Igniter Circuit Testing

Measuring the resistance of an igniter used to detonate an explosive is a little like measuring the length of a live rattlesnake . . . the measurement itself is relatively simple, but there are certain safety aspects that strongly affect the design of the measuring apparatus. In plain English, when you test an igniter circuit, you will die if the test current emitted from the resistance measuring instrument accidentally exceeds “safe” limits. For most electrical instruments the design engineer spends 99% of his time worrying about measurement accuracy and performance features, and then almost as an afterthought, he briefly considers the safety aspects of the tester and the consequences of a malfunction in the circuitry. When designing an igniter circuit tester, the emphasis is just the opposite. Thousands of hours are spent thinking about the safety of the tester. If there is a 1 in 1,000,000 chance that some freak combination of malfunctions and operator errors could cause excess test current, sending an armed ICBM toward New York City, then safeguards must be built into the tester to prevent this condition.

What is an Igniter?

Modern rockets are “set off” by passing a current through a small heating element imbedded in inflammable material. When the temperature of this small heating element reaches a critical value, the “igniter” bursts into flame, initiating the rocket fuel. This same process is used to set off weapons–sometimes the igniter creates a small explosion which triggers the main explosive material; other times the igniter burns, and the resulting gas pressure operates a small cylinder which begins a sequence of mechanical and/or electrical steps leading to the detonation of the warhead. A third process involves the use of “explosive bolts”–fasteners that instantaneously fly apart when a current is passed through the small heating element in the igniter within the bolt. These same bolts are used extensively in space vehicles and underwater devices to rapidly separate large objects without requiring heavy, large mechanisms.

How is an Igniter Tested?

An igniter circuit tester is a special type of ohmmeter that measures low resistance very accurately, using a test current that is as much as 1000 times less than a conventional precision low resistance ohmmeter. The low test current is necessary to prevent ignition of the rocket motor or detonation of the explosive device being tested. Since excess test current could result in bodily injury or death, every possible way of accidentally creating overcurrent must be considered and protected against. This kind of thinking leads to such tests as deliberately failing components to see what will happen (certain resistors must fail open rather than shorted, for example). Quality assurance testing must exceed even the tightest “MIL Spec” procedures, and finally, the designer must be able to second-guess all the mistakes any test operator might make in using the tester. It must not be possible to connect the leads to the tester improperly, and it must be virtually impossible for someone to open the instrument case and modify or bypass the safety circuit elements (which prevent overcurrent in the event of a failure of the active circuits in the tester). All critical elements must be redundant and must fail in a safe way, causing test current to be less than 5% of the value required for ignition or detonation of the part EVEN IF ALL ACTIVE ELEMENTS OF THE CIRCUIT FAIL SIMULTANEOUSLY.

In the many years we have been in the business of testing explosive devices we think we have seen most of what can go wrong–batteries leaking and shorting out circuitry, technicians opening the case of the instrument and bypassing the current limiting devices, inexperienced personnel replacing the internal battery with one of higher voltage or, God forbid, wiring in an AC supply to replace the batteries without realizing that this supply must have an “ultra isolation” shielded transformer and zener fail-safe networks to prevent potential differences between earth ground and AC ground from detonating the part.

Our latest instruments have every safeguard we can think of. Critical fail-safe circuitry is sealed in a potted assembly attached directly to the output connector. The AC charger must be unplugged in order to connect the test leads, making it impossible for a test to be run with the AC still connected. We use a special rechargeable nickel-cadmium battery pack which is totally sealed in a thick plastic case to guard against leakage and has an output connector that does not match any off-the-shelf battery of higher voltage. Instruments are subject to a series of environmental and performance tests, ensuring both accuracy and reliability.

To our knowledge, our instruments are the only automatic igniter circuit testers approved by the military as safe for testing explosive devices.

The Accuracy Problem

It is very important for an igniter circuit tester to be accurate: certain problems such as a weak spring on a safety shorting switch manifest themselves in resistance changes as small as 0.1 ohm. Typical accuracy of a high-quality igniter circuit tester is 0.010 ohm with resolution as high as 0.001 ohm. This kind of accuracy is readily obtainable with a precision lab Kelvin bridge–you just pump enough current through the unknown resistance to get a big voltage drop. The magnitude of current used in a typical lab bridge would set off any igniter. In igniter circuit testers, accuracy must be obtained with voltage drops as small as 10 microvolts. Such requirements are right at the state of the art in low-noise low-drift balanced differential operational amplifiers. In some systems, obscure effects such as thermoelectric voltages become the predominant limitations to accuracy.

Safety Warning

There are some instruments on the market that can explode the device being tested if the instrument malfunctions. Other instruments permit fail-safe circuitry to be bypassed. Be wary of “lab type” instruments that have been modified for igniter circuit testing. Testing explosive devices can be a matter of life and death.

