extension leads

[Fred]

Hi,

About ten years ago I bought an extension lead from Wickes. Not knowing any better at the time, I bought a long one, thinking that a long one could be used when long or short lengths were required, whereas a short one could only be used for short lengths.

Then I came and read here and I learnt that long leads had disadvantages. I can't remember now whether this was because of voltage drop or fault currents, or may be both. Can someone remind me?

Anyway, some of my tools have started switching on and off intermittently when plugged into the lead, and I can only assume that over the years the contacts in the extension socket have been worn and are not making a good connection to the plug.

So it is time to buy a new lead or leads. I seem to remember reading that 1.5mm^2 cores could only be used safely up to 15m and 2.5mm^2 cores (in a caravan power lead) could be used up to 25m. Is there any advantage to buying a short "caravan hook-up" cable to use for lengths under 25m?

Otherwise I'm thinking I could use the cable from the Wickes lead, or possibly use arctic flex as an improvement, to make some short leads.

TIA

[Brian Gaff]

Some of those older sockets were amenable to being taken apart and resprung to make them work again. I found those four way ones tended to have a problem where they connected to the bus bars. its all by pressure and over the years things relax and get tarnished. Brian

[F Murtz]

Replace the sockets instead of buying new leads if they are structurally sound.

[Hugh - Was Invisible]

Biggest problem with modern long leads is that if you use much power you have to unwind the whole lot to prevent overheating. If you use thicker cable you add to the size and weight. Intermittently used power tools are probably not a problem but our wallpaper steamer is.

[Martin Brown]

Or in the case of our local church fete a pair of 3kW kettles on a mostly rolled up mad hot extension lead plugged into the wall. They did it last year as well completely melting the inside of the reel. *SCARY*

I am inclined to the view now that they should include a thermal cutout! The clear warning on the reel has little or no effect on most folk :(

[John Rumm]

The modern 50m JoJo ones do include the cutout these days... presumable for this sort of reason.

[John Rumm]

The answer is both, and also "heat"...

The longer the lead then the more voltage drop. May not be a huge problem for many tools (power tools run a bit slower and with a bit less oomph), but some kit may get upset (things with induction motors that may stall on startup, or run but attempt to compensate for lack of volts by drawing more current, and then overheating)

The second issue is fault current, or to be more accurate lack of it. Long leads will have a higher earth loop impedance, and also a lower prospective short circuit current. So when something goes pair shaped the fuse in the lead takes longer to operate, or in the worst case, does not operate at all in a sensible time. Making sure its fed from a RCD protected socket is obviously a "good thing" (tm).

Lastly, heat. Long leads can only take a small proportion of their full load when wound. Hence for big loads at moderate distances, you may find yourself having to unwind a rats nest of cable to use it safely.

Either that, or the flex has broken and gone intermittent at the ends where it bends a lot.

The socket terminals might allow a bit of cleaning and bending to get a better grip. The flex can have the first and last metre cut off and be re-terminated to get rid of the dodgy bits.

Depends on the loads you need to run partly. If you are doing big ones at moderate distance, or ones for which voltage drop is going to be a problem, then it may be worth it.

[Dave Liquorice]

Both but as others have said volt drop won't upset many things a great deal. Fault currents could be an issue but as you will always be running the extension via an RCD (won't you...) that isn't such a problem either.

If the cable is in otherwise good condition just replace the socket and if the plug isn't an RCD one replace that as well. You may say that all your 13A sockets are RCD protected but what if you use it elsewhere?

A little bell jingles that the length limits are something like that so as not to have too much (FSVO "too much") volt drop at full load.

I have a couple of 25m "caravan hook up" cables, for several reasons:

The cable is bright orange. The cable is nice and flexible and coils nicely. They are 2.5mm^2 cross section.

It is quite chunky stuff to have a 13A plug attached, so I have 16A IEE connectors and adapters to 13A, primary use is from the generator but as the IEE's are bit more water resistant than a 13A the pressure washer has an IEE plug on it.

Unless you are working outside in winter I wouldn't bother with arctic flex, the caravan hook stuff handles well and doesn't fight back a great deal even when near freezing.

[Dave Plowman (News)]

Two 3 Kw kettles on the same extension should have blown the plug fuse and quickly.

[Andy Burns]

A 13A fuse is required to carry 1.6x the rated current without blowing, and only required to blow within 30 minutes for 1.9x rated current, so whether, or how quickly it should blow, depends on the duty cycle of the two kettles ...

[ARWadsworth]

The extension lead Geoff gave me has a thermal cut out and it has operated twice.

Once when a pillock plugged in a 2kW heater without unwinding it (he was told not to) and then again when another pillock decided to charge up two cherry pickers without unwinding it (without asking to use it).

