Revised - 13 Feb 1996 with the addition of pictures! Take a look.
This story uses the background music feature of Internet Explorer.
Editor's note: While driving along highways we often encounter construction areas where explosives are used to dislodge rock and earth. These "blast zones" are always accompanied by signs saying "Blast Zones - Turn Off Two-Way Radios". Some may wonder, "Is there really a hazard?" The answer, as described in this informal story in a most emphatic yes. Do not under any circumstances fool around with radio transmitters in the vicinity of blast zones. This article originally appeared in a different form in the Landmobile Radio digest, answering comments from another reader who asked, basically, "Are two-way radio transmissions really dangerous in blasting zones?"
I crew on fireworks shows part time. And a very good friend of mine, Jack Hershey, is a Licensed Pyrotechnician here in California. Both of us are licensed amateur radio operators. I also have designed and built several firing panels ranging from 50 to 700 shots. I've mentioned the above not as bragging, but just to establish some background. I've been watching the mailing list waiting for someone better qualified to answer the question, but seeing nothing, decided to contribute what I could. I can't speak for professional demolition pyrotechnics, but I suspect the methods and technology used to fire the charges are similar.
The average aerial shell for a fireworks show ranges from 2" in diameter to 8", and special order all the way up to 36". The shells are built with a "lift" charge that is ignited either with an electric igniter or a long "quickmatch" (think of a classic cowboy movie blasting fuse, but inside a protective paper tube about twice the diameter of a soda straw,and burning several hundreds of feet per second). The aerial shell is launched from the bottom of a launching tube made of tubular high-density polyethelene plastic, rolled paper or welded steel and several feet long. The ignition of the lift charge throws the actual shell up in the air (roughly 100 feet up per inch of shell diameter) and at the same time ignites a time fuse inside the shell which is calibrated to detonate the shell at the peak of the arc, or a little before.
Photo 1. Part of the firing pits showing three trenches with 50 tubes in each. The tubes are a mix of steel and rolled cardboard (picture a toilet paper core with a wall that's 1/2" thick). The tubes in the barrels are all 8" steel. The lady on the left is unwinding a squib wire, the white balls on the top of the tubes are the shells. Visible are 7 of the 15 crew members for this show. This is a 400 shot (plus the 100 round finale) 4th of July city show. Setup for this show started at 10am, showtime is 9pm. Barring major problems, the setup is completed by 7pm, with a 2 hour reserve. Dinner is fitted in there somewhere. The show lasts about 25-30 minutes, and knockdown takes about 2-3 hours.
The specifications I've been given on the "squib" - the actual electric igniter, are:
1) Must not fire at under 50ma
2) Must fire at over 150ma
The firing equipment scrupulously avoids the region between 50ma and 150ma, as some may fire on as little as 100ma, it depends on manufacturing tolerances and processes.
The squibs are rated as a current device because they are basically a low-wattage resistor contained in an explosive environment. A regular 9v battery will fire squibs just fine if the cable is not too long.
Photo 2. A long shot showing 30 racks (some with 5 tubes, others with 6). All of the tubes in the picture are rolled paper. The rules have been changed recently to allow tubes made of a high density polyethelene plastic with a wood endplug to be used. 100 rounds were reserved for the finale, the rest was in the main show. The finale rounds are chained together by the factory with quickmatch in sets of 5 or 6. The entire finale was shot in about 5 seconds. 4 racks of 5 shells were on each of 5 cues, with a seperate squib at each end of each rack. The two squibs on each rack were in parallel, insuring that all the shells in each rack would fire.
The "Fire" position of the switch just shorts out the Sonalert, and applies full battery voltage to the selected squib, with the only current limiting being the cable from the firing panel to the squib.
From the firing panel to the shells we use regular 25-pair 24 ga solid wire telephone phone cable that has an additional piece of 18ga "zip" cord paralelled to it with both conductors twisted together- this is the ground wire. They plug into a "firing strip", which is basically a long terminal strip with a hot binding post (from the 25 pair cable) and a ground binding post (from the zip cord) for each mortar/shell/squib.
The 25-pair cables range from 50 to 1500 feet, providing 50 seperate circuits per cable. By running multiple ground wires, and doing a few tricks, we've run a shows of 500 shots with 25 pairs and switching ground wires mid-show. One firing panel I built to order has 25 firing positions wired to two paralleled 25-pair connectors and a rotary switch that selects one of 7 different ground wires.
Naturally, as both amateur radio operators and as pyrotechnicans, we were interested in the RF sensitivity of a squib, and to a lesser extent so was the fireworks company. With that in mind, we experimented with a box of 25 on the company's testing range. Note that the squibs we tested with were the smallest size, and we were NOT testing "blasting caps". I was told that the caps are "about the same sensitivity", whatever that means.
The squibs we tested appear to be a piece of 1/8" x 1/8" double sided PC board, with a piece of 24 gage (solid conductor) zip cord about 3 feet long soldered to the the board and extending off of it to one side, and with a loop of about 30 gage nichrome wire spot-welded to the board and jumpering the two sides together while extending about 1/4" beyond the edge.
The PC board is apparently dipped in some kind of liquid explosive, and the loop of nichrome picks up a large drop, which hardens in the air. The squib is then dipped in red wax or epoxy for waterproofing. The length of zip cord is the only part that is exposed when the squib is installed in the lift charge or into the end of the quickmatch. Interestingly every squib that I have seen is shipped with the zip cord shorted.
As an example of the damage potential of the bare squib we tested, when fired, they make a "pop" that is quite sharp and loud from 6' away, and they will puncture a common 3x5" file card laid on top of the squib, but not when 2" away. At 1" there is an indentation and a slight burn mark. I was told that a very fast movie camera (several hundred frames per second) shows a instantaneous ball of very hot flame about 1/2" to 3/4" in diameter.
Photo 5.
Our tests were limited by the fact that they were very impromptu - apparently somebody had raised a question about the RF sensitivity of modern squibs, the company knew that we were licensed amateur radio operators, there were some squibs and a test range handy and some free time, and somebody put two and two and two together....
We were limited to the available equipment that we had with us that day, both on our belts and in our cars, and we didn't have much. Also we were testing only the company's standard squib: two brands of squibs, with identical specifications. Perhaps the squibs from a different manufacturer are more sensitive?
When positioned in a similar manner with a 25w UHF transmitter on the same antenna, it did not pop. A 35w 2m radio also did not pop the squib 1" from a 2m whip antenna.
We tested handhelds by holding the squib wires paralell to the flexible antenna, and also wrapped around it. They were keyed with the PTT button on the speaker/microphone. We tested a 5w UHF, a 5w high band, and a 5w 900mhz handheld and were not able to fire any squibs no matterwhat we did.
We did not have any HF equipment at the test, but I would find it interesting to find out how much power transfer could be created from a HF mobile parked so the antenna is near a fireworks cable... I think that under optimum conditions you could create 150+ma. in a cable.
We've proven to our own satisfaction that you've got to WORK at setting off a fireworks squib with a VHF radio transmitter - that there's a one-in-a-million chance of doing it accidentally. But we don't want anybody at our shows or in our crew to be a victim of that one millionth time - especially when it's so easy to prevent.
You see, the smallest shell we handle and fire - a 2" - can kill you long before you can blink...
Editor's note: The bottom line is that RF can indeed induce sufficient current to ignite explosive devices. From Mike's write up it appears likely that a great safety margin is built in to restrictions - however, RF can cause ignition to occur. Therefore, two way radio transmitters should never be operated when "Two-way radio restrictions" are in effect in the vicinity of blast zones.
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