PVC-sheathed cables at elevated temperatures

[5U4GB]

I need to run some PVC-sheathed cables attached to sensors through a region that's subject to elevated temperatures, up to maybe 60-70 degrees C.  PVC has a somewhat higher melting point than that but I'm wondering whether it'll soften and over time weld itself to the stuff it's run through.  I can't replace the cable since it's attached to the sensors in a sealed unit, I'm thinking as a precaution maybe to put some heatshrink around the section that's exposed to higher temperatures to avoid the welding problem?

The proper option would no doubt be PET sleeving but I've got a ton of heatshrink and no PET lying around.

[bdunham7]

Is it multiconductor or single conductor wires?  I think heatshrinking them will be fine if the temperature is really 60-70C and they aren't subject to any physical strain.  Much higher than that, or if they are compressed, twisted or otherwise stressed then I'd worry they they'd possibly creep enough to allow the internal conductors to touch.

[Poroit]

I think you should be safe using PVC as you have plenty of margin between the Ambient Temperature of the Hot space and the Melting point of PVC.

60-70°C is often the internal temperature of a lot of switchboards in Oz.

How much Air Space do you have around the cables passing through the hot zone? It will all help.

[5U4GB]

It's multiconductor but I'm not too worried about creep, there's no stress or strain on it, the cable is just lying there loosely.

In terms of air space, it's essentially zero, just enough to draw the cable through.  Thus the concern with it welding itself to the equipment, if it does do that there's no easy way to get it back out again. So about 20-odd cm of cable will be at 60+ degrees most of the time.

[Berni]

PVC doesn't typically become sticky like that. However if you are worried about it you could always cover the wire inside a heat resistant woven glass fiber sleeve. That stuff is used around heating elements and can survive 400°C no problem.

Other option is switching to a wire type that is more heat resistant like PTFE insulated, but yeah it costs more. Hard to beat the cheap cost of PVC wire, hence why it is used everywhere.

[tooki]

I need to run some PVC-sheathed cables attached to sensors through a region that's subject to elevated temperatures, up to maybe 60-70 degrees C.  PVC has a somewhat higher melting point than that but I'm wondering whether it'll soften and over time weld itself to the stuff it's run through.  I can't replace the cable since it's attached to the sensors in a sealed unit, I'm thinking as a precaution maybe to put some heatshrink around the section that's exposed to higher temperatures to avoid the welding problem?

The proper option would no doubt be PET sleeving but I've got a ton of heatshrink and no PET lying around.

Most standard PVC cables are rated for continuous operating temperatures of 70 or 80C. Note that the temperature range for fixed and moving applications may be different.

Why not just check the specs of the cable you have or plan to use?

It's multiconductor but I'm not too worried about creep, there's no stress or strain on it, the cable is just lying there loosely.

In terms of air space, it's essentially zero, just enough to draw the cable through.  Thus the concern with it welding itself to the equipment, if it does do that there's no easy way to get it back out again. So about 20-odd cm of cable will be at 60+ degrees most of the time.

It shouldn’t become sticky at 80C. You have to heat it well beyond that to get it to that point.

FWIW, the wire that behaves best in this regard in my own testing are silicone and crosslinked polyolefin (like Huber+Suhner Radox 125 and Helukabel Helutherm 145). Even when heated well beyond their operating temperature range, they don’t get sticky — even direct contact with a 330C soldering iron tip!

PVC doesn't typically become sticky like that. However if you are worried about it you could always cover the wire inside a heat resistant woven glass fiber sleeve. That stuff is used around heating elements and can survive 400°C no problem.

Just beware that whatever is inside still needs to be heat resistant. I was actually just dealing with this topic the other day, and decided to do some testing with the silicone-coated fiberglass sleeves. While they definitely slow heating, they don’t stop it, and within 5 minutes or so, a temperature probe inside such a sleeve, in contact with a 250C hotplate, reached over 200C.

[5U4GB]

Why not just check the specs of the cable you have or plan to use?

It's moulded onto the sensors and, the last time I looked (I don't have any at hand ATM to check) was unbranded so who knows what it is.  Possibly even overplasticised PVC pretending to be silicone, ugh.  However given the other comments here I'm assuming it'll be OK, and will shrink a bit of polyolefin over the at-risk area, as you point out that stuff never seems to get sticky.