Beware of the Old Wheatstone-Bridge-Type Testers

In the 1950’s, most igniter circuit testers consisted of a Wheatstone bridge with a current limiting resistor in series with the internal battery. Measurements were tedious with this type of tester (you had to null the bridge for each measurement and then subtract the test lead resistance–these were two-wire type bridges); but most important, THESE INSTRUMENTS WERE NOT VERY SAFE. On these old testers, a technician could easily open the instrument case and modify the circuit, inadvertently bypassing the current limiting resistor. This has resulted in detonation of an explosive device in a number of occasions over the last 20 years. Unfortunately, many of these old instruments are still in use. They present one particularly dangerous hazard–if a replacement battery is not available (the old testers did not have rechargeable batteries), technicians sometimes wire a different battery into the unit. Since the current limiting resistor on these old testers was attached to the battery case, the bare exposed terminal on the outside of the case is

“downstream” of the safety circuitry. Almost anyone looking at the inside of the tester would think this terminal is the output from the battery; but, in fact, if a battery is connected to this point, the igniter circuit tester will output over 100 MA, detonating most parts. This tester had other problems. Inexperienced users could misunderstand the operation of the instrument and fail to null the meter on the correct range. Or they would forget to subtract test lead resistance (Space Electronics testers do this automatically using a true four-wire “Kelvin” circuit).

Igniter Circuit Tests

There are two basic tests that are performed on all igniter circuits:

A. Continuity. Most igniters have a resistance of about 1 ohm. After the igniter is installed in its final configuration, it is necessary to verify that all the associated circuits (including the igniter itself) are functional. Obviously, the system cannot be operated to confirm its performance (press the button and if the bomb goes off, everything was working). The best test is to measure the total resistance of the igniter circuitry. A very accurate ohmmeter is required, since a short might only change the total resistance from 1.8 ohm (for example) to 0.8 ohm. Even more critical is the contact resistance of the switch elements in the circuit–a 0.2 ohm change can often give a clue to a damaged contact spring or improperly installed mechanism.

There are actually at least two low resistance measurements made on an igniter circuit. In addition to the series resistance of the igniter element, it is also necessary to measure the resistance of the safety shorting switch which is part of the design of almost every rocket igniter or warhead detonator. This shorting switch is in parallel with the igniter; a variety of conditions must exist before this switch is opened, allowing current to flow through the igniter (for example, a projectile must experience acceleration and then deceleration before the switch opens on the warhead igniter). Since this switch is in parallel with the igniter, test current must be safe limited–in the event the switch were accidentally open (the condition being tested for), excess test current would set off the explosive.

B. Open Circuit Test. In addition to continuity checks, it is essential to verify that the igniter is NOT inadvertently connected to some other circuit, either by virtue of a wiring error, or by the igniter lead wire being pinched under a mounting bracket or any of the thousands of other mishaps that can occur in, for example, the circuit that fires the explosive separation bolts in a two-stage rocket. This test generally has a MINIMUM acceptable resistance between 100K and 2 Megohms. Usually this test involves checking each input to the igniter circuit relative to the ground connection or case of the device.

Safety Guidelines

Although the safety of igniter circuit testers is extraordinarily high, nothing is perfect in this world and there are some common-sense rules that I would recommend:

1. If the part can kill, test it behind a barrier even if you are certain the test is 100% safe.
2. If you can’t see the tester while you are installing the leads on the test part, use a shorting switch that connects all the test leads together while you are working near the part (Space Electronics makes a key-operated switch attached to the test cable; you can take the key with you and not worry about who is fooling around with the tester buttons while
3. you are connecting leads to a bomb that can blow a 15-foot hole in the ground).
4. Don’t ground anything in the circuit; or if you must, ground only one point. Earth ground can differ from AC ground by as much as 5 volts: this can result in a current of amps in some instances (Our testers are battery-operated, so this problem isn’t a safety consideration).
5. Always check “Open Circuit” as well as continuity, whether the test is required or not. It’s your life that is at stake.
6. Do 100% inspection.
7. Test again if ANY modifications are made to the part or if any part is dropped or jarred. If the part is disassembled and put back together, test it again.
8. Wherever possible, connect the tester to the part using a mating connector that has been carefully wired and inspected. Alligator clips can give false readings if they accidentally short against the case–I have seen instances where a defective part passed a test only because the test clips shorted in such a way to result in a resistance that happened to fall within acceptable limits.
9. Check any explosive device at every stage of production if you can, but at least check the entire arming circuit with the device unloaded, before making the final check with the full explosive installed. Aside from safety considerations, this procedure greatly simplifies the disposition of the part if it fails the test.
10. If a test fails, evacuate the area, and go through the full disarmament routine. Don’t wander behind the barrier to see if the test leads are tight (you should have done this when you installed them).
11. Test small groups of live explosives at one time. Then move this group out of the area before starting another series of tests. I have seen test areas where two weeks’ worth of explosives were piled to the ceiling because the usual material handler had been sick.
12. If the test part leads are not color-coded according to the drawing or the way they were last week, stop the test and get positive clarification. Don’t assume a new vendor uses other color codes or that the tester is so fail-safe that you can experiment with the part. The new vendor may have shipped you 5,000 warheads designed with an igniter whose safe current is 40 microamps, and you may have an igniter circuit tester that puts out 5 milliamps. Every one of the warheads will detonate when you test it.
13. Don’t test during lightning storms.
14. If you are using an AC power pack option with your tester (not recommended for high-risk parts), measure ground potential differences between AC ground pin and earth ground before doing any testing. If it’s more than 0.1 volt, then something is wrong