of the D2. “Airlectric” is described here:

 

I haven’t been able to find any indication (in the Auto-Sleepers Symbol handbook or elsewhere) that an Airtronic/Airlectric system might be 230V polarity sensitive, though I note that the Sargent PSU senses polarity and warns if it is reversed (Page 7-19)

 

As Brian says (and as I said in the Caravan Chat thread) it would be logical to expect an Eberspacher system to be polarity insensitive, but the air-heating system in spospe’s Duetto is evidently not.

 

Gut-feeling leads me to suspect that the reason for spospe’s heater acting as it does lies in the installation (and is possibly linked to the Sargent system) and is not a normal limitation for an Eberspacher Airtronic/Airlectric combination.

 

There's also the possibility that the way spospe's Eberspacher system behaves is unique to his Duetto. It would be worth knowing if he has explored this with Auto-Sleepers and/or owners of other A-S motorhomes having this Eberspacher arrangement.

 

Some answers to the above points are:

 

The Airtronic and Airlectric are installed as described, but the power unit is a "Volta" made by Plug-In Systems, as is the rest of the 12 volt system (no Sargent kit at all).

 

I agree that the polarity sensitivity is probably linked to the installation and I suspect that this is caused by the Eberspacher mains control system comparing the Live and Neutral legs to Earth and throwing its (electronic) hands up in horror and not connecting to 'reversed' mains.

 

I have not spoken to AS about this as I suspect their answer would be, "we make vehicles that are supposed to be connected to a correctly supplied mains system and do not recommend connection to an unsafe (reversed) supply".

 

This somewhat unexpected behaviour of the heater is the only example I have personally experienced of reversed mains causing any operational problems (I have been caravanning / motorhoming since 1977).

[spospe]

Brian Kirby - 2013-12-03 3:12 PM

 

But, was it only this once, Michael? If you have a polartty reversing lead, have you tried firing-up the heater with polarity deliberately reversed, to see if it makes a difference? Jon's suggestion, that for some reason the heater locked out, seems a more feasible explanation, but equally may indicate an as yet unidentified underlying fault.

 

I'm intrigued that you said the heater initially fired up OK, but had turned off by morning, so presumably you'd left it running overnight? If RP were the cause of the failure, it seem odd it would at first run. Might something (return air intake obstructed by bedding, for example) have caused it to overheat and turn it off? Would disconnecting/reconnecting then re-set an over-temperature cut out?

 

Re that switch, how did you discover the chassis was live? :-)

 

Yes I have tried a deliberately reversed lead and the heater did not work on mains.

 

I think you have misunderstood what I wrote (or I misunderstood what I was writing) about the heater firing up. It was switched on and as the van was connected to site mains, was expected to run on mains, but it did not, it started on and ran on, diesel.

 

I tried re starting the heater / switching the supply on and off etc. (which made no difference at all), but did not try my reversed lead until the morning as it was cold, wet and dark at the time. With the reversed lead, the heater worked as it should on mains. The Eberspacher normally automatically runs on mains if present and only runs on diesel if mains is absent (it can be manually switched to run on either, except when polarity is reversed!)

 

I am 69, if I did not as a matter of routine test all electrical circuits before touching them, I would not be 69 ;-)

[Derek Uzzell]

rupert123 - 2013-12-03 4:29 PM

 

...Why does this keep on coming up, reverse polarity is not a problem, never has been, never will be.

 

It keeps coming up because reverse polarity (RP) is mentioned in publications and articles from authoritarian and well-respected sources. For example, Page 14 of this 2008 Caravan Club information leaflet

 

http://www.thomson-caravans.co.uk/advice/diyprojects/pdf/mainselectricalInstallations.pdf

 

and these two magazine articles

 

http://www.venture-caravans.com/pdf/guides/Reversed-Polarity-Guide.pdf

 

http://www.practicalcaravan.com/advice/reverse-polarity-how-cope-tour

 

The risk relates not to a leisure-vehicle failing to "work perfectly", but to a potential danger if an appliance plugged into an electrical socket within the vehicle malfunctions.

 

What your habits are regarding checking 230V hook-up power-supplies is academic. I don't normally check them either (unless they look iffy or don't work) but I'm certainly not going to discourage other people from checking and 'correcting' RP if they encounter it. Even if the impact of RP were so insignificant that it could safely be ignored, checking the state of a 230V hook-up power-supply is good practice in its own right.