[Berni]

PVC doesn't typically become sticky like that. However if you are worried about it you could always cover the wire inside a heat resistant woven glass fiber sleeve. That stuff is used around heating elements and can survive 400°C no problem.

Just beware that whatever is inside still needs to be heat resistant. I was actually just dealing with this topic the other day, and decided to do some testing with the silicone-coated fiberglass sleeves. While they definitely slow heating, they don’t stop it, and within 5 minutes or so, a temperature probe inside such a sleeve, in contact with a 250C hotplate, reached over 200C.

Yeah they wouldn't help to protect the actual wire inside from the heat. The stuff is typically used to protect just 1 single conductor, so even if the insulation fails there is no short.

Here i was just suggesting it as a extra layer to help the cable not get stuck. Those 60°C is not that hot for PVC to be in danger. But extended heating like that could lead to accelerated aging. Various manufacturers mix in all sorts of different chemicals to improve the properties of the plastic, so i can't for sure say it is impossible for that particular cable to not glue itself to the tube. Tho typically what happens to PVC is that the plastic insulation might get very stiff with age and possibly crack (sometime far in the future). It is most likely going to be perfectly fine.

[coppercone2]

https://en.wikipedia.org/wiki/Mineral-insulated_copper-clad_cable

There is mineral insulated cable. I thought it might help the discussion to show what the 'upper bound' and its specs might be


If you really don't want a short, they sell ceramic segments with two holes in it, used for thermocouple feed through.

[coppercone2]

I need to run some PVC-sheathed cables attached to sensors through a region that's subject to elevated temperatures, up to maybe 60-70 degrees C.  PVC has a somewhat higher melting point than that but I'm wondering whether it'll soften and over time weld itself to the stuff it's run through.  I can't replace the cable since it's attached to the sensors in a sealed unit, I'm thinking as a precaution maybe to put some heatshrink around the section that's exposed to higher temperatures to avoid the welding problem?

The proper option would no doubt be PET sleeving but I've got a ton of heatshrink and no PET lying around.

Most standard PVC cables are rated for continuous operating temperatures of 70 or 80C. Note that the temperature range for fixed and moving applications may be different.

Why not just check the specs of the cable you have or plan to use?

It's multiconductor but I'm not too worried about creep, there's no stress or strain on it, the cable is just lying there loosely.

In terms of air space, it's essentially zero, just enough to draw the cable through.  Thus the concern with it welding itself to the equipment, if it does do that there's no easy way to get it back out again. So about 20-odd cm of cable will be at 60+ degrees most of the time.

It shouldn’t become sticky at 80C. You have to heat it well beyond that to get it to that point.

FWIW, the wire that behaves best in this regard in my own testing are silicone and crosslinked polyolefin (like Huber+Suhner Radox 125 and Helukabel Helutherm 145). Even when heated well beyond their operating temperature range, they don’t get sticky — even direct contact with a 330C soldering iron tip!

PVC doesn't typically become sticky like that. However if you are worried about it you could always cover the wire inside a heat resistant woven glass fiber sleeve. That stuff is used around heating elements and can survive 400°C no problem.

Just beware that whatever is inside still needs to be heat resistant. I was actually just dealing with this topic the other day, and decided to do some testing with the silicone-coated fiberglass sleeves. While they definitely slow heating, they don’t stop it, and within 5 minutes or so, a temperature probe inside such a sleeve, in contact with a 250C hotplate, reached over 200C.

I believe the idea behind that is to let the cable survive long enough for a protection circuit to trip

[tooki]

Why not just check the specs of the cable you have or plan to use?

It's moulded onto the sensors and, the last time I looked (I don't have any at hand ATM to check) was unbranded so who knows what it is.  Possibly even overplasticised PVC pretending to be silicone, ugh.  However given the other comments here I'm assuming it'll be OK, and will shrink a bit of polyolefin over the at-risk area, as you point out that stuff never seems to get sticky.

Heat-shrink may or may not become sticky (though not at 70C), it depends on the extent it is cross-linked. The polyolefin insulations I mentioned are fully cross-linked (with the Radox retaining more abrasion resistance while hot than the Helutherm.)

