Electrical extension cables

[creswell]

I welcome your thoughts on extension sockets, i am constantly finding extensions plugged into other extensions,
i always point out this practice must stop, however i am being told on the other hand that the appliances plugged in are low voltage and of no risk, this obviously cannot be good practice, you thoughts please

[John Webb]

The minimum number of joints in any electrical circuit is always 'best practice', so in my opinion it is far better to have a reasonable number of sockets in every room to eliminate the use of extension cables. With extension cables it is important that:
They are fused to the correct capacity
They are unwound if the reel type, especially if powering anything of more than a few watts consumption
They are not laid under carpets or any other floor covering because of the risk of unseen damage, overheating and possible trip hazard.

However, I'd far rather people used extension cables with multiple socket outlets and a single plug in the wall socket than any form of 'plug-in adaptor'. These latter items should be banned outright. I have seen a number of examples of overheating and fire caused because:
(1) the adaptor is not fused and has been electrically overloaded by too many high-wattage appliances being attached to it;
(2) poor contact developing between adaptor and socket because the socket is mechanically overloaded by the weight of plugs and cables plugged into the adaptor.

[Tony]

Extension leads plugged into extension leads is not good.  Each plug/socket connection is a week point - if you need a longer lead, BUY one.  Or better still, have extra points wired in where needed.  Do these multiple leads go from room to room?  Or worse still, outside?  Both are no-goes in my view.

John, surely it would be better to have a single, overload protected, multiple outlet powerboard plugged into the wall socket?  Rather than adaptors plugged into the end of an extension lead?  As fire safety is your field, I'll bow to your view here, especially as things could be very different over there.  Here, it would be a powerboard and minimum use of extensions (and if any lead goes outdoors, an extra RCD even if the switchboard was protected).  And preferably get an electrician in to add additional points.

Whatever.

[John Webb]

VolFirie, Yes, I quite agree with you on all counts.
I think you're outside the UK; you may not be aware that we have a system of 'Ring Mains' in which a loop from fusebox (distribution board) runs out around a building, or part of one, back to the board and joins up with itself. Sockets to BS 1363 (a British Standard) are placed on the loop. Every appliance plug has a fuse in it, normally 3Amp for radio, TV and other low-wattage appliances and 13A for heaters, washing machines etc. Fixed appliances like the latter can be fed from a permanently wired, switched and fused connection box.

The bext extension cables are those that end with at least two, sometimes up to six socket outlets, usually in a shatter-proof semi-rigid plastic or rubber material. They are protected from overloading by the fuse in the plug and therefore are reasonably safe.
The 'Plug-in' adaptors I'd like to see banned are a rigid plastic cube-shaped 'box' which plugs straight into the wall socket and may contain up to four sockets on it's various faces. It cannot be built to the BS, because the BS does not recognise them. It may or may not have a fuse; in the latter case it is very easy for two or more high-wattage heaters to be plugged in and used - electrically overloading the plug on what are already mechanically strained socket contacts and plug pin contacts. Eventual overheating is almost bound to happen. (Electrical fires were the first subject I was introduced to when joining the fire safety world nearly 40 years ago! It has remained a subject of interest ever since.)

[Tony]

Thanks John, very interesting.  And yes, as you can now see from my name block, I'm not in the UK!  

A ring-main for power - here it's more like the spaghetti that runs from the back of some pumpers...  Each line is seperately fused (more commonly a circuit breaker now) at the fuse box.  An RCD is also required on new premises, I keep meaning to ask a sparky in the Brigade to come and fit one for me.

I've seen extension leads like the one you described, but only at a building site!  They're rare, perhaps home-made by the individual electrician?

And those adaptors?  I agree, ban them!  Too many people here STILL piggyback them, even though protected powerboards like I mentioned work out cheaper!  

I'm always fascinated by the different ways of different countries.   Sorry for hijacking your thread cresswell...

[John Webb]

....I've seen extension leads like the one you described, but only at a building site!  They're rare, perhaps home-made by the individual electrician?.....

They are available here 'off the shelf' either prewired, usually with 2, 5 or 10m of cable, or without wire for people to make up their their own cable lengths.
On British building sites there is legal compulsion to use 110 volts supplied from isolating transformers to provide as much safety as possible, and 16amp unfused plugs to an international standard are used so there cannot be any accidental misconnection between 110 volt equipment and the 230 volt domestic mains.

My apologies to Creswell too, but this subject is one dear to my heart.....!

[kurnal]

Apologies for gatecrashing this thread- I would be interested to pick up on Volfirie's description of the Australian wiring systems which appears to be based on a radial wiring system as used to be common in the UK before the days of ring mains and fused plugs on appliances.  Do you have fused plugs on appliances or different sized plugs for different fittings?

In the old days in the UK we used to have small two pin 5 amp plugs, and bigger three pin earthed 15 amp plugs. Householders were expected to know the difference.  

Nowadays with each appliance plug being identical and individually fused, the ring main socket outlet being capable delivering 13 amps on a circuit capable  of maintaining up to 30 amps (arguably) we rely on that plug being correctly fused for fire safety.

