Dangerous usage of coiled up electric cables
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I'm sorry but you are incorrect. The reason the coiled cable will overheat is because it becomes an "auto transformer" increasing the induced voltage and with it the temperature- similar to the old fashioned HT coil on a car. The size of conductor will not prevent this. Ex Electrical Engineer 🤓
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I think that there’s a little too much emphasis on the theory of coils here. Electricity passing through a wire creates heat - always. Since the heat produced is in proportion to the cable’s resistance, the rule of thumb is the thicker the conductor the better, because the resistance is less. Nonetheless some heat is still produced. In most cases the heat is shed into the surrounding material, Hence the feed to your home cooker loses heat into the plaster which surrounds it. It’s a thick cable, so there’s not a lot of heat to lose anyway.
The reason that we have unsightly pylons stitched across our countryside is that unwanted heat can be readily shed into the atmosphere, which would be less true if the cable were buried.
The reason that caravan EHU leads aren’t supplied on drums is that winding it onto a drum would reduce its ability to shed heat. A large lose coil of 2.5mm EHU cable, lying under your van will have sufficient heat shedding capability to be used safely. A home made solution of winding surplus cable, and worse still, thinner cable, onto a drum, to keep things tidy, may reduce heat loss enough to melt the cable. Since the highest continuous demands for electricity are likely to occur on the coldest days, nature provides a convenient increase in cable heat loss.
In summary, there’s no need to fret about safety when you see a bundle of lose orange, thick cable under someone’s van. The time for concern is when you spot a tightly coiled cable with little or no airflow around it.
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That’s quite correct. It is simply that cables passing current produce heat and coiled cables have no way of dissipating the heat which then builds up and may melt the insulation causing fire or the potential for electric shock.
Some years ago there were scare stories about coiled cables for a slightly different reason. The coil produces an inductance (ac resistance) which acts to restrict current flow so under fault conditions insufficient current would flow to blow the fuse in the plug so the fault would not be cleared and therefore there would be the possibility of electric shock. I did some calculations and discovered that for the kinds of cable that we are discussing the inductance is nowhere near high enough to create the situation described. Inductance by the way doesn’t produce heat, only resistance (“I squared R”).
The most dangerous fault is live to earth where, for example, metalwork becomes live, however, nowadays we use RCDs (earth fault protection). The resistance in the cable would have to be extremely high for the RCD not to trip.
Sorry if this is a bit too technical but I just wanted to put to bed the dubious theories about auto transformers etc. It’s simply the heat built up of the cables natural resistance and with the standard UK hook up cables there is unlikely to be a problem except when tightly spooled, at full current, in hot conditions.
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Re the dubious theory of auto transformers I have seen evidence of a lightly coiled extension cable burnt out whilst supplying a 40 watt soldering iron which would demand 0.16 amps. A 50 Hz supply WILL generate an emf which WILL create an auto transformer effect with the associated heat build up. Ignore this at your peril.
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On site surrounded by continental motorhomes and caravans, they all have drums in use. I don't know about regulations but they seem to have a 10 inch separation rule between units, not 10 feet. 1650 pitches and not a colour coded peg in sight. Not two outfits parked the same way............bliss. Can't wait to get back to our beautiful over regulated sites full of home grown experts.
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Our household cable reel rated for 13 amps says it can be used fully coiled at 8 amps. So as they only often get 10 max they should be OK with one of our higher rated cables. However, some of the continental cables seem a bit on the thin side, so who knows.
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I had mine on a H shaped bracket, tightly coiled for 3 years. Only enough cable released to reach the bollard. I lived to tell the tale👍🏻. PS-when packing away I never noticed any heat in the cable🤔
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Coiled cable on a drum when conected to the electric supply weather black or red creates a magnetic field the tighter the coil the hotter the cable gets someting to do with radial or linier waves well that is what I learned in physics many year ago.
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The outer colour of the cable has no bearing on the business bit on the inside. Ours is blue artic cable. Hmm, blue means cold - we’ll be OK then😂😂😂
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Physics are great, all those letters, numbers & stuff but real life experience proves all those ‘physics dictate’ quotes wasted really-sorry☹️
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Interesting. Transformers do suffer from core loss heating due to eddy currents in the magnetic core (the heat being caused by the resistance in the metal). In the case of your soldering iron it seems like something else is going on because there is no metal core in a coiled cable. Many strange things happen with electricity!
As has been said above, continental hook-up leads are often much smaller diameter than UK (1.5mm2 is quite common) and you often see them on drums, however, Club site 16 amp supplies are unique and it is quite rare to see anything much more than 6 amps on the continent, especially as you go further south. One site in Italy that we go to regularly is about 4 amps. Others say that 10 amps is common but I see that very rarely.
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The thinner cable is probably fine for over there at 10a, BB. I contemplated changing ours for the lighter thinner variety but it would be borderline for 16a. Instead, I have two cables of different lengths for ease of use.
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The outside diameter of a cable (refered to above as "thickness") is no indication of the size of the metal (usually copper but might be aluminium) cores inside that carry the electricity. If the insulation in the cable is not all that good at insulating more of it will be needed. The continental black cables may just have thicker copper and thinner better insulation.
And just to continue with spreading usefull (in the pub quiz) technical information, electricity does not flow "through" the metal of the cores. The electron movement is concentrated on the surface of the strands. So a core made up of thinner, but more of them, strands will have the capabaility to carry a greater current. The continental black cables may just have thinner strands.
And lastly, just for the pedants, if the cable is doubled back on itself from the mid point, and the mid point is wound onto the drum first, the inductance effect will be self neutralised leaving only resistance to melt the insulation. Maybe the black cables have higher melting point insulation.
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Purely out of curiosity I have just looked at the 3 domestic extension leads I have in the shed. all 3 have two different ratings printed on them. one use when still coiled around the drum, the other for fully extended. Ball park figure is the maximum load is reduced by about a 1/3rd on each of them if used coiled up.
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