Pyro control on a Multi?

[Neil Hampson]

Each year at Panto-time, I work on a Village hall panto, which usually includes a couple of Pryos. Due to the design of the stage and the general layout all of the control is at the back of the hall, this means each year I have to lay a load of cable (2xAudioline, 1xComms, 1xDMX and 2xPyro) As I have no multicore these are currently individual cables, over the top fire exits, round the Pros arch, under the stage etc...

I have aquired a small drum of Beldon Data cable. The cable is a individually shielded, 3 pair, with overall shield (I am not sure at the moment if the shields of each of the pairs are insulated from each other), I have enough to run the length of the hall twice as a permanent install, running in conduits terminating at a stagebox at either end. I am happy that the cable will be OK for each signal type in isolation, however can I mix any of these with the Pryo control? The Audio will be mixer to amp, the comms is a standard TecPro ring. The Pyro is a LeMaitre Pyroflash 6/24

I am planning:

Cable 1

Pr1: Audio Left to Amp (XLR3F - XLR3M)

Pr2: Audio Right to Amp (XLR3F-XLR3M)

Pr3: Comms (XLR3M-XLR3M)

Cable 2

Pr1: DMX to Dimmers (XLR5M-XLR5F)

Pr2: Pryo Line 1 (Bulgin 3 pin F-F)

Pr3: Pryo Line 2 (Bulgin 3 pin F-F)

 

As I will use the Bulgin connectors for the Pryo, I am not worried about cross connecting the equipment (I.e Comms to Pryo etc.) but my main concern is is there any possible chance that a DMX, Comms or Audio signal could ever set off a pyro? but I suppose just as importantly, can the pulse from the Pryo firing cause a problem in the DMX or the Audio signal. (I am not sure at the moment if the shields of each of the pairs are insulated from each other)

[David A]

It would probably be O.K. but probably isn't safe enough when dealing with pyros, the consequences of a misfire are too serious.Don't do it and keep the wiring separate.

[Brian]

Best practice is not to fire pyro from remote locations. Ideally the operator should be close to the devices to give him the best possible view of what is going on. If you are running lx (and sound?) as well as pyro then it will be difficult to give the pyro the attention it needs.

[Neil Hampson]

Best practice is not to fire pyro from remote locations. Ideally the operator should be close to the devices to give him the best possible view of what is going on.

 

All pyros are fired at the front edge of the apron ( the safety zone extends quite a way into the public area of the room, but still away from the seating), add to this most are fired in 'tabs' scenes there is no way the pyro can be fired from backstage. The tech desk is situated about 10 metres away, sat on top of a steeldeck, from this vantage point the operator can see the whole of the safety zone, including the area that is of the stage.

 

If you are running lx (and sound?) as well as pyro then it will be difficult to give the pyro the attention it needs.

 

There are always at least two operators (nominally LX and Sound) If there is only two of us generally the LX op will fire the Pyro and the sound op will cover the LX board for the time between the Pyro standby and go, (this can work the other way around if there are a few Lx cues and no sound)

 

[Vent frustration] (sits cowering in the corner waiting flames)

I don't want anyone to take offence or take the following personally (esp. Brian and David A)

I appreciate people comments on what is a safety related topic, however it would be good if someone could answer with comments such as 'No, don't do this because {Insert resonable argument here}' rather than a knee jerk reaction of 'No don't do this as it sounds a bit dodgy' although I do understand why the comments have been made and repect that peple who made the comments. As a the friend of someone who tried his very best to blow his own hand off using (stock) pryos, I do understand the dangers involved using pyro, but I would not dismiss the idea straight away until I had some reasonable evidence that there is a chance of a mis-fire. My main concern was actually the other way, in terms of will a pryo firing create a 'pop' in the audio signal?

[/Vent Over]

[Brian]

I don't want anyone to take offence or take the following personally (esp. Brian and David A)

None taken.

 

My main concern was actually the other way, in terms of will a pryo firing create a 'pop' in the audio signal?

