Smokes and sheathing the interconnect

tobybuk · 18 Feb 2019

When wiring in smoke alarms with an interconnect does the the interconnect need sheathing?

I'm asking as the manual for alarm states that the interconnect should be treated as live. Obviously if I sheath the interconnect it will not be obvious to someone later on which is which unless I leave the sheath short so the wire can still be partly seen.

Thanks

LearningSpark · 18 Feb 2019

Do you mean sleeving to identify it is a live conductor or are you using the bare earth in T&E as the inter connect?

tobybuk · 18 Feb 2019

Sleeving to identify it is a live conductor. Its a three core +earth cable and the third (GREY) is the interconnect.

JohnW2 · 18 Feb 2019

tobybuk said:
Sleeving to identify it is a live conductor. Its a three core +earth cable and the third (GREY) is the interconnect.

Yes, in a single-phase installation, strictly speaking, any conductor other than brown which is used as a 'line'/phase (a.k.a 'live') conductor should be over-sleeved with brown at the terminations. Assuming that you are using the black as neutral, that should be over-sleeved with blue at its terminations.

tobybuk said:
I'm asking as the manual for alarm states that the interconnect should be treated as live. Obviously if I sheath the interconnect it will not be obvious to someone later on which is which unless I leave the sheath short so the wire can still be partly seen.

Yes, that's what one normally does - use a bit of over-sleeving short enough that the underlying insulation can also be seen.

Kind Regards, John

EFLImpudence · 18 Feb 2019

Yes, you only have to 'identify' what the conductor is being used for. you don't have to completely hide its original colour.

It could be argued that the conductor is identified by the terminal markings - L, N and whatever mark identifies the signal wire.
If not, you wouldn't know which to connect to what in the first place.

JohnW2 · 18 Feb 2019

EFLImpudence said:
Yes, you only have to 'identify' what the conductor is being used for. you don't have to completely hide its original colour.

Indeed.

EFLImpudence said:
It could be argued that the conductor is identified by the terminal markings - L, N and whatever mark identifies the signal wire. If not, you wouldn't know which to connect to what in the first place.

Interesting. I must say that I've never thought of that or heard anyone trying to use that argument. I had always presumed (as I also presumed others did) that the intent was that the conductors of the cable itself should be 'identified', so that the identification remains even when the cable is disconnected from terminals - but I accept that the regs do not seem to say that explicitly.

Kind Regards, John

tobybuk · 18 Feb 2019

Thanks guys, much appreciated.

Adam_151 · 18 Feb 2019

It doesn't need sleeving, Its not eactly a switched-live as such (think about the alarms signalling to each other in a power cut), table 51 gives a number of different colours acceptable in a control circuit. Although it should be noted that it is standard practice to use the black as interconnect, and then to sleeve the grey as blue and use as neutral.

I beleive the manufacturers instructions are probably trying to make the point that it should be treated a live conductor (i.e. not earthed, or part of an electrically separate system), rather than as a line conductor, which would need to be identified as such

JohnW2 · 18 Feb 2019

Is the 'interconnect' control voltage of a mains-powered smoke alarm LV or ELV? I would rather presume the latter, since they presumably have to be able to 'link', as well as 'work' when running from their backup battery?

Kind Regards, John

Risteard · 18 Feb 2019

As stated keeping it black is the best as black is a permissible colour. It should not be identified brown as it is not a 230V phase conductor.

JohnW2 · 18 Feb 2019

Risteard said:
As stated keeping it black is the best as black is a permissible colour.

I think this is all getting a bit confused/confusing. Black is a permissible colour for identifying a control circuit line conductor, but Andy suggested that it was not a line conductor, and ...

.... It should not be identified brown as it is not a 230V phase conductor.

... but, just like black, brown is also permissible colour for identifying a control circuit line conductor - so, if we are going to call this a 'control circuit' (which I think is probably the main thing which is confusing the issue) brown would be no less 'permissible' than black, would it?

Kind Regards, John

plugwash · 18 Feb 2019

JohnW2 said:
Is the 'interconnect' control voltage of a mains-powered smoke alarm LV or ELV? I would rather presume the latter, since they presumably have to be able to 'link', as well as 'work' when running from their backup battery?

I woudl presume it's ELV voltages but not safety-isolated from the mains. So it has to be treated like mains from an insulation point of view.

Risteard · 18 Feb 2019

JohnW2 said:
I think this is all getting a bit confused/confusing. Black is a permissible colour for identifying a control circuit line conductor, but Andy suggested that it was not a line conductor, and ...
... but, just like black, brown is also permissible colour for identifying a control circuit line conductor - so, if we are going to call this a 'control circuit' (which I think is probably the main thing which is confusing the issue) brown would be no less 'permissible' than black, would it?

Kind Regards, John

It's supposed to be distinguishable from the 230V phase though, so brown isn't strictly correct.

JohnW2 · 19 Feb 2019

plugwash said:
I woudl presume it's ELV voltages but not safety-isolated from the mains. So it has to be treated like mains from an insulation point of view.

Yes, that's what I thought/assumed, on both counts.

Kind Regards, John

JohnW2 · 19 Feb 2019

Risteard said:
It's supposed to be distinguishable from the 230V phase though, so brown isn't strictly correct.

.. but you can't really have it both ways. You say it should be black, because BS7671 says that is 'permissible' for the line conductor of a control circuit, but also say it can't be brown, even though BS7671 says that is also 'permissible' for the line conductor of a control circuit.

Kind Regards, John

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Smokes and sheathing the interconnect

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