Polarity reversal switch

Grehan

Grehan

Well-Known Member
Joined
11 Jun 2001
Messages
3,729
Location
Inland France + Oxon.
www.french-waterways.com
I understand there is a way to wire a DPDT (double pole, double throw) switch so as to act to reverse polarity. But the largest capacity 240v switch I can find, Googling, is rated 15A. Would this be man enough to cope with boat mains intake (say maximum 2kw heaters, electric kettle, immersion)? Or could I find (say) a 20A version (at an affordable cost) somewhere?
Any other comments or suggestions welcome.

BTW I know I can use a 'reversal pigtail' and I've got one and been using it for years, but I'd like to have the potential simplicity of a switch.
 
Firstly the amp rating of the dock power point will limit the amount of power you can suck. 15 amps is the usual fuse rating for power point here in Oz which gives 4.25 kilowatts which is a fair bit of power.

A DPDT switch is arranged so that either the input pair go to the moving arm of the 2 DT switches. The up position of one switch and the down position of the other are then connected together and go to one output wire while the down of one switch and the up of the other go to the other output. Sorry I am not good at producing diagrams.

However you will notice that this puts the AC mains across the 2 options of the DT switch and likewise across the adjacent switch. I just feel that the close proximity of the 2 sides of the mains could mean any arc when switching with current flow could lead to an arc from active to neutral. This would see your switch in flames in milliseconds. So unless the switch had really big spacing between contacts and between poles I wouldn't dream of trying to use a switch. No I have no experience but just think 240v at 15 amps would invite problems. Just use your cross over cable. olewill
 
Sorry I am not good at producing diagrams.
Click to expand...

How's this ?

Use a switch with a centre OFF position to ensure that one position is disconnected before the other is connected.

A proper 240 volt switch should take live and neutral on adjacent terminals without problem.

If you use a 15amp switch be sure the circuit is fused at 15 amps or less.

I'd think I'd prefer the pigtail.
 
Don't do it!

I absolutely, guarantee that one day someone will flick it / catch it by mistake / get confused as to which way to switch it etc... and break everything!

It WILL happen... :)
 
I one day someone will flick it / catch it by mistake / get confused as to which way to switch it etc... and break everything
Click to expand...

Why will it break everything.
The reversed "polarity" will means a reduction in safety because British appliances dont usually have double pole mains on/off switches but I cannot see that it will break anything.

Dont like the idea though and like everyone else would stick to the pigtail idea.
 
OK. That seems like a bit of a thumbs down, then. :p
No point in asking a question if you then ignore the answers. Thanks guys and/or girls.
[edit] ah, I note the names Vic, Alan, William, Ross . . perhaps the 'thanks girls' is not appropriate ;)
 
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My 'fuse board' on board is protected by a double pole switch and the rcd is the same. So for me as long as the earth is not incorrectly wired it is not a problem with the equipment that I use on board - so far ! If you are concerned the way is to have a 'tail' with wires crossed over as others have mentioned.
 
Just to make it absolutely clear folks. I do have a polarity indicator and a reversal pigtail and I do use it. I do have a DP mains switch MCB on the 'fuseboard'.
I just idly wondered about a polarity reversal switch and how practical that was.
 
FWIW I don't see anything wrong with what you're proposing as long as you use a 'break before make' switch, or one with a central 'off' position.

e.g. 257-9654 from RS - 20A 2P C/O with off £11.58 +VAT

Rather confusingly these types of switches are listed under 'Control Gear', not 'Switches and |Relays' :confused:

Andy
 
Very simply...

make up a very short (<0.5m) interconnecting lead with a female plug one end and a male the other but switch the blue with the brown in one end. mark it "POLARITY CHANGER" and use it as required. check ok/not ok before using with a neon connected between earth and neutral on input. if it lights polarity is wrong so use changer. if it doesn't light all is OK
Chas on Kentrina's login so not necessarily her point of view
 
Ken_Irby said:
make up a very short (<0.5m) interconnecting lead with a female plug one end and a male the other but switch the blue with the brown in one end. mark it "POLARITY CHANGER" and use it as required. check ok/not ok before using with a neon connected between earth and neutral on input. if it lights polarity is wrong so use changer. if it doesn't light all is OK
Chas on Kentrina's login so not necessarily her point of view
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Thank you, but as I've made clear twice . . that's what I ALREADY HAVE. :rolleyes:

Many thanks for the switch link, misterg, looks the ticket.
 
Why Complicate matters and introduce a weak spot?