If you have good quality balanced audio gear then it's likely you'll get away with it. It's not possible to give a better answer than that as many external factors such as how your whole setup is earthed will influence things.

 

...but my main concern is is there any possible chance that a DMX, Comms or Audio signal could ever set off a pyro?

There have been a couple of incidents in recent years where pyro devices have mis-fired which are suspected to be due to coupling in from other sources. As always, after the event, it was never possible to say for definite what happened.

 

Pyro devices are designed with defined 'no-fire' and 'must-fire' currents with a large gap between the 2 figures. In most cases it is not possible to get a 'must-fire' current from normal signal sources however, there are a few exceptions to this...

 

Scroller circuits as they have 24v power on a pair of pins

Comm circuits for the same reason

DMX circuits which use pins 4 and 5 for power

Telephone circuits - dc plus ringer

 

...plus a few obscure ones.

 

In your proposed pair allocation you have DMX running with your pyro signals. Now, in normal operation that wouldn't be a problem but what happens when something shorts circuits a dimmer output to ground? Can you guarantee that the normally benign earth reference on your DMX line isn't suddenly going to jump 240 volts above ground? Or have to carry a fault current which might induce a current across into your pyro lines? The fact is that no one can know for sure what will happen.

 

To be honest, if you are running in conduit for the job then it's no real hardship to run in dedicated cables for your pyro. You still run the risk of cross coupling but you stand a better chance of getting away with it.

 

I have worked out a whole load of scenarios of pyro cable running with other services, calculating induced currents etc, but the cases are all so unique that it's not possible to say that it will or won't happen. I've worked examples where I can fire pyro by bumping a 2k lamp on a different circuit but I've also got examples where you can do things which on the face of it look dangerous but aren't.

 

Oh, and if you do decide to go with it then think very carefully about which pin you use on the bulgin to connect to the screen of your pair. Hint - the E pin might not be your best choice.

[dbuckley]

I'd not recommend this on two counts

 

a) crosstalk - pyro control signals are not "normal" digital signals and could potentially blat any of the others. In your specific configuration the DMX512 is physically the closest, and the worst affected would probably be the comms, as it's unbalanced audio, but it's only for the crew, so what the hell :huh:

 

b) I'm not convinced you are using the right cable for DMX512, despite my previous attestations that DMX512 will work happily over T&E, mower extension cable and worse.

 

I don't have an issue with the sound of the type of cable you are proposing to use, and would suggest that you can use it for pyro, but just not with something else. A seperate DMX512 cable would be my recommendation.

[Peter Chivers]

One issue that has not actually been mentioned is that a pyro cartridge draws a reasonable current as it is firing. I don't have any figures to hand, and am not prepared to try and remember and get it wrong. Most 'screened' multicore cables are not rated to handle anything more than a current measured in milliamps - even comms gets iffy at times. I strongly suspect that you will find that the cable resistance will either prevent the pyro firing, or the cable will get excessively hot very quickly! :huh:

 

Peter

[Neil Hampson]

b) I'm not convinced you are using the right cable for DMX512, despite my previous attestations that DMX512 will work happily over T&E, mower extension cable and worse.

 

The cable in question is actually an RS485 cable (Quite a chunky on at that, I don't have the p/n but it's a Beldon Z-fold Datatwist in about 0.5mmSq). so if anything it would be the audio that suffered as it's not exactly OFC. then again its only going 10 metres.

 

Thanks for all of the replies on this. I have been given a length of single pair cable this afternoon that will reach, therefore my gameplan has changed slightly, follow me on this...

Multi 1

Pr1 Audio left

Pr2 Audio Right

Pr3 DMX

 

Multi 2

Pr1 Pyro

Pr2 Pyro2

Pr3 Nothing

 

Single pair: Comms

 

Brian, as the pyro is a 6/24, I assume that th 'e' pin is common to all 4 possible channels regardless of polarity. Is there a way of doing this if the shields were commonned together. (I have not seen this cable yet so the screens may yet be insulated from each other.) How is it done in an XLR wired system?