DMX interference

[spinmaster1]

Just a quickie,

Could someone tell me if DMX running alongside power will have interference or other problems. It will be running along with power in the same trunking box for only 3 metres. The power in the trunking includes 24 channels of dimming power, and 4 x 63a cable feeding the dimmers. Hope this is enough info.

 

Thanks,

Rob.

[tokm]

Never had an issue like this with 40M+ runs of DMX and dimmed & hot soca loomed together, so doubt 3m will give you any trouble!

 

HTH, T

[DonkiDonki]

DMX cable is pretty resistant to mains noise like that, you are likely to have problems if you use mic cable though; so as long as you get decent cable there should be no issue.

[PaulDF]

I seem to remember there is a section of the iee regs regarding mixing band 1 and band 2 voltage systems, think it says about placing them in separate compartments, making sure the band 1 cables are rated to the highest present voltage or using screened cable (screen needs to have the same current carrying capacity of the largest circuit).

[lightsguydave]

What's the difference between 3 pin DMX and 3 pin Mic signal cables? At work they're fairly interchangeable since FOH DMX rigging gets done with mic signal cable because it's black, and all our DMX is luminous yellow!

 

I know this problem could be easily solved with the purchase of black DMX but that's boss man's decision.

[pritch]

What's the difference between 3 pin DMX and 3 pin Mic signal cables?

 

Impedance. DMX cable should be 120 ohm (although Cat5 is mentioned in the DMX specs, and is 100 ohm). Mic cable has a considerably lower impedance.

 

Now... it might not make a difference, and you may get away with it. But, on the other hand, it's something that's outside the spec, and has the potential to cause issues.

[PaulDF]

Also I don't believe 3 pin XLRs are mentioned in the specification for DMX. The proper termination is a 5 pin XLR - shame the manufacturers of equipment don't seem to agree.

[timmeh2]

I seem to remember there is a section of the iee regs regarding mixing band 1 and band 2 voltage systems, think it says about placing them in separate compartments.

 

Hi

 

An interesting point, this is mentioned in the 17th;

The Wiring Regulations (17th Edition) state that voltage Band I (ELV) and Band II (LV) circuits shall not be contained in the same wiring system unless:

 

1. Every cable or conductor is insulated for the highest voltage present.
   
2. Each conductor of a multicore cable is insulated for the highest voltage present in the cable.
   
3. The cables are insulated for the system voltage and installed in a separate compartment of the cable ducting or cable trunking system
   

I suppose the crux of the argument lies in whether you consider data signals to be LV or not. If this were the case, then many, many installations where DMX cabling is not trunked or pass close to power cables they would be effectively illegal. A prime example is on a truss where you've got power and data running all over each other. We all know that a DMX or CAT5 cable won't withstand line voltage being sent through their conductors or their shielding, and even worse is if you add three-phase soca into the mix where you've got 415V between certain pins. Are they now saying that data cables must be rated to this voltage?

 

For me, as usually DMX is not explicitly powering anything, and that you can also classify it as SELV, I'd say that this rule does not apply. This classification would also be correct for fixtures that use PoE (Power-over-Ethernet) or other LED systems that send +24VDC through one (or more) pairs and DMX through another on the same cable.

 

All the best

Timmeh

[Davethsparky]

I think you may be missing the point, the regulations say that they cannot be contained within the same wiring systems not that the cables cannot be near each other. insulated and sheathed cables are considered to be a wiring system in themselves (flex, t+e, SWA, etc) the only time this really affects us is when using wiring systems where you have cables with a separate form of containment (conduit, trunking, tray etc)

 

Also it should be noted that not complying with the 17th edition does NOT render the installation illegal, they are non-statutory regulations. If you deviate from them but you are satisfied and can prove that your installation is in no way unsafe then there is no problem.

 

And yes it is extra low voltage because there is a voltage present and it is below 50 volts AC or 120 volts DC.

 

So to answer the OP, you are unlikely to get interference if you put your DMX cable in the mains trunking but you will not be in compliance with the wiring regulations if that cable is not insulated to the highest voltage present (either 240 or 415 volts)

[sleah]

Dave, are you saying, in a nut shell, that running a DMX cable alongside flex or T&E is OK but running it alongside mains singles is not OK?