Grehan said:
Thank you, but as I've made clear twice . . that's what I ALREADY HAVE. :rolleyes:

Many thanks for the switch link, misterg, looks the ticket.
Click to expand...

Can't see why you want to build in a possible source of failure, personal injury and increased maintenance. leave it as it is and use your polarity changer as and when. simples!
Chas on Kentrina's login so not necessarily her views
 
can someone explain to me why polarity ( L/N ??), needs to be changed
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Because in foreign marinas it is not uncommon to find them the "wrong way round". The reversing switch or pigtail is so that they can be changed to the "right way"

yes live and neutral, not strictly polarity maybe, but can you think of an alternative word.

(I believe even the words live and neutral have been replaced by phase and return)
 
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Thanks Vic, it did vaguely cross my mind, but I'd dismissed it as so unlikely in these days of EEC standardisation etc, I really am surprised.
A relative of mine once suffered quite severe injury. A re-wire of her house had crossed over L/N on a socket used for an electric fire, there was "operator error" as well, but it wouldn't have happened with correct wiring.
 
VicS said:
Because in foreign marinas it is not uncommon to find them the "wrong way round". The reversing switch or pigtail is so that they can be changed to the "right way"

yes live and neutral, not strictly polarity maybe, but can you think of an alternative word.

(I believe even the words live and neutral have been replaced by phase and return)
Click to expand...

I think the problem goes a bit deeper than that doesnt it? Yes, I always check the polarity of an unknown supply for safety reasons, but in some countries (France for example) BOTH cables are 'live' to earth, supply at +110volts, return at -110volts.

In Uk our supply is +240v on the live or phase supply, while the 'neutral' or return is at 0v. It has to do with the way 3 phase grid supply transformers are wound.

The French regard their supply as 'safer' because the grid supply transformers are set up so that the supply cables carry at most 110v which is less likely to be lethal if someone touches it. But it does mean their supply does not differentiate between +phase and -return as we do and may quite likely be reversed.

Does it matter? Yes, because UK appliances that have a single switch in the +240v line are at 0v within the appliance. If reversed then the appliance remains 'live' the wrong side of the switch.

On board, that means the boat mains circuit would be at mains potential even though switched off, unless isolated through a double pole switch.
 
think the problem goes a bit deeper than that doesn't it? Yes, I always check the polarity of an unknown supply for safety reasons, but in some countries (France for example) BOTH cables are 'live' to earth, supply at +110volts, return at -110volts
Click to expand...

So in France then you cannot have a reversed "polarity"... because the two conductors are the same namely 110 volts from earth, So a "polarity" reversing tail, or switch would be pointless as reversing the "polarity" would have no effect. Both conductors at still 110 volt from earth.

So no point either in checking the "polarity"


?????????????????? :confused:
 
In the U.K. our distribution transformers are wound delta/star.
The star wound 3 phase secondary looks like a Mercedes star and the centre or "star point" is 0 volts, both "Neutral" and bonded to "Earth" at the point of supply for domestic 240V.
Thus the "Live" is 230-240V - "Neutral" is 0V and "Earth" is also 0V.
However, because the "Neutral" is never bonded to "Earth" after the point of supply whereas the "Earth" is continuously bonded (literally by earth bonding points along the route from house to house) the impedance of the cable produces a voltage drop in the "Neutral" relative to its distance from the supply transformer and the cumulative current that it's carrying.
A house close to the supply may have L = 240V, N = 0V, E = 0V; whereas a house at the far end of the cable may have L = 230V, N = 12V, E = 0V.
Some continental systems were (and maybe still are, I don't know for sure) floating, i.e. neither power conductor is bonded to earth, so the effect of measuring between either conductor and earth was to give 110V.
An old rumour had it that, if you permanently or deliberately grounded one side of such a system, the continental power company would then "blow off" your fault by grounding the other conductor with a much thicker bit of wire than yours thus restoring the system to serene floating balance.

Both floating and bonded systems have their merits and problems.

Both work fine with incorrect polarity.

The safe way to design for both systems is "double insulation" and double pole switches to eliminate the danger, that oldharry pointed out, of having a live conductor in the appliance due to incorrect polarity and a single pole switch.

The simplest polarity check is to have a green neon between L-E and a red one between N-E.

I would always try to maintain correct polarity for safety, I always want to know which conductor is potentially "live".

On the other hand I was taught many years ago "This is the only job where I'll clip yer ear lad if I catch you with your hands out of your pockets" ;)
 
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