Reverse Polarity

[RedFrame]

Most funsters are at best “have a go” amateurs...

Cheers!

Russ

Most time served "Professionals" (electrical, gas, vehicle, painters etc, etc) that I've payed money to over the past few years would qualify as "Funsters" then...

Cheers
Red

 

[Old Gibber]

You could change the so jets for double pole switched ones. Quite a few UK manufacters sockets are double pole switched, MEM, Volex, Contatum are some of them.

Just one minor point. Most caravans and motorhomes don't have switched sockets anyway.

 

[autorouter]

I will make my point again, "As has been said the only danger is if you decide to take an appliance apart and it is still connected to the mains, though even if the mains polarity is correct this is still dangerous."

It may be the only danger you can think of, but there are plenty of other possible scenarios where reversed polarity in a UK MH will be a danger. Of course they are unlikely, but then so is a car accident or a fire, but we still take precautions. In most of these scenarios, the RCD will kick in and save you, but that's not the point you or I am making. The point is that you can have a fault, the fuse/MCB will blow/trip, but there will still be areas of metal that will be live and exposed, AFTER the overcurrent protection devices have activated.

If you want examples, how about the classic toaster in the washing-up bowl, stepladder falling on a hairdryer, mains flex trapped in a camping table's folding metal legs, etc, etc.

 

[Old Gibber]

It may be the only danger you can think of, but there are plenty of other possible scenarios where reversed polarity in a UK MH will be a danger. Of course they are unlikely, but then so is a car accident or a fire, but we still take precautions. In most of these scenarios, the RCD will kick in and save you, but that's not the point you or I am making. The point is that you can have a fault, the fuse/MCB will blow/trip, but there will still be areas of metal that will be live and exposed, AFTER the overcurrent protection devices have activated.

If you want examples, how about the classic toaster in the washing-up bowl, stepladder falling on a hairdryer, mains flex trapped in a camping table's folding metal legs, etc, etc.

There is no reason to assume that a MCB will trip in any of the scenarios that you have outlined. MCBs protect against overcurrent, nothing more. Cut through the live of a mains cable with a pair of insulated cutters and the MCB will not trip.

Most likely the RCD will trip in those scenarios, and the RCD isn't affected in any way by reverse polarity.

 

[autorouter]

Cut through the live of a mains cable with a pair of insulated cutters and the MCB will not trip

Last time I cut through a live mains cable with insulated cutters it certainly tripped the MCB. If you carefully ensure you are only cutting the live then it won't, of course. Try doing that with the folding metal legs of a camping table.

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[Mike800966]

Oh Dear Oh Dear, here we go again. Being realistic reverse polarity is only a problemwhen you try to get the bread out of the toaster with a metal knife whilst holding the metal casing. Just pull the blooming plug out! UK single pole switching only breakes the Live, if its reversed it breakes the neutral. Also dont dance barefoot on wet grass whilst holding the motorhome!. If you look inside a CEE plug or socket the Live and earth holes are marked.
Mike
Edit Perhaps I should have said a BS4343 plug

 

[autorouter]

Oh Dear Oh Dear, here we go again. Being realistic reverse polarity is only a problemwhen you try to get the bread out of the toaster with a metal knife whilst holding the metal casing. Just pull the blooming plug out! UK single pole switching only breakes the Live, if its reversed it breakes the neutral. Also dont dance barefoot on wet grass whilst holding the motorhome!. If you look inside a CEE plug or socket the Live and earth holes are marked.
Mike
Edit Perhaps I should have said a BS4343 plug

OK, let me spell it out. Try this little experiment. Plug in an appliance like a laptop charger that has a 2-core wire, not a 3-core wire. Take a metal kitchen knife and start to cut through it. At some point the knife will cut through and short the live and neutral wires. Either the fuse in the plug will blow, or the MCB will trip. The fuse is in the live wire, so live input will be disconnected, but the neutral will still be connected. The neutral is at or very near the earth voltage, so touching the neutral will not be a safety hazard. That's what happens if the live and neutral are correctly wired, not reversed.

Now try it with the live and neutral reversed. As you cut through the wire, the knife will short the live and neutral wires, as before. Either the fuse will blow or the MCB will trip. The fuse is now in the neutral wire, so the neutral input will be disconnected, but the live will still be connected. So the knife will be live, even though the fuse has blown or the MCB has tripped.

Now I suppose I have to spell it out that I do not suggest that deliberately cutting with a knife is a likely accident scenario. However there are plenty of possible accidents where a metal object can cut into a mains flex, and become live if the live/neutral wires are reversed.

As I've said before, if there's a working RCD in the supply, it will kick in and prevent an electric shock, so most MHs are protected from that point of view. But not all MHs have RCDs.

 

[Nedge68]

OK, let me spell it out. Try this little experiment. Plug in an appliance like a laptop charger that has a 2-core wire, not a 3-core wire. Take a metal kitchen knife and start to cut through it. At some point the knife will cut through and short the live and neutral wires. Either the fuse in the plug will blow, or the MCB will trip. The fuse is in the live wire, so liv…

Nice explanation again from Autorouter, safe isolation of equipment is a daily part of the life of electricians and technicians everywhere, polarity of electrical installations is an important safety check for the reasons given above. MH should be protected by the RCD on the HU, how many test the RCD operates when they plug in? Check the HU cable and plug for damage, unwind the cable reel…

 

[Old Gibber]

As I've said before, if there's a working RCD in the supply, it will kick in and prevent an electric shock, so most MHs are protected from that point of view. But not all MHs have RCDs.

Frankly I would have thought it would make far more sense to fit an RCD if one is not already fitted.

Also, going back to RCDs, how many regularly check their house and motorhome RCDs? Surely this should be emphasised far more than the very remote/non existent problems associated with reverse polarity.

Anyway, now forewarned, I will be really careful not to let the stepladder I don't carry in the motorhome fall on the hair dryer which I don't have.

 

[autorouter]

Frankly I would have thought it would make far more sense to fit an RCD if one is not already fitted.

I think that's one thing we can definitely agree on

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