SELV wiring

[aptsys]

I'm shortly going to be running an RS485 and CAN link to every room in my house for some lighting fixtures and various control panels I've designed using some CAT5. The plan is to also run an 18V supply in conjunction with the CAT5, but having left my OSG at my parents a few weeks ago I can't quickly check what the regs are regarding the types of cable I can use for the low voltage supply.

The supply to each room is electronically current limited to 6A with a 1V/20mA slope to 0V for currents exceeding 6A. There is also a traditional glass fuse rated 6A should the electronics fail for whatever reason.

I'd like to run 1.25mm or 1.5mm flex rather than T&E so that it can easily be differentiated from mains cable, but I cannot remember if it is stated that flex is forbidden for SELV in fixed installations.

Is anyone able to enlighten me?

Many thanks

 

[Adam_151]

I can't see any reason why you can't

OOI, you actually using DMX512? or just making up your own dataformat and chucking it over RS485 ?

 

[breezer]

if it is dmx 512 then you can use a 2 core screened and pick up the mains supply localy

but i some how think it wont be

 

[Adam_151]

Actually, you could do that regardless of what you are actually sending over the RS485 line whether it be DMX or something else, you just need a ground reference (the screen in 99% of cases) in addition to the data pair, then it doesn't matter where you pick up power, of course if you are distributing power from a central psu then you are going to have the ground reference there anyway

 

[aptsys]

The RS485 link isn't using the DMX protocol, although the electrical characteristics of the physical layer are the same, as is the baud rate.

I was previously going to use a DMX type protocol, but decided against it since I felt it was a better use of resources to make the fixtures intelligent. It's a <address>,<command>,<parameter 1>,<parameter n-1>,<parameter n>,<checksum> protocol now.

It was only the power supply cables which I was unsure about because of my preference for using brightly coloured flex so that it could easily be distinguished from anything else.

 

[Space cat]

Have you allowed for voltage drop? At 6 amps you will lose about 150 mV per metre on 1.5 sq mm cable.

 

[aptsys]

Yeah, the voltage drop is no problem, although I might increase the rating of the cable if the budget allows The fixtures have regulators on-board which should theoretically allow operation right down to 4V.

 

[plugwash]

even allowing 2V of drop at 6A that is only 13M before your volt drop is out of spec.

I would seriously advise you to use a higher voltage, 24V is a good choice it's still low enough to be safe but you will have far less volt drop problems.

 

[electronicsuk]

even allowing 2V of drop at 6A that is only 13M before your volt drop is out of spec.

I would seriously advise you to use a higher voltage, 24V is a good choice it's still low enough to be safe but you will have far less volt drop problems.

How do you figure? OP states that p.d. at source is 18v, at 150mV drop per metre and an absolute minimum of 4v required at the terminating end then that allows (18-4)/0.15=93 metres of cable.

FWIW, have you considered using POE or developing your own similar standard? A POE midspan that injects power over the spare pairs doesn't care what goes over the data pairs as all the negotiation is done over the spare cores. It only takes a classification resistor of the correct value and a few discrete components to make a pass switch and you'll have something that a standard POE midspan will happily source power to. The only problem then is designing a regulator that will give you 5v (or whatever you need) from 48v, because a standard linear regulator isn't gonna cut it

 

[plugwash]

sorry looks like I misread the post, i'm sure I saw 6V in there somewhere.

 

[aptsys]

The regulators in the fixtures are switchers, but the choice of components has meant that the input voltage can't be greater than 32V. I may end up using a 24V supply which will reduce the voltage drop on the cable to 112.5mV/meter and reduce the maximum current demand to about 4.5A.

I'd considered POE, but decided against it because I wanted to have separate power supply circuits to each room while keeping the RS485 link intact.