# reverse polarity



## billym (Dec 17, 2005)

I have confused myself. We have arrived on a site where we will stay all summer ( probably ). We have reverse polarity. Does it matter ?
If so..................................
I have a lead from my

rv connected to a 25 metre EU cable connected to another 25 metre EU cable connected to a EU fuse box. Which wires do I need to reverse is it on all the cables or just the one connected to the fuse box? Help PLEASE

P.S. get to Bordeaux.. its absolutely boiling


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## gaspode (May 9, 2005)

Hi Billym

Just reverse the live and neutral wires on one of your plugs, probably easiest to do it on the one that plugs into the hook-up distribution board. Don't forget to swap it back when you move on!


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## billym (Dec 17, 2005)

Thanks Gaspode. Thais is why I like this site so much.


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## gromett (May 9, 2005)

Just to refresh peoples memory on this. If you get a phase swapped situation everything should work ok. The problem is that in england only live is switched. So if you have the phases swapped when abroad and you switch something off, parts of the equipment will still have live attached. This is a safety issue.

Regards Karl


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## autostratus (May 9, 2005)

gromett said:


> Just to refresh peoples memory on this. If you get a phase swapped situation everything should work ok. The problem is that in england only live is switched. So if you have the phases swapped when abroad and you switch something off, parts of the equipment will still have live attached. This is a safety issue.
> 
> Regards Karl


Why does this constitute a danger?

If the polarity reverse isn't corrected would I be electrocuted or would the RCD system in all modern British caravans/motorhomes save me from shock in the event of touching a live wire.


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## gromett (May 9, 2005)

The RCD should be a system of last resort.

If you switch off your electrical device you expect there to be no power to it. In a reverse phase situation there will still be live to the device. You are then relying on the RCD to protect you. 

An RCD will not protect you from a live-neutral fault as the same amount of current is going in and then out from you (you are basically the device)
The RCD will only trip if you become earthed somehow.

Cheers
Karl


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## Rapide561 (Oct 1, 2005)

*Reverse polarity*

Hi

This will sound silly - but is swiftching the live and neutral wires OK to do?

Also, how do I know if I have reverse polarity or not?

Rapide561


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## DABurleigh (May 9, 2005)

Rapide,

1) Yes. 
(Must confess going by the original query I don't know what the practice is in US RVs. Similarly I presume 100% continental vans such as a Hymer shouldn't have to worry about reverse polarity because both their live and neutral lines should be switched. The problem is a British equipment one, but whether uniquely British I don't know).

2) By using a mains tester available at any caravan accessory shop.

Dave


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## gromett (May 9, 2005)

Ok, this is a bit complicated. I have written this out a few times and each time deleted as I thought it may confuse. I am trying to keep it simple while at the same time not introducing technical innaccuracies.

AC power is floating unless one of the connections is grounded. In this case the other connection will go from -120v to +120V relative to the grounded connection. The connection that is varying is called the live. For an RCD to work the Neutral needs to be bonded to earth to provide an alternate route for current to flow in case of a fault.

In the case of a swapped phase. The RCD would still work in the event of a live earth fault. However...

For safety sake, when you switch your hairdryer or TV off. You expect it to be totally off. For example. If you have a TV and you switch it off using the button on the front (old style switch). The earth to the device is still connected and because of the phase swap live is still present inside.
Neutral has been switched off, but remember earth is bonded to neutral.

This means that inside your TV there is both 240V live and a 0V earth potential difference. If the phase was the correct way round when you switched it off, there would be no live inside the chassis.

If a fault occurs there is a good chance the RCD will protect you BUT freak scenario's can occur. Faulty RCD. Or say you are not on hookup and the grounding relay on your inverter hasn't kicked in properly. Or someone chucked a bag full of xy or z into the cupboard that holds the RCD and it jams the RCD in the on position etc etc etc. Lots of possibilities.

The RCD is a device of last resort, and you really should ensure that the electrics in the van are right as a matter of safety. After all electricity doesn't just electrocute it can cause fires as well.

A socket tester can cost as little as £3.99 and the crossover cable around £10.00 and if it means your RCD doesn't have to do any work 
8) 8) 8) 

Also remember to test your RCD once a month or so as they can and do fail.

One of my electrical enginees taught us three rules of electrical installation.
1) Just because it works doesn't mean its right (or safe)
2) Always keep one hand in your pocket when working on live gear
3) Never trust anyone elses work.

Cheers
Karl


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## sallytrafic (Jan 17, 2006)

One reason that you shouldn't rely on an rcd is that it is a mechanical device (although electronically triggered) and therefore takes a finite time to operate, so for that period you are subjected to the 230V. That is why you should excercise it by pressing the test button - once a month would be good. 

