Shielded vs Balanced for control lines

[Zeff]

Forgive me if this has been covered elsewhere, but I'd like some clarification about maximizing RF & EMF suppression in pyro control lines.

 

I've heard and seen it stressed that shielded cables should be used, but the LeMaitre controller we're using utilizes pins 1 (Shield) and 2 (hot) making the balanced cables little more than expensive unbalanced lines.

 

Would it not be better to use pins 2&3 to take advantage of common-mode rejection and also have the additional safety of the grounded shield?

 

It seems to me that shielded unbalanced lines (because the shield is integral to the circuit) is more susceptible to interference than balanced UNshielded.

 

Cheers

Zeff

[Jivemaster]

It's a North America speciality to use XLR3 for pyro. Probably simply because there are usually plenty of mic leads, probably with no thought about interference and stray currents. In the UK pyro is usually wired in twin flex.

 

However I'd think twice about changing and apparent "convention" for your locality.

[Brian]

I've heard and seen it stressed that shielded cables should be used,...

 

By whom?

 

 

...but the LeMaitre controller we're using utilizes pins 1 (Shield) and 2 (hot) making the balanced cables little more than expensive unbalanced lines.

 

Really? The controllers make no such assumption. Maybe someone has made cables using shielded cable?

 

 

Would it not be better to use pins 2&3 to take advantage of common-mode rejection and also have the additional safety of the grounded shield?

 

But where would that ground come from? Which controller do you have?

 

 

There are some scenarios, including a serious incident in the US, where the very presence of a earth ground connection was enough to cause unintended ignition of pyro.

 

You might want to start by drawing out some conceptual schematics and trying to cause 500mA to flow through an igniter from an induced source.

[Zeff]

 

 

I've heard and seen it stressed that shielded cables should be used,...

 

 

 

 

By whom?

 

 

My ERD-certified instructor strongly suggested shielded. I have also seen it suggested on several other pyrotechnics websites.

 

 

 

...but the LeMaitre controller we're using utilizes pins 1 (Shield) and 2 (hot) making the balanced cables little more than expensive unbalanced lines.

 

 

 

 

Really? The controllers make no such assumption.

 

 

I'm not sure how the Bulgin conns are wired with respect to pyro controllers (perhaps you can fill me in), but in audio, where we commonly ground devices like mics & amplifiers (with some exceptions for signal ground) so that we DON'T kill people, pin 1 is shield. It's our convention. In my search for answers I found no mention of wiring convention for pyro controllers and since having an induction differential on the conductors didn't make sense to me, I posted here. I thought I was posting to the identical-looking U.S. forum, but here I am.

 

 

Maybe someone has made cables using shielded cable?

 

 

The NRCan-produced Special Effect Pyrotechnics Manual Edition3/2014 describes using "audio-type XLR cables". The controllers we just happen to be using (LeMaitre Pyroflash 6 Way and Pyroflash 2) use 3pin XLR. We were trained on it and another (I don't remember the make/model) using 4-pin. It too, used the shield as a conductor, and the other three pins allowed daisy chaining to three device addresses.

 

 

 

Would it not be better to use pins 2&3 to take advantage of common-mode rejection and also have the additional safety of the grounded shield?

 

 

 

 

But where would that ground come from?

 

 

Not having access to LeMaitre schematics and having neither time nor inclination to crack open the controller, I assumed pin 1 would be earth or chassis ground. The shell of the fixture or device (flashpod) would be grounded back to the controller via the shield.

 

 

 

 

There are some scenarios, including a serious incident in the US, where the very presence of a earth ground connection was enough to cause unintended ignition of pyro.

 

 

What is the return path for the DCV, Brian? And on the event(s) where having grounded shields proved dangerous, can you elaborate specifically on what the issue was or provide a link to a description of the event? I'd like to know what to avoid.

 

 

You might want to start by drawing out some conceptual schematics and trying to cause 500mA to flow through an igniter from an induced source.

 

 

I'll try that, thanks for your time.

 

Cheers

[Shez]

I'm a little puzzled by the use of the term balanced. You are aware that just using pins 2 & 3 of an audio XLR type cable doesn't make a circuit balanced? That's determined entirely by what's at either end of the cable and I can't conceive of any sensible output circuit topology that would result in the connection being balanced. The impedance in one or both lines will inevitably change depending on whether the circuit is off, on test or being fired.

[Brian]

@Zeff

 

I'll do a longer more detailed post later but, for the US accident, have a search on 'treasure island birket'

 

 

 

And, as if we need reminding how dangerous pyro can be...

 

2 dead, 4 injured in explosion

[Jivemaster]

For High Explosives all round the world, and for Pyro on all continents except North America firing current is supplied down simple twin core wire. BUT in North American pyro you are stuck with an odd system, but if it's "Standard" there then you are stuck with it. I have my doubts that ANY cable system is perfect at preventing accidental firing, and when the reality is such a low failure rate due to wiring, it's hard to justify rigorous enforcement of either system.

[scjb]

For High Explosives all round the world, and for Pyro on all continents except North America firing current is supplied down simple twin core wire. BUT in North American pyro you are stuck with an odd system

Ummmmm... really?