reverse polairty

[Alanb]

Brian Kirby - 2018-01-27 6:23 PM

 

Alanb - 2018-01-26 10:32 PM.......................

As the neutral is bonded to earth at the local transformer (substation), and the earth is not carrying current, the voltage drop along the neutral will appear between earth and neutral at the remote end. That is at the consumer premises. If this voltage is sufficiently high to impart an electric shock, there is something wrong with the 230V supply network.

 

The reason for the safety earth is to prevent any metal enclosures or other metalwork becoming live in a fault situation. By bonding such metalwork to earth, a fault to the enclosure effectively becomes a live to neutral fault via the earth connection, thus causing the fuse to blow, or MCB to trip. This method of earth fault protection has been superseded by RCDs, but still provides a second layer of protection.

 

Alan

Thank you Alan, and no need to apologise. As I said I'm not an electrician or engineer, and was trying to remember what I hade been told years ago.

 

So, is your first paragraph above to be understood as meaning that Michael's statement that with the live switched "off the circuit is dead" is a reliable guide to safety. If so, I owe Michael an apology for mis-correcting him! :-)

 

Where I live the mains cables are old, and are generally thought to be running a bit "hot" these days due to the greater number of electrical appliances we all now use. Are we therefore safe in assuming that the 230V network will be maintained in a fault free condition? Our earth is provided via the armouring of the incoming main (which is terminated in our house but branches on to supply next door). I'm not aware that anyone has checked the actual resistance since we moved in, over 30 years ago.

 

My comment regarding the design assumption of a safety earth being that its resistance will generally be lower than that on the neutral was not intended to query the reason it is provided, just to try to explain the circumstances, as I remembered them, under which a risk of shock might arise of only the live had been isolated.

 

I am aware that a great number of houses dating from the 1930's (and later) contain remnants of very old wiring, still have fuse-boxes, and definitely don't have ELCB or RCD mains switches. As it happens, ours is on an ELCB, for which I believe a good earth is still essential? I appreciate this strays from motorhomes insofar as their internal installations are concerned, but many folk connect them to house supplies from time to time, so has some relevance.

 

Brian,

 

I as I am not an elelectrical distribution engineer, or electrician. I cannot claim to be an expert on the matter, so if anyone finds fault with my answers, please say so in the interests of safety.

 

For your first question the answer must be yes. I did suggest two abnormal conditions that might cause a dangerous situation. These were both fault conditions, the first being for a very short time. The second involved damage to the neutral conductor and not the live, not impossible but very improbable. Also you would have to be touching both earthed metalwork, and the neutral.

 

Your second question relates to older supply networks. In my opinion your local electricity distribution company would be failing in it's duty of car,e if it failed to take reasonable care to maintain the network.

 

I would not expect the safety earth conductor to have a lower resistance than the neutral. Think about your domestic wiring in this respect.

 

I have only met ELCBs where the ground conductivity was poor e.g. rocky areas. In these circumstances the trip coil of the ELCB is connected beteen the consumer's earth terminal and an earth rod, which was required to be remote from any parallel earth. From memory, I think that the ELCB, was required to trip before the voltage across the coil exceeded 40V. So in your domestic installation you earth via the cable armour, is in fact monitored by the ELCB. A good earth is alwars desireable, but in this situation not essential.

 

The ELCB has been superseded by Residual Circuit Breakers (RCB), Residual Current Circuit Breaker (RCCB) or Residual Current Device (RCD). Different names, but all check that currents in live and neutral are equal in magnitude. Their great advantge is that they will trip without a solid earth connection, and hopefully prevent fatal electric shock if a conntact is made with the live conductor.

 

The retrospective fitment of a RCD is generally recommended, but split load systems where the RCD only protects the power circuits may be preferred. With split load, you are not plunged into possibly dangerous darkness, if your domestic water heater, or iron goes on the blink.

 

More applicable to motorhomes is the use of an RCBO. My first MH, a Pilote R390 was sold as being fitted with a RCD. This was not true. It only had a double pole 16A MCB. It was soon replaced with a 16A RCBO (Residual Circuit Breaker + Overload). May I suggest that that anyone with an older MH having only an MCB does likewise. (An RCD is identified by having a test button.)

 

Simple definition as I am worried about upsetting a "L'aime Duck" -- Residual current = difference between current in live and neutral conductors. That is the bit that leaks to earth by whatever route.

 

On a closing note a RCD should still trip if supply polarity is reversed, even in motorhomes.

 

Alan

 

 

 

 

 

 

[rupert123]

Ye Gods, the original posters head must be going round in circles by now. He asked a simple question and some of the answers would make an experienced electrician go round in circles.Look it is really simple, reversed polarity is no problem at all if you use even the slightest bit of common sense. Plug the van in and forget about it. The common sense bit, if you are not an experienced electrician do not go poking around inside electrical gear without turning the supply of at the mains or unplugging the item. It seems to me that some on here just love bits of gear, no matter how useless and love fiddling with them, fair enough, but lets not pretend extra leads and meters to test and correct reverse polarity are needed, throw them away. What are my qualifications for saying this. Well five year electrical fitter apprenticeship in HM dockyard Chatham three years as the electrical officer on BP tankers, several years working as an electrician. This is I admit a fair while ago but I have tried to keep up and stuff like reverse polarity is pretty basic. Perhaps some others would care to state there qualifications on here we then get some idea who to take notice of. I am aware a few have but not enough.

