Cyprus-Forum.com

[Eric dayi]

I saw this thread a while back and did not get a chance to add something important for extension lead users....

Many people make a huge mistake in not unravelling the whole of the cable from the lead. What they usually do is undo just about as much as they need and then use the lead like that. This is fine when you are using small loads but at higher loads the cable just becomes a coil of electromagnetic currents and will heat up very very quickly. This can be to a point of if a child or someone even picks up the cable it will melt in their hands and the rest I think you can make out for yourselves. Remember this guys...It is very important. The other things that could happen is that the soldered joints could melt....Better that than a burned hand or a fire.

Zan, i think he was referring to an extention block on a short cable rather than the reel that you are referring to.
Also to get to the pint of melting the pvc, you would need a very heavy load for a period of time to achive this. Have never seen this in practive, but i would suspect that domestic electrical installations wouldn't allow such a large load without a fuse or trip going.

Notwithstanding this, it is always advisable to un-reel all of the cable anyway.

Cheers

Just thought I would tell the dangers of leads and it was not a direct comment on what happened.

Have you ever tried to leave an extension lead coiled and run near to the 13A limit that it can take....It literally takes seconds for back EMF and Eddie Currents to raise the temperature and create even more heat. It works just like a super heater in giant factories. Giant coils can induce heat in metal in less than second that will burn your hand off.

I went to a party once and the host had done exactly that, he left the extension lead coiled up and connected an electrical heater on it. A little while later I saw smoke coming from the coil and told him about it, you should have seen the guy run. Luckily nothing happened and we can laugh about it now but just imagine what could have happened if no one had noticed it?

[zan]

Thanks mate....My point exactly!

[devil]

Have you ever tried to leave an extension lead coiled and run near to the 13A limit that it can take....It literally takes seconds for back EMF and Eddie Currents to raise the temperature and create even more heat. It works just like a super heater in giant factories. Giant coils can induce heat in metal in less than second that will burn your hand off.

Whereas I agree that the temperature rises, this has nothing to do with eddy currents. Why? Because the magnetic field round a flex is negligible because that generated by the current along the phase wire is cancelled by that along the neutral wire. It is caused by simple IR losses and the heat cannot dissipate easily because of the surrounding wires and so the temperature builds up. In any case, even if it were a single wire with no compensating wire, it would be difficult for any significant heating to occur at 50 Hz. Industrial induction heating is always done at high frequencies. Please don't try to blind people with bad science.

[zan]

Have you ever tried to leave an extension lead coiled and run near to the 13A limit that it can take....It literally takes seconds for back EMF and Eddie Currents to raise the temperature and create even more heat. It works just like a super heater in giant factories. Giant coils can induce heat in metal in less than second that will burn your hand off.

Whereas I agree that the temperature rises, this has nothing to do with eddy currents. Why? Because the magnetic field round a flex is negligible because that generated by the current along the phase wire is cancelled by that along the neutral wire. It is caused by simple IR losses and the heat cannot dissipate easily because of the surrounding wires and so the temperature builds up. In any case, even if it were a single wire with no compensating wire, it would be difficult for any significant heating to occur at 50 Hz. Industrial induction heating is always done at high frequencies. Please don't try to blind people with bad science.

`One extreme I used Devil that's ALL. So the collective Eddie currents do not cause an effect in cable then to add to the resistance and cause more heat. I agree with the heat loss as well of course but all the factors together..... What happens to the cable that is unrolled and that too gets hot enough to melt the cable. This is not something I am guessing at but have remembered my college days as well as seeing at first hand. Add to that bad workmanship and loose connections and the resistance goes mad.

I again can understand if the cable was correctly sized and the connections perfect and the three pin plugs at their best but collectively would you recommend the same as me or not. I think we should not confuse people with differing ideas and just expert advise and as you are the better qualified then I accept that as your role..I am more interested in peoples safety as I am sure you are.

[devil]

If safety were an issue, then we would not have the UK type plugs and sockets/ring mains, which can lead to downright dangerous situations, as I know from experience. Did you know there are more per capita electrocutions in the UK than in most other countries?

[zan]

If safety were an issue, then we would not have the UK type plugs and sockets/ring mains, which can lead to downright dangerous situations, as I know from experience. Did you know there are more per capita electrocutions in the UK than in most other countries?

To be honest with you, I don't really know much about other systems. I got into a system of wirering heating systems so long ago that I think I have lost interest in furthering my knowledge. What system would you say is the best? I did not know about the UK either and am quite shocked to be honest...No pun intended!!! What is wrong with a ringmain????

[devil]

The main problem with ring mains is that a fault in the wiring can go undetected. Hypothetical example: you have, say, 5 sockets on a ring and you have three appliances drawing 11 A each on sockets 1, 2 and 3. No problem, as the cut-out is rated at 35 A. Now, let's say the electrician installing the wiring accidentally broke the ring at 4, so that sockets 4 and 5 are isolated from 1, 2 and 3. That means all the IR losses, causing heating, are on an inadequate wire section for the total current and no one would know, because all the sockets appear to work fine. AFAIK, only the British system uses ring mains in the world.