[kitlane]

What's the difference between 3 pin DMX and 3 pin Mic signal cables? At work they're fairly interchangeable since FOH DMX rigging gets done with mic signal cable because it's black, and all our DMX is luminous yellow!

 

I know this problem could be easily solved with the purchase of black DMX but that's boss man's decision.

 

DMX is a 'square' wave running at 250kbaud (250kHz). A decent mic cable is designed to pass the lower, audio frequencies (22kHz and below) and possibly attenuate the higher frequencies. It is those higher frequency harmonics that make a square wave.

 

There is a danger that the mic cable will distort the DMX signal. If that happens you are relying on the receiving equipment to have sufficiently good electronics to 'rebuild' the wave correctly. Start putting a few splitters in there and things can get a whole lot worse.

[DonkiDonki]

What's the difference between 3 pin DMX and 3 pin Mic signal cables? At work they're fairly interchangeable since FOH DMX rigging gets done with mic signal cable because it's black, and all our DMX is luminous yellow!

 

I know this problem could be easily solved with the purchase of black DMX but that's boss man's decision.

 

 

DMX cable relies upon the physical characteristics of the cable for it's "error correction" system to work, it uses a simple voltage checking system rather than anything software based; this is why un-compliant cable such as mic-cable will work a lot of the time (or at least the errors are not bad enough to be noticed) but quickly falls over when faced with interference issues.

 

If you really want to use cheap cable, some cat5 can be a better choice than mic cable.

 

If your interested to actually see the difference the cable makes, run a universe of DMX into something which shows the entire output on a screen (the entec pro widgets have a utility for this) set all the channels to a static value and then experiment with different cable types; try deliberately running along side mains for dimmers, air-con, etc and watch the screen when the other equipment causes interferance.

 

This is also handy to see what happens when you simulate damged cables by deliberately un-terminating and causing open circuits, etc.

[Davethsparky]

Dave, are you saying, in a nut shell, that running a DMX cable alongside flex or T&E is OK but running it alongside mains singles is not OK?

 

Nearly but not quite. What I'm trying to say is that as far as the regulations are concerned running DMX cables alongside flex or t+e does not present a problem in terms of the voltage rating of the insulation.

I say this because sheathed cables are considered to be a wiring system in their own right and that rule is concerned with cables being contained within the same wiring system. It would however be unacceptable to run DMX cables alongside the T+E if that were also contained within another wiring system, trunking or tray for example.

 

I apologize if I did not make my original point clear.

[KevinE]

On the same subject, maybe someone with both DMX and NICEIC-level competence can help me resolve an argument.

 

This week we've been installing a DMX lighting rig in a swanky highstreet bar. We've been working alongside an NICEIC contractor who is installing mains powered exit signs and 240V halogen downlighters in the plasterboard ceiling.

 

We wanted to pass a DMX cable through a hole in a joist through which is already threaded a 1mm t&e lighting cable that goes to an eyeball light. The hole is accessed now only through an eyeball-light-sized handhole and drilling another hole in the joist is impossible.

 

The electrical contractor told us that if we put our DMX cable through that joist hole, he would not sign the installation off because we would be breaching the 500V insulation barrier. I told him we wouldn't, since the 1mm t&e is already double insulated and if it's safe for people to touch in a surface laid situation, then its safe enough to share a hole with a DMX cable. The argument went on and eventually he said he wouldnt be responsible for his actions if he meggered the circuit and it blew up our lights because datacables wouldnt be 500V rated. I said it wouldn't, the bar owner came in and gave us permission to put the cable in, and the sparky walked off in a huff after threatening to kick away the base of my ladder I was up!

 

We left the cable dangling with instructions for the bar owner to put the cable through if he wanted to.

[PaulDF]

The twin and earth isn't really double insulated, it is a sheathed insulated electrical cable. I very much doubt it will cause any bother with it being in close proximity but technically it shouldn't be run parallel to a mains cable, there should be a separation distance.