The second reason is that when in perfect working order and regularily excercised the maximum current that can pass through you is 30mA and it will be only for a short time. That is chosen because most people will not suffer cardiac arrest with 30mA passing across their chest. The difficulty is that 'most' doesn't mean 'all'. BTW If they made it trip at much less than 30mA it would trip randomly and this nuisance tripping might be a hazard in itself (it would certainly raise my blood pressure!  )

So an rcd is there to protect you but it should be treated as the last resort not the first defence. 


Regards Frank


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## sallytrafic (Jan 17, 2006)

gromett said:


> For an RCD to work the *Neutral needs to be bonded to earth* to provide an alternate route for current to flow in case of a fault.
> Cheers
> Karl


Not strictly true, the modern rcd circuit detects the difference in current flow in the two wires live and neutral. If they are not identical to within 30mA then this is detected, and the inbalance used to trip the contactor disconnecting both live and neutral. How did the currents come to differ? Well one way is by some current taking a live to earth path through you and the earth connection back to the supply companies transformer which is referenced to earth! but other paths are possible and will trip the RCD if above 30mA.

For example if you turn off the live supply by say switching off a single pole switch or circuit breaker then when working on the appliance (say an immersion heater) inadvertently short neutral to earth the rcd will trip. This is because in a domestic supply neutral floats a bit away from earth potential, just a few volts normally (but I have seen nearly 30V in an overhead supplied house at an extreme distance from the substation.) The small current that then flows between neutral and the local earth is enough to trip the rcd.

Regards Frank


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## gromett (May 9, 2005)

I did say I was trying to keep it simple Frank :wink: 

Karl


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## sallytrafic (Jan 17, 2006)

gromett said:


> I did say I was trying to keep it simple Frank :wink:
> 
> Karl


Yes agreed and you did a very good job but the only trouble with the earth bonded to neutral argument is it lodges in the mind of the layman and can cause no end of bother especially with those who use generators. Even my explanation glosses over some fairly important issues.

Regards Frank


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## gromett (May 9, 2005)

Ok, I do take your point but when you simplify anything to make it easier to understand there will always be technical details that are not relevent for the explanation that get pushed to one side.

I stand by my statement that the earth is bonded to the neutral. Ok, it may not be done at the socket, the house or the van and may be done at the substation or even power station but the fact remains that earth is bonded to Neutral. As to the voltages seen on Neutral, I seem to remember this is some thing to do with the fact the domestic supplies are only one phase of a 3 phase supply and inbalances on the loading of the phases from a substation can pull the reference away. Anyway getting too technical on this again I feel.

The point I was trying to make in my post was that 
a) phase swapping will not stop things working but has definate safety issues.
b) An RCD is a vital safety device but should not be relied on to the exclusions of all others.
c) Its better to be safe now than sorry later. For the sake of £15 check the phase and fix it.

Cheers
Karl


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## billym (Dec 17, 2005)

I am sorry to have caused all this work for you all but I was happy with the answers I got. Yes its dangerous and change the wires round at the connection. I suppose if we were all the same it would be boring. I am going outside right now to sort it out.


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## DABurleigh (May 9, 2005)

Perhaps a mod would move this from the Satellite TV forum.


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## autostratus (May 9, 2005)

Just to correct any misconceptions.

We do check the polarity when connecting to site mains by use of a plug-in tester.
We do have a short cable for reversing any reversed polarity site mains we find.
I often ask questions when I believe the answers may be of general interest even when I already know some or part of the answer.

Finally thank you for the concise nature of the answers in what can be a complicated and confusing subject to many.


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## autostratus (May 9, 2005)

DABurleigh said:


> Perhaps a mod would move this from the Satellite TV forum.


Done.


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## DABurleigh (May 9, 2005)

Gillian,

Your RCD question is sort of what I was getting at here:
http://www.motorhomefacts.com/ftopicp-96262.html#96262

The problem is the lousy engineers we have ;-)
(only joking guys, I had my teasing then)

Dave


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## autostratus (May 9, 2005)

I agree.

I've moved that as well!


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## gromett (May 9, 2005)

I have been tempted for some time to create a device that not only checks a socket but automatically fixes common problems such as swapped phase.

My theoretical device would
1) check the supply and in the case of
2) swapped phase -> swap them back again
3) if the earth is missing it would bond neutral to earth and sound a buzzer

It would be wired between the vans socket and the input RCD.
It would have no moving parts. (relays etc)
It would have a few recessed dip switches to configure settings. So that you could disable certain auto-fix features if you were not happy about them.

I keep meaning to do this each time I have to go out in the rain or cold to insert the phase swapping adapter.

Any electrical engineers want to collaborate on this please give me a shout. I did talk to kands and artona about this on the last rally and these threads may be just the motivation I need to get my electronics head on.

Cheers
Karl


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## sallytrafic (Jan 17, 2006)

gromett said:


> I have been tempted for some time to create a device that not only checks a socket but automatically fixes common problems such as swapped phase.
> 
> My theoretical device would
> 1) check the supply and in the case of
> ...


