Routing some bell wire

[Tim+]

I’ve got a new motorised garage door and the mains supply for it is clipped to the garage wall high up (and not in a conduit). It’s an old supply cable for a halogen light that’s been repurposed.

I’ve added a simple bell push low voltage switch to trigger the door opening and closing to save the faff of messing with a wireless remote. It connects directly into the low voltage motor control circuits.

I’m wondering how best to run the bell wire back to along the route of the mains cable. Does it need to be clipped to the wall away from the mains cable or can I just cable tie it alongside the existing cable?

Clipping, if required, would be a bit of a pain as the internal garage wall was once a outside wall and is rendered.

Stick it down with gobs of silicone maybe? ;-)

Tim

[ARW]

Cable tie it.

It's only a switch wire not a data cable.

[Tim+]

Cheers. Thought that might be the case but wasn’t sure.

Tim

[Brian Gaff]

Well, I'd be a bit wary of just using ordinary bell wire outside as Rodents love it. I had some feeding a thermometer high on a fence some years ago, and after a couple of years it failed. it had been gnawed through and had suffered damage in other places. As to securing such wire, well, maybe rethink it and house the psu at the garage end and then make the wiring a lot shorter, which could be clipped to anything you like I guess. Brian

[Scott]

I was told once that a DC cable cannot be run alongside an AC supply because of inductive interference. Is this incorrect?

[ARW]

There will be no interference on a 12V switch that is just a break or make contact.

[Tim+]

I can confirm that the garage door opener works just fine with the DC cable strapped to the mains cable.

Tim

[Joe]

There will be an induced voltage, depending on the AC current, but it will be microvolts. Certainly not enough to cause a 12V DC switch to arc over.

If the 12V line is isolated from ground, the pair may pick up some tens of volts by capacitive effects, but at an extremely high impedance, and the differential voltage between the two wires will be tiny. Basically how balanced-line audio works.

[SteveW]

The cable to power the electronics of my boiler (in the kitchen), has to come down in the hall, alongside other cables. If the FCU for the central heating (upstairs) is turned off, the boiler supply floats at about 90V - and it certainly gives you a jolt if you touch it.

[Chris Green]

I was going to say this earlier but isn't there a requirement to space ELV (which the bell wire to the switch presumably is) by at least 50mm from mains wiring? It's a wiring regulations safety thing rather than anything to do with electrical interference. Maybe it only applies to fixed wiring in conduit and such though.

[Scott]

That's what I thought. An electrician once told me the two had to be separated. He said it was because of induction, but maybe this reason was incorrect.

[Tim+]

If Adam says it’s okay, that’s good enough for me. ;-)

Tim

[Pancho]

I assumed it was a precaution against a fault, such as rodents gnawing, dampness, or a cut, causing the mains wire and data/bell wire to form a circuit and for the data/bell wire to become live?

[Chris Green]

I think that's right, I think that "mechanical separation" can be an alternative, i.e. a conduit with separate channels for mains and 'not mains'.

[ARW]

There is. Especially for data and telecommunication cables - 50mm.

However in this case although in theory there should be form of segregation it's not going to cause a problem.

If Tim had used 2 core mains flex cable tied to the T&E he would be completely with in the regs - and surely that is just as likely to pick up interference as bell wire in this case.