Reverse Polarity

[Corrinne]

Just been trying to understand why my motorhome keeps displaying “reverse polarity detected”. Don’t understand electricary and what I read went right over my head. It seemed to be saying that the EHU in Europe is often wired differently from that needed for a British motorhome. Does anyone know whether reverse polarity is dangerous?

 

[SuperMike]

 

[tonyidle]

Just one snag with your post. Your first sentence is quite correct, however, don't get hung up on the idea of an earth, because running off an un-earthed genny is not the same as the mains. You need to consider the circuit path. If there is no connection to earth anywhere within the genny or the electrical system then touching L and earth will not make a circuit and therefore will not give someone a shock. You cannot have a "representation" of earth, either you have a circuit or you don't. Its no different to standing on a well insulated mat in dry conditions and touching the live.

I meant what you have just said. I'm not hung up on earth and by representation I meant an earth conductor rather than what we're standing on: the two are very different but their interaction with safety devices can be the same. Standing clear of earth and touching the Live isn't possible: it's only Live when measured against something - if there's nothing to complete the circuit then it isn't live (as far as measuring a voltage or touching it is concerned) - it's simply one of the line conductors. A portable generator is a special case: for example what part of it would you connect to true earth (and how would you decide whether the ground is of sufficiently low resistance to be of use)? Neither line out of the generator is Live or Neutral unless one of them is connected to the generator frame and the frame is connected to a good ground. If both conditions were fulfilled you would have what exists at the Mains Generator or Sub-station.

 

[Mike800966]

I wrote a piece about this just recently and in a nutshell "it doesnt matter" UNLESS you start to modify items or work inside them- the example I gave was poking about inside a metal cased toaster.
There are some inline RCB's which are single pole ( god help us)
I was surprised in France recently to note sites where the LN was reversed even on BS4343 (Blue plugs) which are clearly marked L .
So dont worry especially and dont go poking about inside the toaster!
Hope the Oh gets better soon.

Mike

 

[thebriars]

Actually the genny is a bit like a drop box on a building site. That not only reduces the voltage to 110v, but also isolates both the live and neutral from eart, making it much safer. I only use my genny to power the central heating if we have a power cut, never with the mh. Having said that, every time we've hired a mh in the US or Canada they come with a genny, and they certainly don't worry about earthing. Even the last one, a brand new but quite rare top spec Fiat PVC had a genny slung under the back! On reflection I don't remember seeing too many dead tourists who had electrocuted themselves.

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[bigtwin]

SO.... if your Live and Neutral connections are reversed, its not a good idea really because there is a slim chance your appliance's casing (like a metal cased electric toaster or kettle) could still be live

Why do you say their is a slim chance?

Under what specific circumstances do you expect the casing of an appliance to be live?

Ian

 

[bigtwin]

I wrote a piece about this just recently and in a nutshell "it doesnt matter" UNLESS you start to modify items or work inside them- the example I gave was poking about inside a metal cased toasterMike

Agreed

Ian

 

[Camping Gaza]

So dont worry especially and dont go poking about inside the toaster!

I think the Original Poster has long gone by now if she has any sense!!!

 

[Camping Gaza]

Under what specific circumstances do you expect the casing of an appliance to be live?

A breakdown of internal electrical insulation. especially on movable appliances, terminals fail, screws come loose, wires break . components short. This does happen and that's the reason for neutral earth bonding in the first place

 

[bigtwin]

A breakdown of internal electrical insulation. especially on movable appliances, terminals fail, screws come loose, wires break . components short. This does happen and that's the reason for neutral earth bonding in the first place

Wouldn’t the examples you give not also cause a problem, irrespective of whether or not live and neutral are transposed, when the user was using the device normally?

Ian

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[Camping Gaza]

Wouldn’t the examples you give not also cause a problem, irrespective of whether or not live and neutral are transposed, when the user was using the device normally?

Ian

A good question, but you have to know how earth bonding works, instead of the live going through your heart and to earth via your feet or hands touching something connected to earth, it goes down the neutral then to earth (least resistive path) at the sub station. you will get a shock but not lethal

If the poles are reversed, the neutral is not bonded to earth and the current flows as if there is no earth, but now there is cos you touched the live casing.... YOU are now the earth in the circuit. boom! (thats how I understand it anyways)

I know, Have a word with your local institute of electrical engineers, you can google them for an address. They will I am sure point out all the positives and negatives (or is that the live's and neutrals) of electrical safety.

You will get better answers there I am sure. In the mean time I am on to my final carton of popcorn

 

[bigtwin]

A good question, but you have to know how earth bonding works,

I know, Have a word with your local institute of electrical engineers, you can google them for an address. They will I am sure point out all the positives and negatives (or is that the live's and neutrals) of electrical safety.

You will get better answers there I am sure. In the mean time I am on to my final carton of popcorn

I don’t see an answer in your reply.

Are you saying that you know the answer to my question and that for me to understand the answer I must first understand earth bonding?

If you do know the answer to my question, why would you not respond and, instead, direct me to the IEE?

Ian

 

[Camping Gaza]

I don’t see an answer in your reply.

Are you saying that you know the answer to my question and that for me to understand the answer I must first understand earth bonding?

If you do know the answer to my question, why would you not respond and, instead, direct me to the IEE?

Ian

Simply because you will get a better answer.

 

[bigtwin]

Simply because you will get a better answer.

Since we’re already corresponding on the matter, I’d, nonetheless, be interested in your answer.

Ian

 

[Camping Gaza]

read the post again, its been edited.

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[bigtwin]

A good question, but you have to know how earth bonding works, instead of the live going through your heart and to earth via your feet or hands touching something connected to earth, it goes down the neutral then to earth (least resistive path) at the sub station. you will get a shock but not lethal

If the poles are reversed, the neutral is not bonded to earth and the current flows as if there is no earth, but now there is cos you touched the live casing.... YOU are now the earth in the circuit. boom! (thats how I understand it anyways)

Happy with your 1st para explanation but struggling a little with the second.
If the L and N are transposed, is the ‘L’ line not then effectively bonded to earth?

The L and N lines effectively complete a circuit through the appliance back to earth, are you not effectively adding another fault (loss of earth bonding) into the fault scenario and it is this (lack of an earth (path)) that means you become the path to earth? In which case we’re looking at multiple (2 faults (internal short to the casing AND loss of earth bonding) as well as ‘reverse polarity’) before any danger occurs - this would certainly justify your use of the “slim chance” in your earlier post.

Ian

 

[Camping Gaza]

See if this diagram makes sense ignore the HOT, it means live. back at the substation transformer, down the end of your street the neutral and earth wire is connected (and to ground)

if you swap the 3 pin SOCKET so the hot (live) is on neutral and neutral is on live, back at the substation the live is now connected to your ground via the neutral which is connected to your case. and your case is live

you can often feel this if you lightly run your fingertips across the case of a reverse polarity appliance with a metal case. It produces a vibration in your finger tips.

see the image and reverse in your mind the hot and neutral at the 3 prong SOCKET (not the plug on the appliance that's wired correctly) and imagine the flow back to the case via the ground wire and you touching it with no resistance at all to earth. It will not kill you due to your bodys resistance, but if the earth wire was not connected...

 

[thebriars]

 

[Camping Gaza]