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I'm curious about the fire alarm cable that's being installed in a new building next to my office.

In the fairly distant past, I learned that fire alarm cable was "pryo" cable. Under the red jacket it was a copper tube containing two conductors, all mutually insulated by a white mineral powder. Apparently it would stay working unless a fire melted the copper, because there was nothing inside that could burn or decompose.

What they are using today appears to have layers of polymers. Under the red outer layer is a stiff white jacket. Inside is a bare (earth?) wire and two wires insulated with blue and brown polymer.

What are these polymer materials, and what happens to them in a fire? Was the "pyro" cable deemed overkill and used only for lack of alternatives to PVC in past decades?

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  • \$\begingroup\$ hm, what does you finding this type of cable now have to do with "pyro cable" being deemed overkill? \$\endgroup\$ – Marcus Müller Dec 13 '18 at 10:21
  • \$\begingroup\$ Perhaps that "new" cable has a sufficiently high melting point that people are already dead before the cable melts... At that point the alarm looses its usefulness... \$\endgroup\$ – Solar Mike Dec 13 '18 at 10:22
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    \$\begingroup\$ @nigel222 point is that the copper tube thing might still have its place! For example, you really don't need to make the cable to a single alarm button resistant up to 1000°C – no one is going to press that button if the last 5m to that button are that high. If you, however, have some sort of central cable distribution, it would make a lot of sense if temperatures that high in that specific room wouldn't end the ability of someone to sound the fire alarm. So, maybe your pyro cable is overkill for one application, but not for another. \$\endgroup\$ – Marcus Müller Dec 13 '18 at 11:04
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    \$\begingroup\$ @nigel222 Alarms are usually connected in a loop, each end of the loop connected to the central. If the cable breaks in the middle, it doesn't affect functionality because each end has a connection back to the central. It's only a problem if it breaks in two places, every sensor in between the two breaks are now isolated. \$\endgroup\$ – pipe Dec 13 '18 at 11:52
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    \$\begingroup\$ @nigel222 If it's shorted out it's also detected as "defective", the sensors we used were designed so that they would internally disconnect the shorted out segment so it would be handed roughly in the same way as a break. But of course, it's better if the cable is healthy as long as possible... \$\endgroup\$ – pipe Dec 13 '18 at 12:52
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That sounds like FP200 or similar.

The copper tube with mineral insulation was MICC cable.

MICC is a pain in the arse to terminate which is why no installer likes doing it if they have any choice, and the modern synthetics have good enough properties in a fire to make them compliant with the appropriate standards and are not hydroscopic if incorrectly terminated.

MICC tended to fail insulation test after a while if the person fitting the potting gland on the end was not taking sufficient care.

There are multiple vendors of modern fire alarm cables and they all have their own trademarks and 'secret sauce' chemistry.

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  • \$\begingroup\$ "secret sauce" ... so that's why my Googling failed! \$\endgroup\$ – nigel222 Dec 13 '18 at 16:12
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Fire alarm cable can use a variety of insulations depending on application. Some is made from ordinary PVC and others of XLPE (cross linked polyethylene) or XLVA (look it up), silicone and even Teflon.
Some cable is made to fail in a fire. The insulation melts, the conductors touch and that triggers the alarm.

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The flexible cables (image below from this link) you saw probably have a layer of Mica as insulator in there (which is a 960°C safe mineral isolator). Some of them can withstand close to 1000°C for up to 3 hours.

Cable from above catalogue

However, that's under the assumption that no external pressure (e.g. from parts of the cable housing falling onto them) is applied to them.

One way to ensure that would be putting them in safe metal tube – and that's basically what the mineral powder filled tubes are. If you, however, ensure that there won't be other things failing and scraping the mica insulation off the cable, then you don't have need for any copper tubing!

Also, if you look at the picture above, you can see an aluminium "wire armour"– practically your tube-to-go!

So, if you have a metal cable conduct, then you're pretty safe: most of the other cables' isolation will be gone the moment the flame-retardiant silicone rubber of your fire alarm cable gives in, so these cable will have reached a somewhat "stationary" pose. The risk of one of them changing position sharply enough to scrape of insulation is small. Same if the cable is running alone on some plaster wall.

So: fire-proofing cables is a process, not only a material :) Your flexible cables might be very well as safe as metal tubes filled with mineral powder for some use cases, moreso in others (think about non-fire-related failures, like water ingress), less in mechanically difficult situations. It's maybe less about over-kill, but about safety of installation.

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  • \$\begingroup\$ No, this isn't what I saw. No armour. Outer jacket is a quite stiff white material (coloured red on exterior),, has aluminium foil on inner surface. Two inner wires insulated brown and blue with a very elastic material (silicone rubber at a guess) with a single copper core. Third uninsulated wire. \$\endgroup\$ – nigel222 Dec 13 '18 at 12:34

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