Fire When Ready – A Pyro-Electric Safety Primer
BY Daniel Birket
Birket Engineering, Inc., February 2002
Pyro-technicians are trained to handle pyrotechnic devices with great care in part because “you never know when it might go off.” The basis of this prudent uncertainty can be traced in part to a lack of information and sometimes to inappropriate techniques. This document attempts to illuminate some obscure areas of pyrotechnics and list some practices that can reduce the risk of unexpected ignition.
To Fire or Not to Fire…
Pyrotechnics are a much-anticipated component of shows at theme-parks, theatres, and on tour. When the “fireworks” occasionally fail to fire or work, it disappoints the audience, stirs up the management, and stresses-out the pyro crew. But any pyro-technician (who would like to continue to count to ten on his fingers) knows that the problem of devices that don’t ignite when you expect is nothing compared to the hazard of ones that do when you don’t. The lack of a pyrotechnic accent may detract from the artistic presentation, but a charge that explodes unexpectedly will shut down the entire show whether or not it harms anyone. This problem of unexpected ignition is the focus of this document.
There is a fairly widespread notion that it’s easy to make a system that will ignite pyro-electric devices. After all, you only need a medium-size battery to ignite an electric match (often called a “squib”). Anyone able to fix a flashlight can build a system that fires an electric match. This is true: it is easy to ignite pyro. In fact, it’s hard to make a system that won’t light pyro – until you want to – and that is the question.
Ohms and Amperes and Watts, Oh my!
Voltage, current, resistance, and power determine whether or not an electric match will ignite and a good engineer will thoroughly analyze these parameters when designing a pyro-electric control system. But it’s not necessary to know Ohm’s Law to know how to avoid unexpected ignition – just a few rules of thumb.
An electric match will ignite if enough power is sent through it to heat it to its ignition temperature. A tiny amount of power won’t do it. When a pyro controller tests for continuity, it uses extremely low power to safely test for the presence of the match. All commercial pyro controllers take great care with the continuity check circuitry because it is an obvious area of risk – a little too much power during the test might ignite a match instead of just testing it.
Stopping Unexpected Power
There are two ways to avoid sending unwanted power through the electric match:
- Block any unwanted power from reaching the match, and
- Route any unwanted power away from the match.
Blocking unwanted power involves placing insulators and shields between possible power sources and the match. Similarly, routing unwanted power away from the match involves putting conductors between the match and a safe place to dump the unwanted power. With a little thought, we can usually block the unwanted power sources from reaching the match. For the remaining sources, including those we don’t expect, we can try to route the unwanted power away to a safe place.
There are several kinds of unwanted electrical power that might find their way to an electric match. Most are fairly obvious and easily avoided, but some are quite devious.
While everyone is used to wires guiding electricity along the circuits we want, circuits can and do form anywhere different voltages find a way to connect. Current can flow through catwalks, conduits, pipes, even damp stains on surfaces. Have you ever felt a tingle when touching some equipment? That electrical source might have been able to light an electric match.
Obviously, we don’t want the pyrotechnic wiring to come in contact with wiring for lighting, audio, or other power. There is more than enough power in a sound system to light an electric match. Cross-wires between ignition channels are another common cause of unexpected ignition. Cross-wires may occur where there are many channels of pyro or other wiring grouped together. A pyro controller that tests for channel cross-wires as it performs its continuity check will reduce this risk.
One unexpected, but very common, route for unwanted power is the ground – that is, the dirt under our feet. In almost any electrical power wiring system you’ll find “ground” wires that lead ultimately to the earth. Metal frame structures are often tied to the earth with special wiring. “Lightning rods” are wired to the ground with thick cables to dissipate the natural voltage of passing rain clouds. Practically every electrical system uses “ground” on one side of its circuits. This is a good safe practice for most electrical wiring, but bad news for a pyro-electric system.