My experience is that most table lamps. hi-fis tv sets etc invariably have a 3 amp cord and  householders will eventually fit a 13 amp fuse when the factory fitted 3 or 5 amp fuse fails. This is a significant fire hazard in my opinion and a major weakness of the BS1363 plug design. We should have had fuses of different shapes and sizes.

[Tony]

No worries Kurnal.  I suppose 'radial' does sound better than pumper spaghetti doesn't it?  Yes, that's what we use, and 240VAC  as well.  I'd never heard of the ring main method before this.

Plugs.  Not fused. Same size for all appliances, whether they require earth or are double insulated.  The double insulated types mostly have a dummy earth pin now.  Each separate main protected at the fuse box (or power box now) by a circuit breaker or fuse.

I can see the issue of users replacing fuses in the plugs, there'll always be people who get sick of fuses blowing and replace with a larger one.  Sort of like people here who used to (and some probably still do) replace constantly blowing fuses in the fuse box with a piece of fencing wire or a 3 inch nail...

One thing with electrical wiring.  Here it can ONLY be installed/repaired/replaced by a qualified electrician.  And if unlicenced wiring can be proven as the cause of fire, the insurance company is laughing.  Same as when people don't have smoke alarms, or have removed the batteries (something we always have to look for and report on).

And to get back on thrread again?  Even wiring up an extension lead can only be done here by a sparky.

[kurnal]

Back to Cresswels thread the use of extension leads brings a number of potential problems.

One common very bad practice is to daisy chain a number of leads together- sometimes with a 5 amp lead in line with a 13 amp lead. Obvious potential for overload.

Another problem is the increased risk of mechanical damage to the extension lead as it runs along the floor, under desks and causing additional trip hazards along the way.

As the length of the lead increases so does the electrical resistance and so for earthed appliances the level of protection provided by earthing diminishes as the lead gets longer  and the potential for damage to the earth conductor  or poor connection increases as the lead gets longer. And as for indoor type sockets used outside- lets not go there.

Recently found a coiled up lead rated at 4 amps wound 6 amps unwound feeding a kettle, and a three way block adaptor - unfused-  feeding a kettle 3000 watts, microwave 1400 watts, toaster 1200 watts in a factory canteen. Total 5600 watts  or about 23 amps if all used together .

On to Volfiries latest thread it would be intewresting to know if there are more fires in Australia caused by defecticve  electrical apliances than in the UK. Here were are all very hung up on fitting the correct fuse in the plug - but in Australia and indeed much of Europe where unfused plugs abound, a table lamp with a very thin 3amp rated flex will be plugged into a power outlet say of 15amp rating without a fuse at all.

As our ring mains are fused (or MCB protected)  at  30 amps we obviously need at least a 13 amp fuse in the appliance plug, but does it really matter if we have a 13 amp fuse in the table lamp? What current would the 3 amp flex and other unprotected components  have to pass in order to overheat to the extent to cause a fire? Do we worry too much in the UK about fuse ratings when much of the rest of the world have no fuses at all?

[John Webb]

....Recently found a coiled up lead rated at 4 amps wound 6 amps unwound feeding a kettle, and a three way block adaptor - unfused-  feeding a kettle 3000 watts, microwave 1400 watts, toaster 1200 watts in a factory canteen. Total 5600 watts  or about 23 amps if all used together....

....As our ring mains are fused (or MCB protected)  at  30 amps we obviously need at least a 13 amp fuse in the appliance plug, but does it really matter if we have a 13 amp fuse in the table lamp? What current would the 3 amp flex and other unprotected components  have to pass in order to overheat to the extent to cause a fire? Do we worry too much in the UK about fuse ratings when much of the rest of the world have no fuses at all?

Your example is a very clear breach of the Electricity at Work Regs and indicates that there is a either a failure of the company's H&S@W policy or they don't have one! I haven't seen quite such a bad example myself, I am grateful to say. But the 13A fuse should blow in a second or less at twice the rated current. (Did you look inside the plugs to see if the fuses were replaced with brass rod? That I have come across!)

Table lamp with probably 0.5sq mm size cable and say 3m long - overall resistance will be around 0.6 ohm - a short circuit at the table lamp end could allow a current of 400 amps = rapidly blown 13A fuse and not really a problem.
Damaged cable allowing 13A to flow but not allowing fuse to trip - some 50Watts per m of cable to dissipate - cable gets very warm and if laid under a carpet..... high potential for eventual ignition, I suspect. But a 3A fuse would have operated at around 6A.
Conclusion: Fusing has a fire-prevention function and needs to be considered.

(Cable resistance calculated from Table 4H3B of the IEE Rules (16th Edition) (BS7671))

The same publication says that cable on a ventilated reel should be derated to:
85% of rated current for 1 layer
65% for 2 layers
45% for 3 layers
35% for 4 layers.

Most cable reels are the solid type and unventilated..... need I say more!

[John Webb]

Beautiful example of how not to do it this afternoon. At the end of my road there is a garage that services vehicles and who often leave them at the end of our road while they are working on others. Clearly the battery in one of these parked vehicles need a charge - no less than three extension cables , two on reels, coming out of the garage office, diagonally across my road and to the vehicle concerned. One reel was almost in the middle of the said public highway!