dimming ballast: ELV and mains in close proximity

[sm_jamieson]

Wiring a dimming ballast, I can get a combined mains switch / low voltage potentiometer from multiload.co.uk that fits into a grid switch. Their info:

They have a 3-pole mains terminal for on/off or changeover (2 way) switching of the mains supply up to 10A: push-on/push-off action of the control knob switches the mains supply on these mains terminals. The modules also have a well separated 2 or 3-pole low voltage terminal which produces 2 or 3 wire 0-10V analogue control from the low voltage terminal when the control knob is rotated.

This is obviously to do with PELV / SELV.

1. In the single back box, how should the two types of wiring be connected / separated to pass the regs ?

1. The control cable needs to be run amongst mains cables. Is it sufficient to use double-insulated shielded 2 core cable with the shield connected to earth ? Something like this:

[Tim Watts]

sm_jamieson (sm [e-mail]) wibbled on Wednesday 05 January 2011

11:53:

Whilst it is fine to run LV circuits adjacent to a SELV cable *which is suitably insulated for the higher voltage of the adjacent circuits* (LV/240V) this doesn't solve the problem of mixing them in a single box grid enclosure.

If the ELV needed to truely be SELV (because it went off to a bathroom fitting for instance) I would be deeply unhappy about doing this - there is too much chance of a wire dropping off the ELV and touching an LV terminal or vice versa.

However, this is all fairly moot because you (as I understand) are not really after SELV. You have fittings which require ELV (not SELV) control.

What I would do is regard the 0-10V ELV circuit as mains and rate the cable suitably.

I've just looked at the multiload.co.uk site and skimmed the tech specs of the rotary switch you mention - nowhere does it say the 0-10V is SELV, so I'm pretty sure the control circuit must be regarded as you would all mains wiring.

If you wanted to shield the ELV control cable and ground the shield, don't compromise the cable for this - eg do not start using audio cable - use a mains rated shielded cable, eg SY, fire-alarm cable, Earthshield, whatever you can get the cheapest (SY needs special glands). But I suspect it is not necessary.

But have a look at the installation instructions - all I can see is

"High standard isolation of low voltage (0-10V) wiring and mains wiring by use of a suitable fixing and isolation / separation."

Which appears to mean: do what you can...

As they have a UK phone number - you could phone them - ask for "Technical". Sound like you know what you are doing, but simply say this is the first time you have installed 0-10V systems and do they have practical guidance for cabling and routing the 0-10V circuit?

That is just my opinion, but I believe it to be sound (enough that I would do it). But you should wait for some additional comments...

Cheers

Tim

[sm_jamieson]

1

Yeh. The screened cable idea came from an installation PDF for another similar dimmer switch available from RS. It says: "Use twin core screened cable with the outer screen connected to earth." Maybe this is avoid picking up induced voltages from mains cables nearby.

I could certainly phone them.

Simon.