[michaelmorris]

"Thanks for the link. The article seems to say that reversed polarity shouldn't be a safety issue in day to day use. Also in the vast majority of cases reversed polarity should not interfere with the functioning of equipment attached to it. However, there MAY be a functionally issue with some pre-2002 mains battery chargers and some electronic equipment. Is this correct? "

 

"Yes, that is a fair summary."

 

 

Thanks. Given the nature of some of the electronic equipment I sometimes take away with me, I'll get a test plug and polarity reversing cable just to be 100% sure I don't have any problems. :-D

[Alanb]

Please note that my previous rather lengthy post was only made in direct response to the questions posed on open forum by Brian.

 

l agree with Rupert, and Allan that there is nothing to worry about. Ignore the scaremongers.

 

However in response to Rupert's mention of qualifications, I have always considered it rather immodest to flaunt them in public.

 

Alan

 

 

[aandncaravan]

michaelmorris - 2018-01-28 9:20 PM

 

"Thanks for the link. The article seems to say that reversed polarity shouldn't be a safety issue in day to day use. Also in the vast majority of cases reversed polarity should not interfere with the functioning of equipment attached to it. However, there MAY be a functionally issue with some pre-2002 mains battery chargers and some electronic equipment. Is this correct? "

 

"Yes, that is a fair summary."

 

 

Thanks. Given the nature of some of the electronic equipment I sometimes take away with me, I'll get a test plug and polarity reversing cable just to be 100% sure I don't have any problems. :-D

 

 

 

Michael, if you are carrying around electronic equipment that is over 16 years old, you are not doing yourself any favours as most electronics, like TV's, computers, etc. are rarely viable or reliable after 10 years.

For example most TV's older than about 7 years will have a significantly higher power draw on the 12v battery, compared to newer LED technology that became mainstream around that time. 'Green' energy TV's of the last 3 years improve on that even further.

 

A modern charger will be at least twice as powerful as one from 1999. A TV from that same era is likely to be a CRT design, weigh a ton, have really high power drain and poor quality picture.

 

A review of any outdated electronics might transform the usability of he vehicle, as well as taking away the worry of even thinking about reverse polarity.

 

 

 

[Derek Uzzell]

michaelmorris - 2018-01-28 9:20 PM

 

...Thanks. Given the nature of some of the electronic equipment I sometimes take away with me, I'll get a test plug and polarity reversing cable just to be 100% sure I don't have any problems. :-D

 

If you are going to check whether a campsite 230V outlet’s polarity is reversed (or the outlet is operational or has an earth) it would be logical to do so before connecting the motorhome to the outlet - and to do that you’d need one of these.

 

https://tinyurl.com/ybf9ewbr

 

(Of course there’s always the possibility that any commercially-available ‘adapter’ will have been wired up so that its polarity is reversed...)

[rupert123]

Alanb - 2018-01-28 10:56 PM

 

Please note that my previous rather lengthy post was only made in direct response to the questions posed on open forum by Brian.

 

l agree with Rupert, and Allan that there is nothing to worry about. Ignore the scaremongers.

 

However in response to Rupert's mention of qualifications, I have always considered it rather immodest to flaunt them in public.

 

Alan

 

Fair enough Allan but I have to disagree about qualifications. There are just to many people who post after just looking up an answer on a search engine, we can all do that. However there is no guarantee the answer found is correct and nothing beats personal knowledge. I am not really saying if you do not have a qualification you are not correct, many on here have years of motor home experience so have come across a problem and had it fixed. Passing this knowledge on is often invaluable but commenting on technical stuff with no background expertise or personal experience is a waste of time and dangerous. I would far rather know a persons background so trust their judgement than just have an answer from someone who has done an internet search or heard it from a friend of a friend.

[michaelmorris]

aandncaravan - 2018-01-29 9:02 AM

 

michaelmorris - 2018-01-28 9:20 PM

 

"Thanks for the link. The article seems to say that reversed polarity shouldn't be a safety issue in day to day use. Also in the vast majority of cases reversed polarity should not interfere with the functioning of equipment attached to it. However, there MAY be a functionally issue with some pre-2002 mains battery chargers and some electronic equipment. Is this correct? "

 

"Yes, that is a fair summary."

 

 

Thanks. Given the nature of some of the electronic equipment I sometimes take away with me, I'll get a test plug and polarity reversing cable just to be 100% sure I don't have any problems. :-D

 

 

 

Michael, if you are carrying around electronic equipment that is over 16 years old, you are not doing yourself any favours as most electronics, like TV's, computers, etc. are rarely viable or reliable after 10 years.

For example most TV's older than about 7 years will have a significantly higher power draw on the 12v battery, compared to newer LED technology that became mainstream around that time. 'Green' energy TV's of the last 3 years improve on that even further.

 

A modern charger will be at least twice as powerful as one from 1999. A TV from that same era is likely to be a CRT design, weigh a ton, have really high power drain and poor quality picture.

 

A review of any outdated electronics might transform the usability of he vehicle, as well as taking away the worry of even thinking about reverse polarity.

 

 

The equipment I'm, talking about relates to one of my hobbies (astronomy), not motorhoming. I sometimes take some of this kit with me If we're going to be staying at a site which is nice and dark.