Board logo

Running 24 volt DC & data cables, help please
Andybarbet - 3/12/14 at 10:36 AM

Hi Guys, I need some help.

I am trying to find out if it is ok to run 24v DC power cables near to Cat 6 data cables in the same cable tray & also what type of cable is best for a 24 volt DC feed ?

I really need some definitive info that I can quote to a customer but after a lengthy search I cannot come up with anything set in stone ?? Our equipment supplier is in europe & is giving us sketchy information about this.

We are running some equipment that needs a data cable to send data to a small lcd screen but we also need to power these small screens with 24 volts DC.

We are running Cat 6 cables for the data side & basically we have narrowed it down to either using a:

2 core 1.5mm shielded cable to run the 24v DC power or:

Using another Cat 6 cable & connecting multiple strands together depending on the power consumption of the displays that we are connecting.
(If we go for this Cat 6 option, any idea where I can find out how many strands to use depending on the power consumption of these screens at 24 volts ?)


We had decided to go for the 2 core cable for power, going on previous installs by our supplier but the customer is now worried that it will cause interference because it is not twisted pairs as in a Cat6 type cable.

Does anyone have any experience/info that may help or can anyone point me in the right direction for this ?

Many thanks


wylliezx9r - 3/12/14 at 11:11 AM

I work in aerospace and in a similar situation we have to allow a 1 inch gap between the two. I can't see why it should be any different in your case especially as the power is D.C.


SPYDER - 3/12/14 at 11:34 AM

Anything here to help...

CLICK HERE


Table B at bottom would suggest 0mm if a divider is present or 50mm with no divider. If >120Vdc.
So 24Vdc should be fine at 50mm it would seem.


nick205 - 3/12/14 at 11:46 AM

We have developed a number of devices now that use Power over Ethernet (PoE). PoE enabled switches and routers inject power onto lines in the Ethernet cabling, which suitably equipped devices can use. You can also get in-line PoE injectors (like a laptop charger which the Ethernet connection passes through, picking up power along the way) where the switch/router isn't PoE enabled.

We've used it to power devices in retail, commercial and industrial situations, where it reduces the no. of cable runs, power sockets, failure points etc. It's very effective! I believe it's also used in CCTV applications.

More details here - http://en.wikipedia.org/wiki/Power_over_Ethernet

In your application I can't see any issue with running the DC cable alongside the data cable.


Agriv8 - 3/12/14 at 12:36 PM

My thoughts

PoE is the answer if that's an option (see here http://en.wikipedia.org/wiki/Power_over_Ethernet) you can get power bricks that will add PoE to a cat 5 cable if not wanting to upgrade Switches.

Shielded cat5 data cable used to be an option if running near light fittings other electronic noise.

My gut felling is 1.5mm for power you will be on with 24v Dc don't wrap the Cat6 around the DC or the other way around.


ATB agriv8


Andybarbet - 3/12/14 at 12:52 PM

Unfortunately POE was decided against by our customer, this was the original idea from our supplier.

We have been told that we shouldn't see any interference problems but up until now, none of the experts that I have spoken to have been able to point me in the right direction of some real info, its doing my head in now as we are under a really tight deadline & have to order the cable in the next 24 hours to have any hope of meeting the target install.

Thanks for the info so far, keep it coming :-)


simonrh - 3/12/14 at 02:49 PM

You shouldn't get electrical noise from a pure DC power cable, there is no alternating field to create it (the same reason there is no such thing as a DC transformer). It depends on what the load is as to whether it can create noise on the cable as it runs.

The worst you should get is a single pulse when the DC is turned on and a single pulse when it is turned off but that is it.

Ethernet cable relies on the voltage difference between a pair of cores to transmit a signal rather than shielding to protect it (it used to be called UTP or unshielded, twisted pair when I dealt with it). This means if you get a voltage or noise induced on one conductor in the pair it also gets induced on the other equally (because they are twisted tightly round each other. The voltage difference remains the same and so the signal remains relatively clean even in a noisy environment


Andybarbet - 3/12/14 at 03:56 PM

Thanks Simon, that makes sense to my untrained brain.

The initial load is maximum 2 amps & then it drops to 300mA when running on one of the sceens, the larger one's im still awaiting the complete spec for.

This is really helping so far guys, Thanks


stevebubs - 3/12/14 at 10:00 PM

Try and get a copy of the documents for BS EN 50174

As above, DC shouldn't be particularly harmful but if you want to play it safe, I'd allow 2 inches which (I think) is what is recommended for Mains separation...


stevebubs - 3/12/14 at 10:04 PM

perhaps relevant and free...

http://www.anacom.pt/streaming/1mike_gilmore.pdf?contentId=718859&field=ATTACHED_FILE


ashg - 4/12/14 at 12:40 AM

it will be fine i work for a medical equipment manufacturer and we do similar all the time. if the customer wont accept your recommendation then their only real alternative is either to build a prototype and submit it to an EMC test house to test the configuration and produce the required paperwork or use fibre optic network cabling. both are expensive options.

as previously said, dc is pretty noise free but if your device is noisy (e.g. cheap inverter boards to run the screen backlight) it will feed back down the line, thats not the end of the world as a good power supply, suppressor and ferrite rings should solve most feedback. you really need to get the emc test results for your screen/device. the manufacturer will have them as they cannot get a ce mark without submitting samples for emc testing.


are you running data over the network cable or a video signal?


Andybarbet - 4/12/14 at 09:33 AM

Hi Ash,

We are running data, 4-20mA signals from some environmental monitoring units.

I will contact the supplier for EMC results too,

Cheers


rgrs - 4/12/14 at 10:20 AM

Have a look at the specifications for Belden 8723

Suspect this will probably satisfy your requirements, i used it a lot for data (RS485) and power both ac and dc @24v.

With an overall screen and drain wire it helps eliminate any issues with interference.

Roger


Agriv8 - 4/12/14 at 11:31 AM

quote:
Originally posted by rgrs
Have a look at the specifications for Belden 8723

Suspect this will probably satisfy your requirements, i used it a lot for data (RS485) and power both ac and dc @24v.

With an overall screen and drain wire it helps eliminate any issues with interference.

Roger


That's a blast from about 20 years ago I must have made 100's of 232 cables with 8 pair to connect dumb terminals to Mux's still got the scars from slipping with the tool to insert the crimped pins into plug !!!

atb agriv8


Disgruntled karter - 4/12/14 at 02:48 PM

OK my four peneth worth

I used to be the product manager for structured cabling for BT and used to run the only independent privately funded test lab, for structured cabling, in the country.

Be aware that any pulses induced into the structured cabling can cause things such as IP phones to drop calls as such we used to insist on having an earthed screen between the structured cabling and any other cables.

The reason being is that although its rare to get problems , when you do they are a nightmare to diagnose as they tend to be intermitent and can cost a fortune to sort out.

You also run the risk that if you have any problem with an application the software supplier will always blame the cabling if you give them a sniff of a chance to point the blame elsewhere