Reversed polarity

[Brian Kirby]

Oh no, not that agin!  Well yes, but with a twist.  Here's a puzzler for the clever and informed ones among you.

We recently stayed on a nice little municipal site at Apt, in France.  (Apt is a nice little Provencal town and the site is just within the town, only five mins walk to shops etc, excellent, and has newish, very clean facilities.  Well worth a visit.)

Plugged in and checked the polarity on arrival, and all OK.  Went for a last toddle before bed and noticed the red reversed polarity light was now on.  On closer examination, this turned out to be both the green "healthy" light and the red "reversed" light.  Both alight at once should indicate no earth, so this seemed in need of attention.  However, this did seem very odd, since it hadn't been so before - but decided to try the polarity reversal lead anyhow.  Inserted the rev pol lead and the green light went out but the red stayed on: thus there was an earth and the polarity definitely wasn't reversed.

It being too late to fiddle, I put the normal lead back in, with both lights back on and decided to see what the morning brought.

Morning broken, I looked at the connection and just the green light was on again, as had been the case when we arrived.

Found a fellow Brit caravanner and asked him if he had noticed anything odd with the electrical supply.  "Oh", said he, "d'you mean the way the polarity reverses at night?"! 

That night the same thing happened again, some time between 9pm and 11pm, but was again "right" by next morning.  Further chat to caravanner and yes, so had his.

Reported the oddity to the nice bright young Guardien and left him scratching his head and muttering about getting the municipal electrician in to check it all out!

Now, logically, what we saw can't happen, can it? 

My best stab was that there was a substantial, time switched load somewhere on the site, possibly the water heaters, that initiated the problem by overloading an undersized cable or a bad connection somewhere downstrean, so raising the resistance of the main return wiring to significantly above that on the earth, and  inducing current to flow through our reversed polarity lights (wired between neutral and earth) to get at the lower resistance on that circuit.

However, thats just a guess.  Anyone got a better one?

Answers on a postcard please!

Reards to all

[twooks]

'tis the piksies and spirits, ghoulies ghosties and poltergeists >:-) >:-) >:-) B-)

[Clive]

Its most likely that the local three phase supply was not well balanced which results in a significant current flowing in the neutral. The volt drop in the neutral can play games with these supply reversal detectors as they use earth as a referance and when phases are unbalanced there can be several volts difference between earth and neutral, enough to light a LED for instance. The supply to your van will be one phase and neutral plus earth. Hope that helps. But why worry? If you van has an unbalanced current main circuit breaker it will protect you against earth leakage whatever the polarity! Sometimes it will even detect an earth fault in an appliance before its switched ON if the incomming supply is reversed.

[david lloyd]

[QUOTE]Clive - 2006-06-28 11:13 PM Its most likely that the local three phase supply was not well balanced which results in a significant current flowing in the neutral. The volt drop in the neutral can play games with these supply reversal detectors as they use earth as a referance and when phases are unbalanced there can be several volts difference between earth and neutral, enough to light a LED for instance. The supply to your van will be one phase and neutral plus earth. Hope that helps. But why worry? If you van has an unbalanced current main circuit breaker it will protect you against earth leakage whatever the polarity! Sometimes it will even detect an earth fault in an appliance before its switched ON if the incomming supply is reversed.[/QUOTE] That's easy for you to say clive :-D regards, david

[johnsandywhite]

:-D I think it is the night watchman pulling the plug on the main feed to plug his adaptor for his TV in and then plugging into the socket the wrong way round. He reverses the process in the morning when his shift ends. (lol) (lol) (lol) (lol) Just joking. But it does happen. (lol)

[Clive]

I just did!

[Brian Kirby]

Clive

Thank you, I wondered if you might respond.  The indicator lights I was using were, in fact, Maplin neons: I'd followed your earlier advice that LEDs were over-sensitive.

I was rather more puzzled than worried, at least after I'd checked with the reversed polarity lead and got a clear indication that the polarity was not reversed and there was an earth.

I've been on quite a few sites over the years, but never seen polarity apparently reversing itself before, so was fishing for a logical explanation for the apparently impossible. 

Just to be on the safe side, I tried the trip button on my RCD, to see if functioned OK.  It did, but unfortunately the process also tripped the main supply pillar RCD as well, agitating the Dutch neighbours who were using the same pillar!  That puzzled me as well, but we left shortly afterwards, so no more experimenting!

Regards

[Clive]

Hi Brian, If you are up to it (qualified or insured) and have access to a voltmeter with an AC range then measure two voltages. 1) Earth to Neutral 2) Earth to live Voltage 1 should ideally be zero but in unbalanced phase conditions up to 10 volts or so might be seen. Voltage 2 should be 230 ish. If these two voltages are AAF then the supply is reversed. Neons take about 50 volts to strike (from memory) C.

[Brian Kirby]

Thanks for that Clive.  No, I don't think I'll mess around with mains voltages and my little multimeter, at least not until someone else has done it first!

Besides, I'd have to go back to Apt to get relevant results.  Pleasant enough idea, but so many much pleasanter things to do while there than get electrocuted!