You could just wrap a turn or two of Kapton tape around the cable. That would also reduce friction while inserting it.

But I strongly suspect you’re making a mountain out of... well not even a molehill, because I suspect there’s no issue whatsoever. The vast majority of cables, even soft PVC, are rated above 60C. And if we are talking industrial sensors, the cables tend to be 80C or higher.

You could always just test your cable by bringing a pot of water to a boil, letting it stand for a couple of minutes without heat, and then immerse a loop of the cable for a few minutes. I’d be very surprised if it got sticky.

[tooki]

Here i was just suggesting it as a extra layer to help the cable not get stuck. Those 60°C is not that hot for PVC to be in danger. But extended heating like that could lead to accelerated aging. Various manufacturers mix in all sorts of different chemicals to improve the properties of the plastic, so i can't for sure say it is impossible for that particular cable to not glue itself to the tube. Tho typically what happens to PVC is that the plastic insulation might get very stiff with age and possibly crack (sometime far in the future). It is most likely going to be perfectly fine.

Absolutely agree. I recently replaced a PVC cable on a bit of equipment where the cable is constantly exposed to steam. The old cable was stiff as wood! I installed polyurethane cable.

[themadhippy]

wots stopping you cutting the sensor off the wire,run in something with a suitable temp rating and joining the 2 back together

There is mineral insulated cable

AKA pyro,an absolute joy to work with,sadly now becoming a rare beasty and lost skill thanks to the likes of fp200.

[coppercone2]

I found a piece of that pyro once a long time ago in a pile of rubbish when I was young and I could not figure out WTF it was. It just did not make sense to me, could it be a heater? but then why does it look like copper. Why is there two conductors. Why is powder pouring out of the tubing. It looked to be made well enough that its not someones weird experiment. Could this really have a use, or did someone make it as a complicated joke, perhaps it some kind of gas filter, or maybe it was just something that is supposed to bend in as weird specific way? 

its up there, along with those capillary copper tubes for measuring heat in mechanical thermostats. It's like there is NO way this makes any sense. Especially if you find it in the trash missing parts. You just think its the weirdest spring ever. Maybe its a alarm of some kind.. it looks like it might be able to pull something if you hit the end with a door. And the thing its connected to happens to look like the atom bomb from gold finger...

[5U4GB]

You could just wrap a turn or two of Kapton tape around the cable. That would also reduce friction while inserting it.

But I strongly suspect you’re making a mountain out of... well not even a molehill, because I suspect there’s no issue whatsoever. The vast majority of cables, even soft PVC, are rated above 60C. And if we are talking industrial sensors, the cables tend to be 80C or higher.

Ah, yeah, that'd do it too.  I'm not really concerned about short-term issues, more like what state it could end up in after ten years or more.  As others have pointed out it'll probably be fossilised which is fine, as long as I can haul it out again if it needs replacing.

[Jay_Diddy_B]

Hi, I would suggest using foil shielded twisted pair. Belden 8450 is an example of this type of cable.




Ordinary PVC cable will be okay at 70C.



If the meter has a guard terminal, connect the shield to the guard terminal at the meter. If the meter does not have a guard terminal try the negative input or  chassis ground. The shield should only be connected at one end.

You can also try ethernet cable. It comes in unshielded twisted pair, UTP and Foil shielded twisted pair, FTP.


Regards,
Jay_Diddy_B

[tooki]

Hi, I would suggest using foil shielded twisted pair. Belden 8450 is an example of this type of cable.

…
You can also try ethernet cable. It comes in unshielded twisted pair, UTP and Foil shielded twisted pair, FTP.

The original post states clearly:

I can't replace the cable since it's attached to the sensors in a sealed unit

[5U4GB]

In the end I went with heatshrink because it was the easiest to apply, possibly unneccessary but it means I don't get to run an experiment on PVC plasticiser migration at elevated temperatures over long time periods.  Other options would have been Kapton tape as suggested by @tooki, and something else that was already in use where the sensors are deployed, aluminium foil tape, all of which would keep the cable from sticking to its surroundings if it happens to degrade over time due to the heat.

I'll report back in twenty years or so with an update on how things held up.