Thread: BobL or Soundman, how do you check an extension lead

Ian,

Unfortunately it's not that simple as you cannot perform a proper test with a standard multimeter. You really need a proper AC testing device that tests the whole cable at 240V. For example you might test the cable and it has a low resistance or an earth/live connection and it may appear to have no detectable short but bung 240V on it and the insulation or plugs may be cracked and leak like the proverbial.

Some of this may be the voltage and some of this may be due to the the nature of AC. At work we have gear that operates up to 20kV. You should see the sparks you can generate from cable that looks prefectly OK at a a few V or even 250 V when you stick a few kV on it. Higher V cable uses special insulation and specially grounded sockets/plugs.

For the sake of safety if you are unsure about any cable then get it tested properly or for that matter cables are so cheap these days you should throw it away and get a new one.

thanks

Bob is right.

However you use the word " degrade"

Modern cable dosn't "degrade" much at all not like the old rubber stuff used to, it would just rot and perish away inside its own skin.

Modern thermoplastic extension cable is prety damn stable I I don't believe it degrades at all

BUT

What does cause it to become less safe and reliable is damage from a variety of sources.

there are proper tests set down by AS standards some of which require "proper" equipment.
But the single most important part of the inspection process is... the... visual inspection.
It is also a requirement of the law in queensland that ALL users make a visual inspection of an electrical device befor using it.

so
The lead should look to be in every way in good condition.
plugs whole and complete, no splits or cracks.
cable firmly attached to the plugs with no secondary insulation showing.
no cuts, splits, breaks or deformations in the cable sheath.
No sign of heat related damage in the cable or the plugs, no bad smell.
In heavy operation the cable may get warm but there should be no areas that are significantly hotter than else where..

The visual inspection is the best thing an unqualified user can do and will uncover the vast majority of faults.

if in addition you have you cable formaly inspected annulay you will very rarly have a problem.

remember almost all of safety is common sence and looking.

cheers

Originally Posted by soundman

Bob is right.

However you use the word " degrade"

Modern cable dosn't "degrade" much at all not like the old rubber stuff used to, it would just rot and perish away inside its own skin.

Modern thermoplastic extension cable is prety damn stable I I don't believe it degrades at all

BUT

What does cause it to become less safe and reliable is damage from a variety of sources.

there are proper tests set down by AS standards some of which require "proper" equipment.
But the single most important part of the inspection process is... the... visual inspection.
It is also a requirement of the law in queensland that ALL users make a visual inspection of an electrical device befor using it.

so
The lead should look to be in every way in good condition.
plugs whole and complete, no splits or cracks.
cable firmly attached to the plugs with no secondary insulation showing.
no cuts, splits, breaks or deformations in the cable sheath.
No sign of heat related damage in the cable or the plugs, no bad smell.
In heavy operation the cable may get warm but there should be no areas that are significantly hotter than else where..

The visual inspection is the best thing an unqualified user can do and will uncover the vast majority of faults.

if in addition you have you cable formaly inspected annulay you will very rarly have a problem.

remember almost all of safety is common sence and looking.

cheers

just to add to the visual inspection advice, the most common problem i have encountered with modern (cheap) leads is corrosion inside the plug and/or socket. I suspect this is largely galvanic corrosion as the wire is copper, crimped to nickel-plated pins, rather than the brass or copper that used to be mandatory in such things. I've gone through 4 of the cheap Bunnings stripy long leads, 3 of which never saw moisture as they were always inside. The only saving grace is that the clear plastic plugs allow the corrosion to be seen before it starts to cause a problem.

At work we subject electrical leads to some very agressive chemical environments. In several labs we purify, by distillation inside special chambers, some very nasty acids (HCl HF and HNO3). The heating sources inside these chambers results in a continuous air temperature of ~ 50oC.

In the past, the electical cables to all these stills were conventional PVC covered appliance leads which survived for many years although the outer grey PVC sheath did eventually became slightly sticky. These cables were tested regularly and usually did not appear to suffer this sort of treatment. What always gave out first (sometimes within a year or so, were the plugs, sockets and switches which suffered from corrosion and had to be replaced. In some areas we sealed the plugs and sockets with silicone which helped but did not always stop the corrosion completely.

In our new lab we have all the plugs and sockets outside the distillation chambers. The cables are just the standard 10A white plastic domestic type cables. So far (2 years) all is good.

