correct method of stripping twin & earth?

[Evans Electric]

I've never heard of a problem caused from "Stretching " with the "Cheese Wire" method.

This trade is full of myths and personal preferences that get spread around until they start to become fact.

 

[ianmacd]

Anyone who disputes the altering of the cpc using "cheesewire" method really should think about it a lot more. Its a fact! Same can apply to putting any strain on the conductors including (but not limited to), pulling the insulated conductors apart to remove the sheath and excessive drawing of the cable during install.

Every time a conductor is bent, it will alter it.

 

[IzzyS]

Maybe the people to ask for a definitive answer are the cable manufacturers? But then we might all end up paying another annual fee for "calibration" of our cable stripping equipment

There are youtube videos from the learning lounge which show knife, cable strippers, pulling "live" conductors apart, and cheesewire methods as all being acceptable. Personally I use them all except the proper T & E strippers. Whatever method used, don't damage the insulation - more important in my opinion than possible miniscule thinning of the conductors.

 

[ianmacd]

Whatever method used, don't damage the insulation - more important in my opinion than possible miniscule thinning of the conductors.

And you know this how? If the copper has stretched then so has any insulation over it also reducing its thickness and, therefore, effectiveness.

 

[IzzyS]

And you know this how? If the copper has stretched then so has any insulation over it also reducing its thickness and, therefore, effectiveness.

Possibly so. But I've never noticed a reduction in IR (still >299M) , so I don't lose any sleep over it. Slicing through the brown and blue on the other hand...

If stretchy copper is such a problem, why don't reels of T & E come with "WARNING - DO NOT EXCEED xx NEWTONS PULLING FORCE" plastered all over them? As I said earlier, the manufacturers are in a position to make a formal statement on the issue - do you have confirmation from them that it is a valid concern? (I don't, but neither do I have any confirmation that it's not).

 

[electrician786]

But surely the cables must be bending and stretching when they are rolled up onto the cable drums?

 

[Sidewinder]

Copper for conductors has a defined specification under B.S.

This is a very ductile version.

It is very flexible.

However, it has an elastic limit, beyond which and bending forces will affect the crystal structure of the material.

I am not pushing the stretching the cpc bit, I am on about work hardening of the copper.

If you con't believe it can happen, take a piece of solid 2.5 say 6" long and carefully strip the insulation off.

Bend this repeatedly back and fore until is snaps, quickly, you will experience an increase in temperature at the point of fracture and as I say the copper will fracture.

This is due to work hardening of the ductile material and ultimately brittle fracture due to the tensile forces at the outside of the bend radius.

These tensile forces cause cracks in the crystal lattice of the copper at nucleation points, which are minute discontinuities within the crystal structure.

These cracks can start and nucleate at various rates dependent upon the cyclical loads.

They also increase resistance in the material by causing points of high resistance due to these discontinuities.

"Not quite the exact, perfect terminology" as it's a bit rusty now as it has been a few years now since I have done materials work.

Get it so far?

Got to go for my dinner now, also very busy so don't really have time to look this up and provide references etc. so doing it off the top of my head.

 

[ProDave]

This is all very interesting.

But we are talking about (if done carefully) quite a light pull on the CPC and quite a small amount of bending involved.

In the real world, the cable is subject to far more bending and much more stress in the act of pulling the cable through in the first place, and dressing the cable into the accessory, and flexing as you put the accessory into it's box etc.

Now a metallurgist might be able to detect some slight change, but not enough to matter.

From all the cable stripping and forming I have ever done, I have never encountered any cable problems. The biggest single thing likely to cause a problem is a poor termination caused by a nick in the cable core when stripping the conductors or an over tighten termination screw.

So using the CPC as a cheese wire IMO is nothing to get excited about.

If you think copper is fragile, try working with aluminium cable.

 

[Sidewinder]

Dave,

The crux of the issue is in the angle formed between the cable axis and the "cheese wire" if this is past a critical point and the material has already been stressed and is of low quality, which it will be these days due to financial pressures, it will be of the lowest acceptable quality, then this is where the issue arises.