[Brian Kirby]

spospe - 2013-12-03 5:56 PM........................I think you have misunderstood what I wrote (or I misunderstood what I was writing) about the heater firing up. It was switched on and as the van was connected to site mains, was expected to run on mains, but it did not, it started on and ran on, diesel.

Ah! My mistake then. I misread. Apologies.

 

.............. The Eberspacher normally automatically runs on mains if present and only runs on diesel if mains is absent (it can be manually switched to run on either, except when polarity is reversed!)

I wonder if this is the clue? Somewhere on the 230V side, there must be a relay (contactor, I suppose, as it's 230V) that detects the presence of mains power. This must be connected to live to energise, and switch off the 12V power to the gas side, while switching on 230V power to the electric heater. I wonder if that is a single pole contactor, that is switching only the live side. Then, if the polarity were reversed, the contactor would switch OK, but only on what is now the neutral side, so not passing power to the electric heater. Sorry, bit garbled, but I hope clear - ish! :-)

 

I am 69, if I did not as a matter of routine test all electrical circuits before touching them, I would not be 69 ;-)

Excellent practise! Just don't fiddle with that relay with the mains connected! :-D

[spospe]

Brian Kirby - 2013-12-03 6:21 PM

 

I wonder if this is the clue? Somewhere on the 230V side, there must be a relay (contactor, I suppose, as it's 230V) that detects the presence of mains power. This must be connected to live to energise, and switch off the 12V power to the gas side, while switching on 230V power to the electric heater. I wonder if that is a single pole contactor, that is switching only the live side. Then, if the polarity were reversed, the contactor would switch OK, but only on what is now the neutral side, so not passing power to the electric heater. Sorry, bit garbled, but I hope clear - ish! :-)

 

 

That, or something very like it, is my guess.

 

I am not going to pursue the matter further for the simple reason that the heater works perfectly with the mains connected correctly, so why try and make it work with the mains connected incorrectly?

 

[Brian Kirby]

spospe - 2013-12-03 6:53 PM

 

Brian Kirby - 2013-12-03 6:21 PM

 

I wonder if this is the clue? Somewhere on the 230V side, there must be a relay (contactor, I suppose, as it's 230V) that detects the presence of mains power. This must be connected to live to energise, and switch off the 12V power to the gas side, while switching on 230V power to the electric heater. I wonder if that is a single pole contactor, that is switching only the live side. Then, if the polarity were reversed, the contactor would switch OK, but only on what is now the neutral side, so not passing power to the electric heater. Sorry, bit garbled, but I hope clear - ish! :-)

 

 

That, or something very like it, is my guess.

 

I am not going to pursue the matter further for the simple reason that the heater works perfectly with the mains connected correctly, so why try and make it work with the mains connected incorrectly?

For the challenge, Michael, for the challenge! :-D

[Derek Uzzell]

spospe - 2013-12-03 6:53 PM

 

Brian Kirby - 2013-12-03 6:21 PM

 

I wonder if this is the clue? Somewhere on the 230V side, there must be a relay (contactor, I suppose, as it's 230V) that detects the presence of mains power. This must be connected to live to energise, and switch off the 12V power to the gas side, while switching on 230V power to the electric heater. I wonder if that is a single pole contactor, that is switching only the live side. Then, if the polarity were reversed, the contactor would switch OK, but only on what is now the neutral side, so not passing power to the electric heater. Sorry, bit garbled, but I hope clear - ish! :-)

 

 

That, or something very like it, is my guess.

 

I am not going to pursue the matter further for the simple reason that the heater works perfectly with the mains connected correctly, so why try and make it work with the mains connected incorrectly?

 

An Airlectric unit is, essentially, just a 230v heating-element inserted into the air-ducting downstream of an Airtronic heater. When Airlectric is operating the Airtronic unit still needs to be electrically 'alive', as it's the Airtronic's fan that forces air through Airlectric where it's warmed by the 230V element.

 

As spospe explains, when provided with a 230V mains power-supply an Airtronic/Airlectric combination system should automatically choose 230V heating. If no 230V mains power-supply is available an Airtronic/Airlectric system should automatically choose diesel-fuelled heating. When connected to a reverse-polarity 230V power-supply, spospe's system fails to choose 230V heating but, when the polarity is corrected, 230V heating is automatically selected.

