Best way to use a CAT5 as a switch circuit? | Page 2 | on ElectriciansForums

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Rockingit

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I’ve got a bunch of 230v stuff in one place, a 230v device In another and an inherited data cable in a duct between them.

Don’t think it’s an option to draw a new mains rated cable due to damage to the duct but the data cable is good. So how do I use a relay contact one end to trigger another relay at the other? 5v psu and a 5v relay at the receiving end??
 
He's only trying to create a 240V switch cct using a data cable, with a relay we will all be retired before it needs any maintenance.
But will you have retired before the next periodic inspection when it gets flagged as a bodge? There’s no protective conductor on a run of Cat5 so this needs to be SELV.

Rather than overload and fuse the cable you have I’d look into RS485/modbus control gear which is designed for exactly this. You can get small din rail mounted units one for each end and configure one as master. Talk to audon.co.uk for MOD-2I2O. Chinese options bare board like XTW1-0095 are similar. They can all do 300m on one pair of Cat5 and no networking or PC stuff.

You can also just buy a decent RF switch and forget the cable?
 
But will you have retired before the next periodic inspection when it gets flagged as a bodge? There’s no protective conductor on a run of Cat5 so this needs to be SELV.

Rather than overload and fuse the cable you have I’d look into RS485/modbus control gear which is designed for exactly this. You can get small din rail mounted units one for each end and configure one as master. Talk to audon.co.uk for MOD-2I2O. Chinese options bare board like XTW1-0095 are similar. They can all do 300m on one pair of Cat5 and no networking or PC stuff.

You can also just buy a decent RF switch and forget the cable?

Seems a bit excessive to switch one relay coil on and off!

24VDC down the cable, then a suitable relay at the other end.
 
But will you have retired before the next periodic inspection when it gets flagged as a bodge? There’s no protective conductor on a run of Cat5 so this needs to be SELV.
What part of a 24VDC system wouldn't be SELV, then?
 
Spec for Cat5e is DC loop resistance ≤ 0.188 (Category 5 cable - Wikipedia - https://en.wikipedia.org/wiki/Category_5_cable) which will be for a single pair. So multiply 0.188 by cable length, and that's the max resistance for an in-spec cable. Picking a relay (not quite) at random, coil spec is 12-24V, 1.25W on DC. So 52mA at 24V DC, and around 460 ohm coil.
With a 24V supply, and assuming the coil can indeed operate down to 12V, then you can have up to 460 ohms of cable resistance, which would mean 2.4km of cable. And of course, if you did have significant voltage drop, you could simply use a correspondingly higher power supply voltage so you get your required (e.g. 24V) voltage at the far end.
 
The page lists DC loop resistance so I assume already "there and back" - in much the same way as BS7671 lists volt drop for a circuit rather than a single conductor in the cable tables.
Oh, OK. Overlooked that.
Apologies ?

Though looking back at post #9 from pc1966, he shows the Farnell cable spec which gives "cable resistance" as 0.145 ohms per metre. I'm confused ?

Either way, it's between 1.5 and 2.5km of cable, so "lots" ?
 
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