An
Unintentional Pyro Ignition Experience
Eleven years ago, at
Sometime after the show opened
two actors were injured as they stood near a charge that wasn’t supposed to go
off for another couple of minutes. As I understood it, one was burned on his
arm; the other lost his vision temporarily. Both made a full recovery. They were
very lucky. We were very lucky. Our industry was very lucky.
I got on an airplane and spent
most of two weeks considering every possible reason why this could have
happened. Of course I started looking in the wrong places: software
and controller problems, then EMI, then galvanic effects. Gradually I concluded
that it did not happen because of a legitimate command from the system. Then I
gradually concluded that there existed no other sufficient source of energy such
as EMI or a naturally occurring voltaic cell. That only left our own firing
supply - but how, since I had concluded that the match was a shunted at the time
that it ignited. So, I started taking our firing circuit apart one ohm at a
time. First, by redrawing the circuit showing the resistance of the wire I
realized that it is quite possible to ignite a shunted match if there is enough
current and voltage available, depending on how close the shunt is to the match,
etc. Then, finally, the “two ground fault” thought hit me between the eyes.
I realized that the requirement for an isolated supply allows ground faults to
exist without penalty. So they do, at times. Given enough time, the right two
ground faults will be present, allowing a deliberately ignited match to take
another match with it, unexpectedly. Careful study of the circuit shows that
there are other possible faults such as “fire on arm” and “spontaneous
fire”.
Having reached this awareness,
all I had to do was to walk the site with an ohm meter while shaking and pulling
on wires to find the intermittent ground faults. Both ground faults were clearly
a result of changes the local staff had innocently made using available wire.
One was wire with low temperature insulation run through a conduit next to a
very hot gas flame effect. The other was zip cord across an expanded metal
surface that could be walked on. (I’m told that the blocking of the show had
also changed, placing the actors closer to the errant pyro device than
originally intended.) We already knew that the errant pyro ignited at the time
as another legitimate ignition of a device in an other area. I found
intermittent ground faults in the temporary wiring on the positive side of both
of these match circuits. I was then able to convincingly explain and
conclusively demonstrate to Thaine Morris, Jules Lauve, Bob Bauer, and June
Fields exactly what had happened. As a solution, we implemented double pole
contacts in the areas of concern. We would have liked to have grounded the
supply but the code did not permit that. Everyone was satisfied that the
problem had been identified, and a solution implemented.
In the weeks that followed,
being unable to ground the supply, I attempted to implement a ground fault
detection system for that show. I spent several days monitoring ground faults. I
learned that high resistance ground faults are always present throughout such a
system, and they combine in parallel into the net effect of a low resistance
ground fault. I also found that from show to show it was not unusual to find one
or more points in the system temporarily grounded, probably related to how to
pyro was loaded for that show. It simply is not possible to control what happens
beyond the match terminals after the designer/installer leaves the site. It
should be noted that
With regard to ground fault
monitoring I concluded that while the occasional low resistance ground fault is
often distinguishable, often it is not due to the presence of all of the other
high resistance ground faults that exist because one side of the system is wired
in common all the way out to the match. A GFCI outlet don’t help of course
because it is on the wrong side of an isolation transformer. Thus the only
near-certain solution is double pole relays (or another approach, including
solid state, that breaks both lines to the match). Being unable to ground the
supply, we went back and installed more double pole relays. On future
systems we implemented individual isolated one-per-match firing sources, and
continuous testing for grounds and cross-wire conditions at each match.
I have wondered since why the
industry has been slow to learn from this incident, and why I encounter so much
resistance when I explain it. I have to remind myself that it took me two weeks
to find the problem on this show, and that understanding it requires a solid
grasp of several electrical concepts for which most people have no good use. It
is also too easy to attribute an unintended ignition to EMI or some other
mysterious cause. Only I have had the benefit of a first hand experience, a
successful and exhaustive field investigation, and then hashing this over and
over in my head for eleven years.
On the other side of the fence
is that prevailing (but erroneous in this case) feeling that “isolated”
means “safe” because it is an NEC solution to a shock hazard. Finally,
there is one other factor that I believe to be involved. The industry has
developed very responsible attitude that says "it's pyro, it can do bad
things, so you have to stay away from it or somebody might get hurt."
Unfortunately I’ve heard that idea submitted as the "theory " for an
incident when science does not quickly present an easily digestible theory about
an unintentional ignition. There is no NTSB of pyro. Had it not been for the
support of the four aforementioned individuals, and my willingness at the time
to do nothing else until we understood exactly what happened, the incident at
Treasure Island would have been attributed to the "well, pyro can hurt
people” theory and the solution would have been to “change something until
we think it won’t happen again” and to change the show blocking so
that if it did happen again there would be no one nearby – until complacency
set in again.
Later
John Noonan wrote, “The skeptic
would say that you are trying to change the rules to match your unique product.
Personally, knowing you, I would guess that you are just trying to make the
world a little safer.”
A completely fair question is,
“Since Birket is the only one pushing this, does Birket have something to
gain?" Possibly, but realistically, not much, and it is nothing that is not
available to everyone else in the business. Is it a reason not to call attention
to an important safety issue? No. In fact, I’ll explain exactly what we have
so that others can have it too.
In the months and years after
the Treasure Island
incident we took it upon ourselves to develop a firing system that tests every
match terminal for continuity to ground and to other match terminals using the
same current-limited source used to test match continuity. We were counseled
that it was patentable, but we did not feel it appropriate to pursue exclusivity
on an important safety concept. So, go for it – it is a good idea. Our
system marks each and every fault detected with an LED. We also use a isolated
and pre-charged capacitor as the firing source for each match circuit. No
possibility of cross-talk between circuits. And, we keep every match shunted
until the very moment of intended ignition.
In other words, you bet, we
took what we learned and designed a system to make sure that an unintentional
ignition would never happen again on a system of our design. We feel that
anything else would have been an abdication of responsibility after that event
at
© Birket Engineering, 2004