I was suprised at how long it took to reset the trip/cool down even after I had unwound the lead.

[js.b1]

If the cable is ok & 1.25mm^2 you could just replace the rest...

- Check the plug & strain relief are ok

- Remove any reel, they invite coiled use to avoid a trip hazard only to overheat

- Fit a new duraplug or generic rubber 2-way or 4-way socket strip

If the cable is not ok...

- Buy or make up a short lead (10m) and long lead (20m)

- Do not go so short you need to daisychain leads, that invites other problems

Cable sizing...

- For 13A load IEE guidelines are 1.5mm^2 to 15m, 2.5mm^2 to 25m.

- The aim is to prevent sufficient voltage drop that operation of equipment at the end of the lead is impeded (motor does not start, sits there cooking) or a short circuit does not permit sufficient current flow to blow a fuse (cable sits there melting, cable becomes the fuse)

- Real world is it depends on what you are going to stick on the end of it.

Fridge-freezer.

- A long extension lead may cause sufficient voltage drop to prevent the compressor starting, ruining food or creating a fire hazard.

- However, 25m of 2.5mm^2 flex from a CU is no different to 25m of

2.5mm^2 flat-twin-&-earth from a CU. (The CPC is actually larger in the former, but not my point).

Tumble dryer.

- A 3kW load that melts solid extension leads not fully unwound, or equally just dumped on the floor.

Pressure washer with beefy 1.7kW induction motor.

- Despite the spec, 25m on 1.5mm^2 is fine because it is only a 7A load and the motor is not likely to stall.

Battery charger & light, soldering iron 50m away in shed.

- Gets more interesting when you add a 3kW fan heater or big induction motor saw.

- Reality is the longer the cable, the more permanent its installation, the greater the risk of damage occurring.

Never forget the RCD and test it. Avoid reels, they seem to induce people to keep cable wound up to avoid a trip hazard yet can create an overheating risk. Not all extension reels have thermal cutouts and quality of stuff these days is a bit hit n miss. Warranty claim after the carpet has a reel shaped burn in it is not going to go down well :-)

[NT]

The downsides of long leads are outweighed by the upsides, and both are minor issues anyway. Just fix whatever's broken on your lead.

If you replace plug or socket, cut 6" of flex off that end.

NT

[Graham.]

Similar story, this time a conveyer type toaster in the breakfast room of a Premier Inn. Wound-up extention cable under table geting noticably hot.

I told the maître d?hôtel (joke) that it was dangerous and needed unwinding, and that I was an electrical engineer (a lie) and knew about such things.

It was still there the next morning.

[Martin Brown]

Had a look, but would want a 10m one or maybe 20m and they don't seem to have thermal protect at least according to the summary at Farnell.

Still preferable to ending up with a molten mess of plastic and cable all gummed up together. I was amazed the previous one didn't catch fire!

I also reckon they should be aggressively limited to no more than 20A for 10s to avoid other obvious stupidity like 2 kettles which is an overload condition on a 13A socket even with the cable unwound.

[Martin Brown]

Not quickly enough! About 10 minutes continuous use I would guess and kettles tend to get used with only a small amount of overlap apart from at the beginning when they boil both to get started. I can assure you the 13A plug fuse did not blow in the time the water took to boil.

The fuse also recovers from thermal stress much faster than the bulky reel of wound up and increasingly hotter cable.

I suspect part of the problem *IS* that people think the fuse will always protect them if they are doing something wrong.

[The Natural Philosopher]

Its very easy to melt a coiled up reel of cable at less than 13A. Most of them say exactly that.

[NT]

Inrush of around 3x running current would give in the region of an amp on a modern domestic appliance. Plug fuses don't seem to fussed about motor startup current in practice, perhaps partly because these fuses are mostly rated at well above motor rated i anyway.

NT

[John Rumm]

Although less common on the shorter leads, there are some by the looks of it:

> The extension lead Geoff gave me has a thermal cut out and it has >> operated

Two 3kW kettles probably would take out the plug fuse in under a minute.

[John Rumm]

If there is only a small overlap in use then you would not really expect the fuse to blow. That is diversity in action for you ;-)

Its reaction is also as fast as its recovery though...

However the cable heating aspect is not really something you can expect a fuse to do anything about, since it needs to be sized to provide adequate fault protection for the cable in the first place, and overload protection for the *maximum* rating of the cable in the second place. However with a fully or partially wound lead you will not get anywhere close to the maximum load before it overheats. You ideally need an additional thermal protection mechanism in the drum of the lead itself to cope with this.

Possibly... many don't understand that its there primarily for fault protection any not overload. The first its very good at, the second its "ok" at in severe cases, but a bit patchy in the moderate cases.