I cannot say which is the safest system in the world. I lived in Switzerland for 35 years and they have very few per capita electrocution deaths, even though switches and power sockets are permitted in bathrooms. Same in Germany and France, although different socketry is used (I prefer the Swiss, as it is more compact, but that is a personal preference). It is also DANGEROUS to put switches and fuses on power sockets/in plugs, as it gives a false sense of security should the phase and neutral be inverted. Don't tell me this does not happen. It does and I can tell you that the Electricity Authority of Cyprus made just this mistake wiring up the single-phase electricity meter at this house. It took me a day before I realised that the neutral was 240 V above earth!!! That could have caused severe problems if I hadn't been an electrical engineer and was able to detect the problem; imagine if it had been an 89-y.o. widow switching off an appliance at the power socket (or with the fuse blown in the plug) and the appliance was still live. That is why continental sockets/plugs never have switches or fuses associated with them; you have to physically pull the plug to isolate an appliance, inherently much safer.

[zan]

The main problem with ring mains is that a fault in the wiring can go undetected. Hypothetical example: you have, say, 5 sockets on a ring and you have three appliances drawing 11 A each on sockets 1, 2 and 3. No problem, as the cut-out is rated at 35 A. Now, let's say the electrician installing the wiring accidentally broke the ring at 4, so that sockets 4 and 5 are isolated from 1, 2 and 3. That means all the IR losses, causing heating, are on an inadequate wire section for the total current and no one would know, because all the sockets appear to work fine. AFAIK, only the British system uses ring mains in the world.

I cannot say which is the safest system in the world. I lived in Switzerland for 35 years and they have very few per capita electrocution deaths, even though switches and power sockets are permitted in bathrooms. Same in Germany and France, although different socketry is used (I prefer the Swiss, as it is more compact, but that is a personal preference). It is also DANGEROUS to put switches and fuses on power sockets/in plugs, as it gives a false sense of security should the phase and neutral be inverted. Don't tell me this does not happen. It does and I can tell you that the Electricity Authority of Cyprus made just this mistake wiring up the single-phase electricity meter at this house. It took me a day before I realised that the neutral was 240 V above earth!!! That could have caused severe problems if I hadn't been an electrical engineer and was able to detect the problem; imagine if it had been an 89-y.o. widow switching off an appliance at the power socket (or with the fuse blown in the plug) and the appliance was still live. That is why continental sockets/plugs never have switches or fuses associated with them; you have to physically pull the plug to isolate an appliance, inherently much safer.

I agree with what you are saying here but things are changing to accommodate for these errors. The introduction of MCB's and even more important RCD's is one step closer to safer systems and the new laws concerning domestic installations is also a great move to try and stop DIY'ers from having a go and even more dangerous builders that just throw in a system in an extension that have no idea what they are doing.

I don't quite get where you are coming from on the switching thing because ALL sockets in the UK are double pole so both live conductors are disconnected when switched off. In line switches, if that's what you mean are a different issue and are usually factory fitted and should also be double pole.


The electrocutions thing is a big concern though and even that has been addressed with the introduction of the RCD in all circuits that have outlets for outside use.

I hate the idea of having switches and sockets in the bathroom though and when I see how blase people can be in relatively safe situations, it sends a shiver down my spine to think what they would do with it in such a dangerous location where shock is very likely to cause a death. The RCD may have a part to play in that but in the case of a faulty one or the loss of a good earth then you have no chance. I went to house six months ago and did a perspective fault current test and the external earth was fine...Went back there recently and the ground has dried up so much that the earth rating is half what it was. Scary that the electricity company no longer guarantees the earth in the UK.....One step forward and two back it seems.

[devil]

There are many single pole switches on UK sockets, especially on extension leads.

However, the continental philosophy on bathrooms and suchlike is that if a power socket is not available for hairdryers, heaters etc., then people will bring in extension leads anyway and THAT is infinitely more dangerous than properly designed sockets and switches.

I agree that the copper pole in the ground is a very variable and potentially dangerous earth connection and even more so, now that people rely more on earth leakage current cut-outs. In Switzerland, there is only a single earth connection for a whole district, situated at the base of the step-down transformer and connected to neutral at that point; there is a 5-wire distribution system, 3-ph+N+E to each house, all 5 wires with the same cross-section, not 4 wires like here. One thing that really got me here was that I found that the earth wiring in my house is done with a much smaller cross-section than the phase and neutral. When I was a student, it was drummed into me that the earth wiring should be able to carry at least the same current as the phase so that, in the event of a fault, the earth connection should be the last to fail. I can assure you that if the EAC permitted it, I would have had the house rewired to Swiss standards (and the plumbing done likewise!) long ago.

[Smiler Brian]

When I worked at Bloodhound camp (near Episkopi) part of our weekly servicing was to pour several gallons of water onto the earth spikes. When it was not done for a few weeks we had all sorts of electrical problems.