Although one connection to ground is harmless, it’s then easy for a stray wire strand, a fleck of metal, or even a drop of water to form a second connection somewhere else. A second connection may complete an electric circuit between an unwanted power source (perhaps another ignition channel) and the match through the ground. For this reason, good pyro control systems are isolated from ground to help prevent ground faults. Regular testing or a ground fault detection feature is necessary to insure that the system remains isolated from ground. A ground-fault detection circuit works like a “GFCI” (Ground Fault Circuit Interrupter) safety wall outlet to detect unexpected connections to the earth. (Note: Plugging a pyro controller into a GFCI outlet doesn’t work.)
Electric power is able to “side-step” from one circuit into another even when there is no electrical conductor between. Wherever electricity flows there is a magnetic field that can cause current to flow in another circuit without ever touching. Power transformers use this principle. When it happens unintentionally, its called Electromagnetic Interference or EMI. If you have ever heard a 60-cycle “hum” in the audio system when the lighting system turns on you’ve seen this principle in action. The high-power lighting circuits can easily jump to unshielded audio wiring anywhere the wires are routed together, for example in a cable tray. High power circuits can have the same effect on unshielded pyro channel wiring.
Insulate, Isolate, Shield, Protect, and Verify
Here are some techniques for keeping these unexpected power sources out of the pyro-electric distribution system:
- Grounded metal conduit: In a permanent installation, grounded metal conduit provides excellent physical protection for pyro channel wiring. Wiring is unlikely to be damaged inside the conduit and grounding the conduit helps to route unwanted power away from the pyro devices. Grounded metal conduit also shields the wire from EMI fields that may induce current in the pyro wiring. Note: If water collects in the conduit, it may eventually break through the wire’s insulation and produce a ground fault.
- Heat and Abuse Resistant Insulation: Where pyro wiring is exposed or movable, the wire’s insulation should be able to stand up to burning fallout and rough handling. If the insulation melts or scrapes away, it’s easy to form an unwanted circuit. Teflon is one type of tough insulation.
- Shielded wire: Shielded wire has a web of wire wrapped around the center conductors. When the shield (not the wire) is connected to ground at one end, it will block most Electromagnetic Interference. Note: Take care not to ground the pyro channel wiring by allowing it to touch the shield.
- Twisted-pair wire: When a pair of wires is twisted together, they become less susceptible to EMI. This is good alternative to more expensive shielded wire.
- NO “Zip Cord”: Un-shielded, un-twisted, “zip” or lamp cord generally has a soft thin insulation that is neither heat nor abuse resistant. It may be cheap, but it’s not suitable for permanent wiring. Zip cord wiring is a common factor in many cases of unexpected ignition. Remove any temporary zip cord with the spent pyro.
- Isolation Transformers: If a pyro system uses a low-voltage AC firing current, isolation transformers between zones can help block ground faults and other unwanted circuits from forming. Note: this technique provides only limited protection and is easily compromised unless the isolation is tested regularly.
- Isolated Firing Circuits: One excellent way to avoid unwanted circuits from forming between one pyro channel and another is to isolate every circuit from every other. Good pyro control systems use individual firing capacitors to handle each channel independently.
- Verified Circuits: To insure that every pyro channel is isolated from ground and from every other pyro channel, use a pyro controller that verifies every circuit during the continuity check. A simple continuity check will insure that a match will fire when expected, but ground-fault and cross-wire checks are necessary to insure that no match fires unexpectedly.
After the pyro system designer and the pyro-technician user have taken every reasonable precaution to prevent unwanted power from reaching the electric match, it can still happen. To deal with this problem, pyro control systems “shunt” each pyro channel. The “shunt” is an automatic and/or manual switch on each pyro channel that conducts unwanted power away from the electric match where it will do no harm. This works very well – as long as the channel wiring is heavy enough. If the wiring is too long or too thin, the unwanted power may prefer to flow through the electric match instead of the shunt. It depends on the wire length, but 14-gauge wire is usually heavy enough to insure that channel shunts can provide reasonable protection for the match.
Some pyro systems use a manual shunt that protects the pyro-technician while loading the pyro devices. This is adequate for installations where no-one will go near the pyro after the shunt is removed, but inappropriate for a live-action show. Ideally, every pyro channel should be individually shunted up until the moment it is fired. With this style of controller, the entire cast and crew is protected from unexpected ignition at all times.
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