Best way to use a CAT5 as a switch circuit?

[Rockingit]

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??

 

[plugsandsparks]

Depends how long. You can use 24V DC , then have industry standard stuff. You can double up the cores. Got to work.....

 

[Spoon]

5v psu and a 5v relay at the receiving end??

Depends on cable length. As above mate, I'd go with 24V PSU & relay (depends on how you are sending the signal down the cat5) and 24V relay at the receiving end. Less voltage drop.
There are 8 cables in a cat5, so group 4 for the positive and 4 for the 0V. That's assuming you are only sending 1 signal down it. Stick 24V at one end and measure the voltage at the other, see how much it has dropped. Then get a relay/contactor to suit that voltage and the load going through it.

 

[freddo]

There wont be any volt drop to measure if there is no load on the end of the cable though?

 

[PEG]

True....but not hard to simulate with a couple of known relay coil loads....i've used 12 or 24v lamps of a similar wattage,and read the voltage.
Adapt,overcome,survive.....come on lads,by the time the greens have us all on air source and EV's,there is only gonna be a couple of volts at everyone's incomer...?

 

[brianmoooore]

Short the far ends together and measure the resistance (a) between the two groups of wires at the source end, then look at the coil resistance (b) of possible candidates for the relay. The voltage appearing at the far end of the cable with the relay connected would be b X 24/ a+b. If this is close to the stated coil voltage for the relay being considered, then it will do the job.

 

[littlespark]

If the duct is damaged, are you sure the cat5 is intact?

 

[Marvo]

Some CAT 5 cables are even rated to 250v so you might not need to reduce the voltage. Check the specs of your particular cable.

 

[pc1966]

Some CAT 5 cables are even rated to 250v so you might not need to reduce the voltage. Check the specs of your particular cable.

They might be 250V, but that will be DC and not with the kV or so over-voltage spikes you see on the mains.

I would stick with 24V as a easy option for such a cable. I did a quick search and got 0.145 ohm/m (14.5 ohm/100m) as a cable spec:

http://www.farnell.com/datasheets/1311844.pdf

With 600 ohm for 24V DC coil for something like this:

https://uk.farnell.com/finder/55-33-9-024-0000/relay-3pdt-250vac-10a/dp/1169242

Min voltage is 19.2V so PSU of nominal 24V - 5% = 22.8V, coil is 32mA at min volts, so max cable R is 112.5 ohms (3.6V drop and 32mA), give a bit of margin and say 100 ohms, so even single cat5 cores for 100m should be fine (cable R implies 344m for cores out/back).

 

[pc1966]

Actually just noticed the cat5e cable claims 2.5kV peak voltage limit, but only 72V DC working.

 

[Tequila Dave]

Wow. I am just a techie dealing with Cat5/6 and didn't realise what voltage you can put down that cable.... really? POE fair enough but that much? Do electricians think much about twisted pairs when it comes to data cabling? I have have some contractors who don't! ?

 

[WallaceP]

Arduino or rasp pi and relay board. It wouldn't take long to learn how to code a program to read the state of some switches on one and set the outputs on another board.

 

[Rockingit]

Arduino or rasp pi and relay board. It wouldn't take long to learn how to code a program to read the state of some switches on one and set the outputs on another board.

Why would anyone want to over complicate matters to that level?! Because, after all the tech you’re still left needing to switch a relay!

 

[WallaceP]

Why would anyone want to over complicate matters to that level?! Because, after all the tech you’re still left needing to switch a relay!

Its not that complicated. Its just one option. Sometimes its the right option sometimes it isnt. Really depends on a lot of factors that aren’t given in the original post

 

[plugsandsparks]

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.

 

[Chris S]

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?

 

[DPG]

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.

 

[Rockingit]

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?

 

[Chris S]

What part of a 24VDC system wouldn't be SELV, then?

24V DC is permissible, the earlier suggestions of 240V AC would not be!
RS485 is great if you need feedback, sensor or alarm from the remote end, or to add more outputs later.

 

[Simon47]

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.