I wish you luck. The last time I tried to come up with a circuit to do a similar thing to 2 I came unstuck with the electricity supply regulations. Solid state devices don't provide the required 'legal' isolation (unless someone has come up with a new device). Obviously there has also to be a 'break before make' facility and feedback will need to go back into the electronics to ensure that 'make' doesn't occur unless 'break' is confirmed. I guess you would use a small microprocessor like a PIC to handle the control logic.

I'm not sure that 3 is a desireable objective unless it is combined with a 'neutral connected' detector. Consider the case where the supply earth and neutral both get disconnected at the same time, then your 'earth neutral bonder' will have connected your chassis by a low impedance to the live feed!

Anyway as I said best of luck.

Frank


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## sallytrafic (Jan 17, 2006)

DABurleigh said:


> Gillian,
> 
> Your RCD question is sort of what I was getting at here:
> http://www.motorhomefacts.com/ftopicp-96262.html#96262
> ...


You are of course correct, the trouble is that someone defined isolation as a gap (3mm from memory). A totally solid state device could reduce the time that you could be exposed to a dangerous amount of current to a very short time possibly less than 1ms. Unfortunately no physicist has invented the device that will jump apart the necessary 3mm but us engineers are willing to applaud if you do.

Regards Frank

PS get the IEE (or whatever they have just changed their name to) to change the rules along with a new SI to change the electricity supply regulations and you will get mega applause.


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## DABurleigh (May 9, 2005)

"Unfortunately no physicist has invented the device that will jump apart the necessary 3mm but us engineers are willing to applaud if you do. "

You see that's the problem - engineers look to physicists to cure the problems the engineers have created. Now if you had left the definition of isolation to a physicist as opposed to an engineer, you wouldn't have the silly gap constraint to overcome! 

Dave
(Feeling outnumbered and vulnerable .....)


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## billym (Dec 17, 2005)

Sorry again for putting it under satellite tv. My mistake. I have done as originally suggested. I still cant understand why if I change the wires round on one cable I dont have to on all of them, but there again I failed all my science exams and dont know a watt from an amp from another watt. I do know lots of other things but they are all useless so thanks to all you clever people who look after us. But......if I have these three extension cables why do I only have to change one lot of wires round ?


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## DABurleigh (May 9, 2005)

You're the smart one being esconsced in Bordeaux for the summer .....

I don't mean to confuse, but if you changed all your cables it would be OK, but if just two of them you wouldn't. As you have a polarity tester and obviously plenty of time to kill (green here) you could try it out! 

You simply need to kid your motorhome that the live wire coming into its hookup socket is the neutral wire and vice versa. Now if you swap these on two cables (or any even number), you'll be back where you started.

Dave


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## gromett (May 9, 2005)

sallytrafic said:


> I wish you luck. The last time I tried to come up with a circuit to do a similar thing to 2 I came unstuck with the electricity supply regulations. Solid state devices don't provide the required 'legal' isolation (unless someone has come up with a new device). Obviously there has also to be a 'break before make' facility and feedback will need to go back into the electronics to ensure that 'make' doesn't occur unless 'break' is confirmed. I guess you would use a small microprocessor like a PIC to handle the control logic.


I am not quite sure what regs you are referring to here. I would certainly never use a MP controlled system as it adds too much complexity. I was looking at some solid state relays which have optical isolation between control circuit and the live supply?

In addition they have a 4000V isolation on the switching side. As they are performing a similar task to a two stair switch solution I cannot see any issue from a safety point of view.



sallytrafic said:


> I'm not sure that 3 is a desireable objective unless it is combined with a 'neutral connected' detector. Consider the case where the supply earth and neutral both get disconnected at the same time, then your 'earth neutral bonder' will have connected your chassis by a low impedance to the live feed!


I am not sure this is a desirable feature either. That is why the dip switch thingy so you can disable it if necessary. I have not put an awful lot of thought into the final device. I just want a device that is hard wired into the system that will swap earth and neutral. Fix any other issues that are safe to fix and sound a buzzer if an unfixable fault ocurrs.

In my Van I am fitting two sockets, one for normal shore supply and another for the genny (yet to buy). Each will have different rules for connection. The victron has automatic bonding/ back connection relays that I don't feel are 100% compatible with generators. But that is a whole other kettle of fish.

Back to the device. A simple crossover relay network (not double poles of course) would solve the phase swap issue and could be done automatically with the addition of a handfull of resistors (+1). Although this is simple I want to avoid moving parts, i am therefore looking at solid state relays along with a basic control mechanism. I don't want to go to a mp controlled system as software and mp can feck up in bad ways. The old KISS expression is coming to the front of my mind.

Anyway, too many beers again unfortunately. Frank, if you are interested in developing this device give me a shout you are just the type of technical pedant that would be extremely helpful :wink: and would ensure I don't electrocute myself....

Cheers
Karl


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