By the way, what is an international masseur?  One who massages other nations, or one who massages in other nations?  Other questions ocurr, but this is a family forum.

Regards

[Derek Uzzell]

johnsandywhite: Funnily enough that pretty much describes the incident that caused me to lose interest in the polarity of 230V hook-up supplies. We had connected to a single-outlet 230V pedestal for the night at a free aire de services. I had religiously tested the polarity, found it reversed and inserted a suitable adapter. Another motorhome turned up some time after and the French owner knocked on our motorhome's door asking if it was OK to insert a 'splitter' so we could share the outlet and he could run his TV. In went his splitter and, a little later, I noticed that the mains-tester in our 'van was signalling reverse polarity. As it was now bucketing down with rain there was a real disincentive to going outside and fiddling around with adapters, so I didn't. And, having not bothered once, I never bothered again.

[johnsandywhite]

:-D Hi Derek Uzzell. Read my signature? (lol) (lol) (lol) 8-) I never check polarity either. Some Spanish are wired back to front anyway from one connection to the next. 8-)

[enodreven]

Hi, Can someone confirm the problems or possible problems in using a supply that has reversed polarity ? I was told that it shouldn't be used as on UK applainces the fuse is on the live supply and if it blows it cuts off the live supply to the appliance and if it was to happen on a supply that was reversed and the fuse blew the applaince would still be live ? Is this correct ? Brian

[Brian Kirby]

Brian

The "standard" UK consumer unit has a "positive" feed and a "negative" return, plus an earth.  (However, I understand that some UK manufacturers are now fitting a European standard consumer unit - read on!)  These units have the "fuses" (more commonly miniature circuit breakers [hereafter mcbs]) installed onto the "positive" wiring only, the "negative" being unprotected.  They are inserted at the point from which all the on-board supply circuits (sockets, battery charger, fridge, transformer, lights etc) originate.  Their sole function is to disconnect the supply if the load on the particular circuit exceeds its designed rating: that is to say, to protect the wiring from overload and prevent possible fire.

When the polarity of the incoming supply is reversed, the "positive" feed gets connected to the "negative" side of the consumer unit.  That means that there are no fuses/mcbs between the supply and the on board wiring.  Under these circumstances, if a circuit is overloaded (for example by short circuit) when the polarity is reversed, the cabling can overheat with attendant fire risk.

Continental European practice, as I understand it is to install double pole circuit breakers that monitor the load on both "positive" and "negative" wiring.  Thus, if the polarity is reversed the danger of overload is negated.

Personal protection comes from an RCD type main isolator.  That is the main switch on the consumer unit.  This monitors the total current flowing into, and out of the consumer unit.  If the two currents do not match there is a assumption that electricity is leaking to earth and the whole supply is disconnected. 

That is your protection against electrocution - where a person touches a live connection and provides the earth path - or from a wiring or appliance defect that results in current flowing to earth.  Although the latter would eventually trip the relevant mcb, the RCD is much more sensitive and acts faster.

The greatest protection overall comes from the RCD and, provided this is tested and any reversed polarity is corrected, an on-board installation is as safe as that of any domestic installation. 

However, if there is no RCD, the risk of fire or electrocution is greatly increased.  The risk from reversed polarity is thus relatively small, and seems confined to possible fire in the event of an overload that does not result from an earth fault.  However, it exists and the best general advice remains to correct it.

If your consumer unit has double pole mcbs and an RCD you can basically forget reversed polartiy altogether.

Hope this helps

[Clive]

Sorry but there is no POSITIVE FEED or any NEGATIVE feed from a mains hookup. The polarity (Pos or neg) changes 50 times a second as its AC. The only true POSITIVE and NEGATIVE is that obtained from your battery. From your hookup what you refer to as a POSITIVE is in fact one of the three phases commonly refered to as LIVE. What you refer to as NEGATIVE is the star point of the three phases called NEUTRAL. This NEUTRAL is bonded to earth at the local transformer. This is why the 3 connections of a 13 amp plug are labeled L (Live), N (Neutral) and E (Earth) Here endeth the lesson. C.

[enodreven]

Thanks Brian

[Brian Kirby]

Thanks Clive, and of course you're right.  I'm afraid I resorted to terms I thought might be the most familiar to the greatest number, thinking that may minimise confusion.

However do I agree that it may have been better to refer to "live" (or even "phase") instead of "positive", and "neutral" instead of "negative".

Since the wiring should all be in flexible type cable, I suppose one should perhaps add that for ease of identification the "live" terminals will be the ones with the brown wires attached, and the "neutral" terminals those with the blue wires.

So, if you're still awake, Brian, that about covers it!

I'm still puzzled by this "international messeur" stuff, though.

Regards

[Derek Uzzell]

johnsandywhite: I switched signatures off long ago.

[Clive]

"international messeur" means that I can rub people up the wrong way in several countries. OR I "do" the wife,s stiff neck wherever we may be in the camper. After all I am a buffoon at heart! Derek is the serious fellow. C.