 

I wouldn't fiddle with it either and I probably wouldn't bother contacting Auto-Sleepers about it. I've browsed the MHF and Auto-Sleepers forums, but found nothing useful regarding Airlectric, never mind the possible impact on it of reverse polarity.

 

I expect (if I were in spospe's shoes) I'd try asking about in on other forums, but the Airlectric add-on is quite rare and it's possible that owners of motorhomes with it haven't experienced running it on a reversed-polarity 230V power-supply (or, as I suggested earier, perhaps only spospe's system is polarity sensitive). But I'd try to find out - for curiosity's sake - if 'my' Airtronic/Airlectric system's behaviour is unique or they all do it.

[rupert123]

Derek Uzzell - 2013-12-03 6:05 PM

 

rupert123 - 2013-12-03 4:29 PM

 

...Why does this keep on coming up, reverse polarity is not a problem, never has been, never will be.

 

It keeps coming up because reverse polarity (RP) is mentioned in publications and articles from authoritarian and well-respected sources. For example, Page 14 of this 2008 Caravan Club information leaflet

 

http://www.thomson-caravans.co.uk/advice/diyprojects/pdf/mainselectricalInstallations.pdf

 

and these two magazine articles

 

http://www.venture-caravans.com/pdf/guides/Reversed-Polarity-Guide.pdf

 

http://www.practicalcaravan.com/advice/reverse-polarity-how-cope-tour

 

The risk relates not to a leisure-vehicle failing to "work perfectly", but to a potential danger if an appliance plugged into an electrical socket within the vehicle malfunctions.

 

.

I neither know or care who recommends or mentions it. With regard to your last sentence, explain please, how does reverse polarity make failure of an appliance more dangerous?

[Brian Kirby]

If I may, it is because British SP switched socket outlets will not isolate the appliance if the polarity of the supply is reversed. So, anyone who finds an appliance has stopped working, switches off at the socket without withdrawing the plug, and then opens the appliance to see if it can be fixed, is liable to get a shock. Also, some older caravans had mains lighting, so fumbling to change a light bulb in the dark also presented a risk of shock, even if the supply to the light had first been switched off.

 

You may dismiss such actions as lunacy, but people do silly things when caught on the hop and under pressure. It is a small risk, but potentially fatal - especielly for someone fitted with a pacemaker. It is also an unfamiliar risk to someone brought up within UK wiring practise where, although extremely unwise, they would get away with it, and may have done so before without consequence.

 

It is a classic example of a risk with a very low potential occurrence, but a very severe potential outcome, that is very easily averted. Anyone giving responsible advice, therefore, is bound to advise of the risk, and how to eliminate it.

[rupert123]

Brian as a qualified electrical engineer, although I admit it has been a long time since I practiced the trade, have told you this before, I sort of know that. However I simply refuse to try and account for every idiot in the world and will continue to credit most people with having some sense of self preservation. I have always said if you must try and fix electrical appliances unplug them first, this is the advice I would give rather than going on about reverse polarity. Reverse polarity itself is no threat, as I said never has been, never will be, but how you treat electricity can be if you are seriously stupid. A lot of things in life are like this, you simply cannot avoid them all, for example where I live we have a narrow gauge railway and every crossing has about ten signs warning you their may be a train coming, why not do as the French do on this, one sign, it reads 'danger of death'. Perhaps we should put a sticker on all electrical appliances with this on but what ever would you and the other pendants talk about then (lol)

[Brian Kirby]

Indeed you did, Henry. I had forgotten that when replying, my apologies. But, you did ask where the risks lie, which I took to mean the risks as expounded by the various clubs etc. So, along with your "I neither know or care who recommends or mentions it", I concluded that you had probably not seen what they publish, or the reasons they quote. Now you've reminded me about your qualifications, I'm not really sure why you asked the question, as you clearly knew the answer all along! :-)

 

So no, not trying to teach an MIEE about electricity, nor being pedantic, but just repeating what others - who know more about this stuff than I do, have published, for the benefit of anyone reading.