As Soundman says, the most likely problem by far is physical damage, run over by a vehicle, crushed by a clsoing door, bouncing around in the back of a van, cut by a falling tool, etc. Now if I were to use these in the chemistry lab above . . . . . I think there would be real trouble. Along the same vein, if a physically damaged cable is used in a damp/wet environment the cracked insulation can give you more than a tingle. It reminds me of the time I was plastering up a largish hole in an internal brick wall and got a bit of a tingle. Eventually I worked out I was getting juiced from a power point just below the hole on the other side of the wall . A few minutes before plastering up I had squirted a fair bit of water into the hole and it had penetrated through the bricks and run down the wires into the back of the power point!

Originally Posted by soundman

Modern cable dosn't "degrade" much at all not like the old rubber stuff used to, it would just rot and perish away inside its own skin.

Modern thermoplastic extension cable is prety damn stable I don't believe it degrades at all

Soundman, I must admit I remain to be convinced of this. I think I still have lying around somewhere some "heavy duty" Monster speaker leads with the clear sheath. Sections of the copper conductor (away from the ends) has turned green as you would expect from corrosion, presumably due to moisture penetrating the cable sheath.


ian

Clear plastic insulated cables are a very diferent thing to 240 volt rated and standards specified and legaly regulated cables.

Without exception all clear insulated speaker cables will degrade with age, the outer jacket will go cloudy and sticky and the same will occur inside the cable causing the conductors to go green with corrosion.
This is almost certainly due to plasticiser migration.
Strong detergent or solvent will remove the gunk from the outside of the jacket but there is nothing you can do for the inside and the insulation will continue to very slowly get worse. I have never seen total break down of the plastic and the rate of degradation varies from cable to cable. In my experience the softer and clearer the plastic is to start with the faster it will happen.

Making a durable clear plastic is a lot harder than making one in a solid colour because you cant use any opaque ingriedents.
Opaque ingriedients is what you use to make durable plastics particularly UV stable plastics. opaque ingriedents also make cable stiffer (in general)

The average HIFi buyer equates quality with a cleer cable jacket, thick insulation and soft flexible feel.

Don't compare speaker cable to mains rated cable.

I have to agree with corosion in plug tops being a problem, this is why the cleer plug top is very popular in industrial applications. It extends the effectiveness of the visual inspection.

cheers

Thanks for remembering me in your roll call there.

As a licensed electrician...

Seriously, check the outside of the cable for nicks, cuts and gouges. If the lead looks like a telephone reciever wire (spiral looking thing), then it's been stretched and should be replaced. Also check that it's not scuffed up and generally looks good. If all is well there, you have to assume (yes, making an a$$ and all) that the insulation is good if you can't use a proper insulation resistance tester (known as a Megger).

All the tester does is crank up the voltage to 1,000V, and measure the resistance of the insulation at that voltage. A standard multimeter only runs about about 3V.

At the plugs, if the plug is molded to the lead and looks to be in good order, it should be. If the plug is clear and put onto the lead (a bought plug/socket), then make sure the polarity is right. Looking at the socket, top left is active/brown, top right is neutral/blue and the bottom is earth green with yellow stripe. Check for broken wires (acceptable number of broken strands is zero), poor connections, burning or corrosion. If anything is amiss, replace the thing (use a sparky...). On the plug end, polarity is opposite for active and neutral, so that if you plug the lead into itself, the colours all match. If the colours are not brown, blue and green/yellow, then it's not a 'lead' and shouldn't be used.

Problems with leads occur from nicks cutting wires, pulled out wires, broken wires and other such mechanical defects that are usually (but not always!!!) quite obvious.

If the lead passes a visual inspection, but doesn't work properly, if at all, bin it and get a new one.

The insulation shouldn't break down without giving obvious signs of degradation, like crumbling or falling apart. This occurs typically from UV exposure or a harmful enviroment.


BobL, shoot the sparky you are using. In corrosive enovironments, Clipsal make switches and GPOs suited to such situations. IP66 rating and coloured orange means they are good for wet areas and chemical attack, within specifications. I should know, I have installed enough of them (and know that you take really special care of them, like not letting apprentices near the things!) in medicinal manufacturing plants and packaging plants. Good stuff, about$200 a pop last time I checked. But they don't foul up and can be easily sealed for clean rooms and the like.