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Old 18th Feb 2019, 12:45 am   #1
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Default Silicone cable safety

I was about to do a quick repair using a hot melt glue gun and suddenly discovered that it had what looked like a badly wired plug with the cable pulled part of the way out of the grip saddle.
When I looked more closely the silicone rubber cable had gone all sticky and soft close to the plug while remaining normal along its length.
Once I had removed the plug it was obvious to me that the rubber had rotted most where it contacted the nylon grip saddle. The part that was in contact with the plug body was far less degraded.
These two plastic materials obviously react so If you have mains powered kit with silicone mains cables make sure there is a sleeve fitted under the cord grip saddle.
My estimation based on the age of the glue gun sets the rate of migration of the decay from the nylon into the flex insulation to be about 1mm per year.
I would advise you check your silicone mains cables and fuses in order to save yourself a rood awakening. It is class II so the RCCD will not stop it going BANG if you fail to spot this problem.
I did fit a 3 amp fuse before using it again.
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Old 18th Feb 2019, 1:13 am   #2
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Default Re: Silicone cable safety

That doesn't look like silicone, it looks more like old skool lawnmower cable.
Where the break in the insulation is looks like it's been either pulled too many times or twisted.
Please don't shoot me down, it's just an observation.
Rick.

If an RCD was to be put into action, it would respond to L & N shorting, without a bang occuring.

Last edited by Glowing Bits!; 18th Feb 2019 at 1:14 am. Reason: More info
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Old 18th Feb 2019, 1:23 am   #3
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Default Re: Silicone cable safety

It makes me wonder exactly what the nylon was reacting with.
https://www.calpaclab.com/nylon-chem...ibility-chart/
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Old 18th Feb 2019, 1:24 am   #4
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Default Re: Silicone cable safety

It is not lawn mower cable. It is an early hot melt glue gun with the original plug and cable.
The cable insulation was like putty under the saddle clamp and sticky and mechanically degraded for about an inch in all directions from that point.
If you look carefully at the photo you can see the dust clinging to the insulation up to the point where the stickiness stopped.
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Old 18th Feb 2019, 1:35 am   #5
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Default Re: Silicone cable safety

Quote:
Originally Posted by rambo1152 View Post
It makes me wonder exactly what the nylon was reacting with.
https://www.calpaclab.com/nylon-chem...ibility-chart/
I think the reaction was the other way around.
It looks like one of the setting agents in the nylon that is left behind after manufacture has seeped out of the saddle and into the rubber.
The saddle itself was not mechanically affected.
The chemical involved will most likely be one of the ones you can smell when plastic is new.
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Old 18th Feb 2019, 3:56 am   #6
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Default Re: Silicone cable safety

I've just replaced the charger on this macbook pro for the SECOND time with the synthetic rubber outer of the DC cable going soft, sticky and falling apart, exposing conductors.

It goes just where you hold it to take stress off the connector. So I suspect a reaction with natural skin oils.

An applestore 'genius' dismissed it as mechanical damage causing fraying and not covered by warranty. Seems the internet has plenty of similar tales.

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Old 18th Feb 2019, 9:54 am   #7
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Default Re: Silicone cable safety

I've never had a problem with silicone cable degrading! Only thing I noticed is it's not compatible with hydrocarbons - clean something with white spirit or paraffin, and it swells. But it's reversible.

PVC cable, on the other hand, don't have it in contact with polystyrene (other plastics may be affected). It slowly softens and dissolves them as plasticisers leach out.
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Old 18th Feb 2019, 11:09 am   #8
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Default Re: Silicone cable safety

Could be a high flex pvc cable, sometimes used on cheaper test leads where silicone is too expensive. I've seen the orange stuff before. I'm no expert but it's like it has a lot of plasticiser in it to give it a tough but almost elastic, shiny consistency. Is that the sort of thing?
One of my clients has it on a piece of portable equipment and when the cable is tripped-over, it stretches and breaks the copper, a bit like rubber cable without the smell.
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Old 18th Feb 2019, 11:14 am   #9
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Default Re: Silicone cable safety

Quote:
Originally Posted by Glowing Bits! View Post

If an RCD was to be put into action, it would respond to L & N shorting, without a bang occuring.
How?

Where's the residual current (L-N difference current) going to come from?

