2-core earthed shielded cable, as a 3-core cable

[LinearPancakes]

We're doing first fix for a lot of 24V DC lighting circuits. Some of these are tuneable white, warm-white + cold-white, so we need 3-core cable for this.

 

Since there are a lot of long cable runs from controllers to the led-based lights, and the controllers use PWM at probably 10kHz-20kHz (more than the usual 1kHz of Hue), we requested shielded cable to minimise any radio interference. We're also using aluminium profiles for the strips to help with both heat dissappation and shielding.

 

The electrician told were using 4-core cable as that's what they could source, but on checking the cable positions prior to the plasterers arriving (tomorrow 😳), I've noticed they used only 2-core cable. It does have an aluminium earth that I've learned they were told can be used as another core, but it's in direct contact with the aluminium shielding.

 

In this arrangement it would seem that not only do we not get the requested shielding, but we maximise radio interference by using the shielding as an antenna.

 

I see three options:

1. Use a fixed white tone (only use two of the cores)

2. Stop worrying and use the earth/shielding as well

3. Get them to replace the cables

 

Problem is, I don't like either of the options... The electrician has indicated the lighting cables are away from 240v cables and the different frequency from the WIFI should mean there's no issues. Sure the plastic part of the cable sheathing will attenuate, as will plasterboard, etc. But there's no getting around that there will be a lot more radio waves emitted than if it was a proper 3-core cable with shielding. I'd have preferred unshielded 3-core cable to this arrangement, as surely using the aluminium shielding will help emit. But I don't know enough to know if these are rational concerns.

 

Anyone able to offer any further insight?

 

[ProDave]

That's fire alarm cable.

 

Us the "earth/screen" core as 0V and interference should not be an issue.

 

So why did the electrician tell you he was using a 4 core cable and then fitted a 2 core & earth?

[LinearPancakes]

Thanks @ProDave. That.. is a good question, to which I will seek an answer tomorrow.

 

The controller I'm using has +24, WW and CW terminals (well, and unused RGB terminals). Would the +24 be the 0V in this case? Or perhaps we also need to ground something?

 

I'm curious how this would still allow shielding to do its job. Is it that most of the "power" is going via the main two cores? (Sorry, I tried and failed to get my head around electron/charge flow!)