Low voltage and mains in same duct

[Tim Lamb]

I had assumed that linking sensors in outlying barns to a common burglar alarm panel would simply be a matter of drawing in cables and connecting up!

However, the existing 4" ducts already contain steel wire armoured cables carrying mains supplies.

The alarm salesman was adamant that the only way round (other than radio/second set of ducts) is to insert a metal conduit. This is not practicable because of concreted over bends. Radio looks expensive and range iffy (50m+ and not line of sight).

Could the requirements of the relevant regulations be met by using SWA for the low voltage circuit?

regards

[Stephen Dawson]

Tim,

The way to get around this is to use a mains rated multi core cable. Something such as 1.0mm 8 core SY would be fine.

Regards

Steve

[Tim Lamb]

In message , Stephen Dawson writes

Oh!

Is this because the insulation of the low voltage cores is adequate for exposure to the mains supply?

The IEE regs. (fifteenth edition, I know I'm just old) calls for an earthed metallic screen between the two supplies.

regards

[fred]

In article , Tim Lamb writes

It has certainly been reported here that where one cable in a group is carrying mains then all the cables in the group must be mains rated. This does seem a bit harsh when the mains cable is SWA with armour as earth but I'm not sure whether this would meet earthed metallic screen requirement you describe.

As an aside, I'm not sure where the 'all mains rated' requirement spawned from as the sheath of a mains rated cable is finger safe for contact, why should the contact of a low (safety, extra or whatever) voltage cable require a more strenuous restriction.

[Tim Lamb]

In message , fred writes

I suppose it is not unreasonable that an engineer called in to work on what he believes is a safe *low voltage* system might be at risk from insulation failures. In my case, mice may gain entry to the ducts.

ISTR earlier regulations calling for all cable insulation in a mixed bunch to be rated at the highest voltage encountered.

I think I'll go with the mains rated multicore as suggested unless anyone knows different?

regards

[The Natural Philosopher]

IIRC the issue is to have TWO insulants each capable of taking full mains voltage between HV and LV.

[1501]

If converters are available, have you considered the option of using a fibre optic cable to the barn?

If the strength member is kevlar you would have totally non-metalic cable and hence on problem.

[1501]

If converters are available, have you considered the option of using a fibre optic cable to the barn?

If the strength member is kevlar you would have totally non-metalic cable and hence no problem.

[Tim Lamb]

In message , 1501 writes

Fibre optic is way outside my experience so not considered.

I had wondered about using relays to isolate the sensors from supplies in the ducts but would still have the issue of putting others at risk (an engineer called in to replace a PIR sensor would not expect to find

240V in the control box).

I'm trying to do a cheap job:-)

regards

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[meow2222]

And the swa armour provides that.

The issue with sharing isnt the cable runs per se, but rather what happens where cables are terminated. Terminate both in the same box and you've got a real risk if any cable comes loose. Terminate them in separate boxes and you've got risk if the LV insulated cable touches bare mains terminations, since its not mains rated.

NT

[Tim Lamb]

In message , [e-mail] writes

OK. Not sure I grasp the separate box bit:-)

I envisage feeding 12V DC to energise remote sensors. Other cores will carry alarm signal/anti-tamper whatever.

The existing SWA is supplying 3 ph. to socket outlets and other installed equipment. There is no chance that the LV stuff will be anywhere near mains other than in the duct.

regards

[Andy Wade]

Firstly we need to be clear that low voltage (LV) means mains and that the alarm circuits are ELV.

In my view there is no problem here. In the 16th ed. regs 528-01-02 is relevant and one of the permissible options is:

"(iii) for a multicore cable or cord the cores of the band I [ELV] circuit shall be separated from the cores of the band II [mains] circuit by an earthed metal screen of equivalent current carrying capacity to that of the largest core of the band II circuit."

Provided that we're talking about smallish SWA sizes (certainly up to 16 mm^2 or so) the armour of the mains cables will almost certainly have the necessary current carrying capacity to comply with that and the ELV wiring will not need to be mains insulated.

In the 17th ed. the same voltage band definitions apply and reg. 528.1 (vi) says much the same thing.

If the problem is political rather than technical then using a 300/500 V rated multicore flexible cable (something like 'YY' control cable, 0.75 mm^2) for the alarm would comply with the requirement for both cables to be mains insulated. You don't need armoured multicore for the alarm.

[Tim Lamb]

OK:-)

Good. Probably 16mm^2 max.

OK

I think *political* in that Aegis policy is intended to provide safe working conditions for their staff.

I plan to purchase the various elements and install in stages myself.

Thanks to all who have contributed.

regards

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