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]

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

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]

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.

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]

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.

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]

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.

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]

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

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

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.

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

 

[meow2222]

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

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

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.

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]

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

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]

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

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

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.

Good. Probably 16mm^2 max.

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

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.

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