DMX Safety

[charliemidi]

While mounting the lighting for an event in the theatre where I work, there was a short circuit with some par leds and the lighting desk was damaged by voltage that went via the DMX cable. A visiting lighting technician claimed that it is standard procedure to install in stage lighting systems a DMX splitter or buffer in order to protect or isolate the lighting desk. I know what splitters and buffers are for but I haven't heard about this "safety procedure" before. The desk is an ETC Express 72-144 Does anyone have any info on this?

[Ynot]

Hmmm...

I'd be rather surprised if a simple short circuit would do ANY damage at all to the desk - many reputable desks will have an element of protection on the output anyway.

That's not to say that a short circuit to a higher voltage, with sufficient current capability wouldn't do the desk any good...

 

We'd certainly need more detail about what the fault was on the LED fixture...

[maeterlinck]

As Ynot says. What was the short between?

 

As for standard practice, I wouldn't necessarily say so. It is common practice, especially in fixed installs with some sort of DMX patch as it makes the whole thing easier and betterer. You could argue it's good practice though...

[themadhippy]

The visiting lampy wasnt trying to cover his mistake by any chance?

[charliemidi]

There were 4 par leds mounted on a scaffold. The short circuit was fairly strong and visible to the stage hands working nearby at the time. The par 4 leds were connected to 4 other par leds some distance away using a homemade 4 to 3 pin adaptor cable (probably the source of the short circuit). The par leds and the adaptor cable were rented but supposedly working. The rental company and the lampy are both denying responsibility. The DS75176BN chip on the motherboard of the desk was blown to bits.

The lampy is trying to claim that it is standard practice in theatre lighting sytems in first world countries to install a splitter or buffer when using par leds and/or DMX.

[themadhippy]

The lampy is trying to claim that it is standard practice in theatre lighting sytems in first world countries to install a splitter or buffer when using par leds and/or DMX

Do pass this on to the lampy " stop talking out of your ar5e "

[niclights]

Indeed. It is clearly the fault and responsibility of the guest, not the venue. There is no standard for using buffers and it is not an excuse for damaging your equipment. If they felt a buffer was necessary they should have provided it. If they continue this argument then ask them exactly what protects the console from a faulty buffer. Nonsense.

 

Furthermore, DMX does not carry power and it is not possible to short or cause damage simply by having a bad cable. The only way this could happen is if the connected equipment itself is faulty (or the wrong equipment was connected). Incidentally, I presume you meant 5-3pin adaptor, not 4-pin which are commonly used to carry power and data (ie. colour scrollers).

[dbuckley]

I don't know about the professionals, but this amateur always uses a buffered splitter to protect the computer against damage from electrical badness coming back through the DMX cable.

[David Duffy]

We recently repaired an LED PAR can which had whacked the console as well as several other fixtures. The fault turned out to be an internal bolt that was too long, and had pushed into the toroidal transformer! The irony was that the bolt was for the earth connection. The build quality was poor I must say and reflected the low price of the units.

 

So it is possible for a fixture to belt the bejubus out of a console. The transceiver chips had a chunk blown out of the top in each case.

 

I'd suggest that a venue where outside gear can be plugged into the house system should provide an isolated split. I just installed one this morning at a school in fact. It should help limit damage to one only segment in case of a serious failure. Most venues will have spare dimmer channels and fixtures, but not many have a spare console.

[charliemidi]

Niclights - as for the adaptor from what our lighting techs tell me it was a four pin (power/data) to three pin adaptation, y'know like a mic XLR connector, I guess they thought they'd leave out the earth and use that terminal for the data.

 

Unfortunately in the rush to mount the lights before rehearsals all the possibly faulty gear (leds and cable) was returned, so the true cause was never established.

 

Thanks for the feedback everyone, I'm actually the sound guy at the theatre but I'll pass all these suggestions on.

[david.elsbury]

If it was a 4 pin connector, I wonder if it was from either a scroller power adaptor or from something like a ChromaQ Colourblock or similar? DMX is normally presented on a 5 pin connector, or a 3 pin connector. If my 4 pin theory is the case, the (fairly) standard pinout would be:

 

1- Ground (data and power)

2- Data

3- Data

4- Power (usually +24vDC)

 

and then DMX is

 

1- Ground

2- Data

3- Data

 

In theory a straight wired 4- 3 pin adaptor would work, though I'd hesitate to say if it was standard or not...

 

Of course, without knowing what adaptor was there, what kit was there, etc, it's hard to know.

[back_ache]

The DS75176BN chip on the motherboard of the desk was blown to bits.

 

 

The dimmers my old company did used a combined 12v/data cable and so would have the transceiver chips in a socket so we could treat them as "sacrificial", I had to swap them out many-a-time due to "creative" wiring and would always carry a tube of them with me.

 

In our theatre we use a DMX splitter for our distributed dimming, not because we need to, but by each area having its own feed, diagnostics get easier.

[Ynot]

Hmmm...

 

Regardless of whether a splitter should be used (which is NOT 'standard' practice, regardless of whether it is a wise precaution) the problem you have is a little deeper than that.

 

As I see it the fault has two potential sources:

 

1. You had a faulty LED can which somehow resulted in power (anything up to mains voltage) being shorted to one or more of the DMX pins. That would most certainly have fried the desk in some way if AC mains found it's way onto the DMX out.

 

2. This FOUR pin XLR you have. As has been said, many times the international standard for DMX is 5-pin, but many manufacturers use 3-pin, especially on budget gear. I am aware of NO kit manufacturers who use 4-pin for DMX and suspect that somewhere along the line someone has been sold a turkey. As above, 4-pin IS a standard for use with scrollers and other kit that needs data and power down the same cable. I would question WHY there was a need for this 4-pin at all. Is that some local home-brewed install at your venue? Or was the lampy trying to pick up the DMX element from a scroller PSU output? We have to assume that the LEDs are pukka 3-pin in/out.

You state in the same post above that the adaptor is both home made AND is rented - so which is it??

 

A short circuit across any of the pins in a properly terminated DMX cable (be it presented 3 or 5 pin) will NOT cause a serious fault such as blowing up a desk in the way you describe. HOWEVER, If this 4 to 3 pin adaptor has a short circuit connecting one of the data legs to the power from something like a scroller PSU, then that very possibly could cause serious problems.

 

You need to do some serious investigation to identify what exactly was happening and what went wrong, but it des strike me that if the visiting company's lampy did all the rigging and connecting then ALL the responsibility (and thus costs) fall firmly at his feet.

[Hilary Watts]

The OP states that the short circuit was 'fairly strong and visible to the stage hands working nearby'. That sounds to me like a short circuit on a mains cable, which is consistent with the statement that the cable was a combined power/data cable - the LED cans would need mains power, not 12 or 24volts. If that was indeed the case then I think I would want to make further enquiries of the lampy/hire company concerned - it's possibly a matter of luck that it was only a chip which was fried, rather than a crew member.

[litemog]

I don't know about it being standard procedure, but a lot might depend on the quality of the equipement the lampy was used to working with. One of the places I work in has a set of cheapo LED cans which have to be run on their own leg of a DMX buffer box otherwise they play havoc with anything else DMX controlled.