DMX Safety

[maeterlinck]

The only aspect of this that doesn't weigh up properly in my mind at the moment is if there were several things connected to the DMX stream when the fault occurred why was only the desk affected? And another LED Can next door would offer an easier route to earth than a probable long cable back to the desk.

 

Of course if a 4-3 or 4-5 adapter cable was used then could someone have plugged the DMX feed from the desk into the 4 pin "IN" of a scroller PSU?

 

Again if the short fault was on the scroller side of the PSU then the spike would only go as far as the PSU as the DMX signals from a PSU will be isolated from the main stream.

 

Good luck with this.

[charliemidi]

Lots of good feedback from everyone so I'll post more detailed info on the Leds and cables later today. It seems the Leds are Chinese and a short four pin cable comes with each Led. The three pin mic cable was a hack job to link the four Leds to four others that were on the other side of the stage.

[niclights]

It almost sounds as if the LED's aren't actually DMX at all.

[charliemidi]

The Leds are heavy as hell, Chinese made "Hot Machine" model LP-354, cheapos I reckon, not to say Chinese is cheapo (after all Macs are made there) but judging by the quality of the the packaging/ labelling etc.

 

The four pin system is particular to the damn things, so whoever buys them needs to make an adaptor cable to 5 or 3 pins. These adaptors are "homemade" by the rental company. So someone bought and is renting turkeys.

 

As Ynot (and others) suggested, there was probably "a faulty LED can which somehow resulted in power (anything up to mains voltage) being shorted to one or more of the DMX pins"

 

Unfortunately we'll never know what went wrong because as I said earlier, all the possibly faulty gear (leds and cable) was returned, so the true cause was never established.

 

My main query was whether a splitter is standard as part of the theatre lighting installation, as the lampy was claiming. It seems it isn't but it's a good idea. After the burnout the lampy brought in a "Doug Fleenor Design Isolated Splitter".

 

I'm glad I'm the audio technician here!

[KevinE]

This is the OEM link:

Dongguan Jiawen Co

 

What on earth could anyone do to get mains into the DMX cable, unless the installer was using mic cables with earthed (to XLR shell) cables? Then a flash from mains to an (unearthed) fixture that has its DMX plugged in could conceiveably find its way via the outer case, onto the shell of the XLR, down the XLR screen and into the desk. This could also cause visible sparking if the fixtures were hung on painted or galvanised truss etc, maybe not a reliable earth bond to protect a hard-wired DMX connection.

 

This is a good reason why earthed XLR shells (ie on mic leads) shouldn't be used for DMX !

DMX tranceivers are normally floating, ie their 0V isnt tied anywhere to mains earth.

DMX buffers or repeaters aren't a mandatory part of an LED installation!

[Jivemaster]

With fully compliant DMX kit you should have no problems handling 32 devices on a single DMX line, and you should use a terminator.

 

With CE (= China Export) kit anything may happen devices may interfere or may load the line too much.

[charliemidi]

Judging by the info on the link provided by KevinE they don't seem cheapo after all.

I guess the heavy casing is to make them water resistant if their indicated use is: "Waterfall, spring lighting entertainment lighting or underground lighting, Pool etc" - This would make sense why the rental company bought this model because here in Rio de Janeiro all gear suffers from high humidity.

................I've just checked the "Hot Machine" LP-354 Leds here are slightly different - so with a name like hot machine I suspect they're a cheap copy of the quality stuff KevinE linked above.

[niclights]

More importantly the IP68 rating might explain why 4-pin connectors are used. Naturally I (and presumably everyone else) assumed this was an XLR connector, but these are not waterproof. In fact there are very few suitable connectors and other similar rated units I've seen do indeed use 4-pin harting connectors. Not only that but they often use the same connector for power for the same reason. Normally the genders will be such that they can't be accidentally mixed-up, but perhaps it was possible here?

 

I think it's clear that no-one considers buffers an industry standard simply for protection purposes but they're obviously not a bad idea. They are usually essential for distribution anyway. The point was that damage to your equipment by faulty or incorrectly wired touring gear is not your fault and it was not your responsibility to protect against it.

[KevinE]

A google search of Hot Machine throws up a disco lighting importer from Sao Paulo...so maybe these have rebadged the chinese unit just like UK companies do here.