Electric firing of pyrotechnic displays has become widespread and the predominant method of igni- tion, and for good reasons; there are more and larger fireworks displays than in the past, more displays are choreographed to music and require precise timing, there is more consideration to op- erator safety, and the realization that an electri- cally-fired display is better appreciated by the audience.
Consequently, electric matches ("e-matches", or incorrectly "squibs") are being used by the hun- dreds of thousands, and perhaps millions each year. Correct and safe handling and application is extremely important, but will not be discussed here at this time. However, pyrotechnicians need to know some basics of ratings, measurements and test instruments used with e-matches.
Each e-match type from every manufacturer may have differing characteristics, but several basic electrical specifications should be understood in order to get a practical "feel" for the product.
1) All-fire current. This is the current in Amperes (or milliamperes) that will always result in the ignition of the match. Be aware that this specifi- cation may be different for series or parallel wired circuits. Typical values range between 0.5 and 1.0 Amperes (500 and 1000 milliamperes). Associated with this current is the time duration that the current must persist for ignition to occur, and may be from 5 to 20 milliseconds (.005 to .020 seconds), and depends on the magnitude of cur- rent flowing in the match. Another related specifi- cation which is especially important to designers of C-D, or capacitive-discharge firing systems, is the energy required to ensure firing; this is on the order of 10 millijoules (.01 Joules) per e-match. Note that I have not mentioned "voltage", as it is the current in the e-match that causes it to fire. 2) No-fire current. This is the current for which the e-match will never fire, regardless of how long the current flows. It varies with type and manu- facturer, but is typically about 150 milliamperes (abbreviated 150 mA.)
3) Test current. The test current is the maximum safe current that the manufacturer specifies for electrical testing. Typically, it is a maximum of 50 mA and includes a safety factor when considering
the no-fire current. This value should never be ex- ceeded by any test device, in any test condition, for any period of time.
4) Unspecified current range. This is, as the
name implies, a range of current for which the e- match may or may not fire, and may also be time dependent. Never allow any operation in this range. For the examples above, the range would be between 50 and 150 mA.
TEST DEVICES
Naturally, a measurement instrument for e- matches is electrical in operation. What you are going to measure is the e-match electrical resis- tance (typically 1.8 to 2.6 ohms), before attaching it to the pyrotechnics, and later for continuity or circuit completeness when attached and wired into the circuit. A meaningful firing site measure- ment actually indicates the electrical resistance of the circuit, including the wiring, and its numerical value is expressed in ohms. The test not only veri- fies the integrity of the match, but provides infor- mation for calculating the maximum number of matches you can fire at one time.
Circuit Continuity Testers
The simplest tester is nothing more than a battery and a light bulb of some sort. It cannot accurately measure circuit resistance, only whether or not the circuit is incomplete (open) or complete (closed), with continuity between its wiring and e- matches. It is almost useless for a professional technician, and in fact is likely to be dangerous in use, for reasons to be explained later.
Analog Meters
An analog meter is a test instrument having a pointer-type indicator with a calibrated scale. It usually has several resistance testing ranges which are selected by a switch, and is powered by an internal battery. Common analog test instru- ments are usually multimeters, which also meas- ure AC and DC volts and current. Avoid any type of analog test meter unless it h a s been verified to be safe on all selectable ranges, especially the lowest resistance range. Blasting galvanometers originally were of the analog type, and are, of course, suitable for pyrotechnic use because of their design.
Digital Meters
Digital meters are recognized by the type of readout t h a t you will see, which is a LCD dis- play. Most digital meters are also multimeters, able to test AC a n d DC voltages, resistances, often AC a n d DC current, a n d are powered by an internal battery. These might be safe for py- rotechnic u s e . Radio Shack carries inexpensive pocket-type digital multimeters which are con- venient to u s e in the field.
Built-in Equipment Testers
Firing systems a n d controllers often have built- in test circuits, a n d m u s t be checked to verify that the test current is less t h a n the maximum safe value u n d e r all conditions.
THE HAZARDS
In order for a test i n s t r u m e n t to m e a s u r e your e-match or circuit it m u s t c a u s e a current to flow in the circuit. Ordinary meters were not designed for u s e with pyrotechnics a n d may be dangerous if the test current exceeds the manufacturer's m a x i m u m test current rating. For example, I have inexpensive analog meters which produce a test current of 160 mA, which is more t h a n three times the safe test current. A light bulb type circuit tester is especially d a n - gerous since the initial "cold filament" or i n r u s h current may be as m u c h as 10 times the nor- mal, lighted value, a n d it persists for a period of time long enough to fire the e-match. Firing panels t h a t u s e light b u l b s instead of LEDs for continuity testing are to be avoided. LED test- ers, if properly designed, may be suitable a n d safe although they cannot indicate actual cir- cuit resistance nor distinguish between a good circuit a n d a direct short.
How Can You Tell What Is "Safe"?
First, keep in mind t h a t except for certified blasting galvanometers, all test i n s t r u m e n t s may be h a z a r d o u s . Under no circumstance should personnel be near live pyrotechnic ma- terial during testing, including testing with
blasting "galvos", a n d therefore a s s u m e
NOTHING is really safe.
You can perform a very simple measurement of your tester to actually find out what the test current is. You need a DC current meter that accurately r e a d s 0 to 50 milliamperes; this can even be the s a m e "unsafe" inexpensive analog meter t h a t m u s t not be u s e d to directly test an e-match. Connect the DC milliammeter leads directly to your e-match tester or firing panel, a n d then read the actual test current. Use the lowest resistance m e a s u r i n g range on your e- m a t c h tester, as the lowest range produces the highest test current. It's t h a t simple. Most digital meters test at c u r r e n t s of 4 milliamperes or less, while analog meters are m u c h higher.
Design Hazards
A very good guideline to follow when designing test instrumentation is for the test current to remain below 50 mA even if any one component fails in the worst possible m a n n e r . This m e a n s t h a t if you have only one current limiting com- ponent a n d it fails so as to produce excessive current, a potentially unsafe condition exists. Use multiple components to e n s u r e t h a t any one failure will not result in excessive current. Also consider what would h a p p e n if someone replaces a battery with an incorrect, higher voltage type - the design m u s t be u n a b l e to ac- cept an incorrect battery.
GOOD DESIGN
Summing up all of the above, remember that except for blasting galvos, no commercial test meters are designed with pyrotechnic safety in mind. New designs m u s t limit the test current u n d e r all conditions to less t h a n 50 mA, and in practice, 10 mA is a good design value. Test any meter you intend to u s e by "testing the tester" with a milliammeter. Built-in testers, as in fir- ing systems, m u s t also be tested, a n d any de- sign u s i n g capacitors might retain a charge that could cause an unexpected discharge. In C-D designs you m u s t provide fail-safe switching a n d fast discharge of capacitors so t h a t no fir- ing energy is available, except when the product is armed. Consider the worst case failure of components a n d improper u s e , polarity and connections. SMB