 

Personally, I pay RP little heed, as our van is to continental standards with DP breakers throughout, so is "polarity blind". However, I still think the best advice, for anyone who is unsure, is to test, and rectify if reversed. Then, they don't need to worry about which standards apply to their van, or their appliances. It just seems to me to be the fail-safe option.

[mikejkay]

"a double pole switch failing and switching just the neutral" is to me an intrinsic fault in design......apart from being a potentially deadly hazard. Fail safe!

 

Safety:

1 Identify the hazard.

2 Assess the risk.

3 Either eliminate the hazard or ameliorate the risk.

 

Simples

 

Conclusion - get a plug in mains tester for use on continental trips. And while you're at it go round and check all the sockets in your house. I found several wrongly wired sockets in a newly renovated flat that had been rewired by a "professional, qualified" electrician.

[Brambles]

My conclusion is a bit different. Fit an RCB if you do not have one or test the one you have. Then you can accidently stick your fingers in the mains lamp holders in your van now, unscrew electrical appliances and accidently touch live wires and so on. So simple, why did the industry not think of it to protect us against 'reverse polarity'. Oh wait, they did!

[malc d]

My conclusion is that if you are a qualified electrician you probably won't worry about reversed polarity, but, if you are not, it might be wise to take precautions, if only for peace of mind.

 

;-)

[Brambles]

And that precaution is press the test button on your RCB to make sure it works periodically.

 

By adding an extra short reverse wired extension lead you are adding in the risk of a joint getting wet, and another connection in the earth cable which could fail and so on. I am not sure how to score a risk assement on adding in an extra joint exposed to the weather but am sure the risk factor would be greater than having 'reversed polarity' especially if the cable is made up by some cheap jobbing shop company or an inexperienced 'will have a go chap' reading instructions on a website.

 

I still picture the chap on site unplugging his reverse lead which has a grommet far larger than the cable diameter and channeling rain into the connector, he stood their shaking the water out and then unplugging it while still connected to the mains, surely he has much more chance of an electric shock because he is using the one thing supposedly to reduce risk...go figure that one out.

 

So is adding a reverse lead reducing risk or adding additional risk. Oh hold on, someone has just moved my reverse lead connection out the way to plug his cable in and got a belt of it as it is soaking wet.......... food for thought.

[Derek Uzzell]

It should be practicable to waterproof the joint between the reverse-polarity 'correcting adapter' and the hook-up cable by using different end-connectors. This was discussed in some depth in this 2011 thread relating to water-protection for connected-together cables. (Note that some of links provided will no longer work.)

 

http://www.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/elec-connection-protectors/24393/

 

It can never be considered bad practice to test a mains hook-up power supply. That many people never do, or don't bother to correct reversed polarity if it's discovered, doesn't change this simple fact - it's unarguably 'safer' to test a hook-up power supply than not.

 

For new and prospective motorcaravanners who recognise themselves as being accident-prone, in addition to testing hook-up power supplies, following the advice on this link may provide further protection

 

http://asksistermarymartha.blogspot.co.uk/2011/09/bolt-from-blue.html

[rupert123]

Derek Uzzell - 2013-12-08 5:49 PM

 

It should be practicable to waterproof the joint between the reverse-polarity 'correcting adapter' and the hook-up cable by using different end-connectors. This was discussed in some depth in this 2011 thread relating to water-protection for connected-together cables. (Note that some of links provided will no longer work.)

 

http://www.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/elec-connection-protectors/24393/

 

It can never be considered bad practice to test a mains hook-up power supply. That many people never do, or don't bother to correct reversed polarity if it's discovered, doesn't change this simple fact - it's unarguably 'safer' to test a hook-up power supply than not.

 

For new and prospective motorcaravanners who recognise themselves as being accident-prone, in addition to testing hook-up power supplies, following the advice on this link may provide further protection

 

http://asksistermarymartha.blogspot.co.uk/2011/09/bolt-from-blue.html

It may be practical to waterproof a reverse polarity lead by why would you bother, throw it away. While I would agree it is not bad practice to test a mains hook up socket it is a waste of time. The only thing this does is it means you can carry a bit more kit around with you, if you wish to do this fine but a motorhome has plenty of inbuilt protection in place to not need it. The only thing I would say if your van is so old it does not have this protection rather than mess around with dodgy leads get it sorted out, then as Brambles has said press the button on occasion.