An RCBO would trip but depending on its current rating there could still be quite a BANG.
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Old 18th Feb 2019, 12:47 pm   #10
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Default Re: Silicone cable safety

It almost looks as if chemical contamination from without has initiated the deterioration- it's not a trace of glue residue is it..? Never seen breakdown of silicon cable before.

The ghastlies that you find inside 13A plugs, honestly.. Tinned multi-core copper conductors have a tendency to loosen up under the screws, meaning the older (and higher consumption) an appliance is, the more often the plug needs checking. I should really go through the entire house.

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Old 18th Feb 2019, 12:49 pm   #11
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Default Re: Silicone cable safety

I have not heard of any adverse reaction between nylon and silicone.
Many silicone covered wires and cables are secured by nylon cable ties without problems.

I suspect some other cause in this case, possibly involving oil or grease.

An RCD won't help with a line to neutral short circuit.
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Old 18th Feb 2019, 12:49 pm   #12
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Default Re: Silicone cable safety

Indeed, I have had a couple of accidental severings of the 2-core flex of hedge trimmers where the plug-in RCD didn't trip. The main fuse box had rewirable fuses, The 13A plug was fitted with 5A fuse, and cable severing was accompanied by a sharp "Bang" and slight pitting of the blades. I guess a modern MCB might had broken the circuit, but the short circuit was too brief for the traditional wired fuses to blow.
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Old 18th Feb 2019, 1:03 pm   #13
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Default Re: Silicone cable safety

If that was a Bostik gun from the 1980s it wasn't silicon. I dropped an Antex XS25 on one of them when I was a teenager and it blew the breakers in the house after a few seconds.
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Old 18th Feb 2019, 1:03 pm   #14
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Default Re: Silicone cable safety

A little off the original topic, but I have also experienced 2-core cable severing of the hedge-trimmer variety and it does indeed create quite the bang! (the MCB on the consumer unit did trip in that instance, but as mentioned not without a loud crack!)
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Old 18th Feb 2019, 2:37 pm   #15
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Default Re: Silicone cable safety

It is indeed an old Bostik glue gun from the 1980s and it is not glue contamination on the cable as the gland where the stick enters the body of the gun has not failed in any way and it is made of the same stuff. That gland has had a glue stick in it for many years without a problem.
The sheath had turned to paste under the grip saddle and gone sticky for about an inch in both directions like some types of handle bar grips do on things like power tools and computer mice.
It is a matte texture with a feel like handle bar stuff but has remained good apart from close to the saddle. It does look like plasticiser migration and reaction to me.
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Old 18th Feb 2019, 6:26 pm   #16
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Default Re: Silicone cable safety

Nasty. I have several things which use this type of cable, and will check later on.
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Old 22nd Feb 2019, 2:31 pm   #17
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Default Re: Silicone cable safety

As a railway signalling design engineer I have come to see many cases of “wire deg” (wire degradation) in lineside location cases and relay rooms. We suffer two types on plastic wires from the late ‘70’s onwards. Wet wire deg where the insulation initially goes soft then as more time passes it starts to turn into a soft mush. The other form is dry wire deg where the plastic insulation goes hard, brittle then cracks and falls off. There were a couple of 80’s vintage relay rooms (Healey Mills was one) where you had to close the door very closely! One of the wire types was Pirelli produced within certain dates, I believe it was down to the plasticisers used. Wet wire deg also seems the affect the ends of the wires first / more, as appears to be the case here.

Referring to the lacing thread thread (!) much lineside location case wiring has had the lacing cut off because it has started to permanently indent into the wiring insulation as it ages and decays.

73 Ian
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Old 22nd Feb 2019, 2:53 pm   #18
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Default Re: Silicone cable safety

The wire I found was definitely suffering the wet kind of degradation starting from the grip saddle. It looks like it is triggered by contact with other materials.
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Old 23rd Feb 2019, 11:37 am   #19
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Default Re: Silicone cable safety

Interestingly, the wet wire deg we have in railway signalling wiring occurs, initially at least, at the ends of cables where they go into relay bases and such like, so they are not actually in contact with anything else.

There is another problem with silver migration on some relay bases, which can in some cases cause the risk of a wrong side failure, but that is another story.

Regards Ian
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Old 23rd Feb 2019, 11:52 am   #20
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Default Re: Silicone cable safety

There may have been something in the atmosphere in the relay base area.
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