Cell Phones in Blasting Zones?

Factual Questions
#5

A radio transmitter can definitely induce a current in wires. That’s precisely what radio transmitters do.

According to this PDF (see page 2), blasting caps are designed to have a minimum firing current of 0.25 amps. According to this discussion, blasting cap resistance is on the order of 1-3 ohms, so if power is I2R, then your phone or FRS radio needs to deliver 125 milliwatts of power to the wires connected to the blasting cap. Cell phones transmit with 3 watts at most - in other words, you’d need those blasting cap wires to receive 4% of your phone’s transmitted power. If you’re in a place where the general public is allowed (e.g. on a road passing by a blasting zone), this seems highly unlikely to happen, especially if the blasting crew has done taken the appropriate steps to minimize the blasting circuits susceptibility to radio frequency interference (see guide two paragraphs below).

Having said that, this article reports that premature detonations have happened when blasting crew team members were using 2-way radios, but these were folks who were working on site, likely much closer to the explosives/wires than the general public can get. No further details were provided, but the cited expert appears to eat/sleep/breathe explosives, so you can probably take her at her word.

For further reading, there’s this PDF, “A Guide to Radio Frequency Hazards With Electric Detonators.” Turns out AM radio transmitter antennas are the biggest problem because of their low frequency and high power. Cell phones and mobile radios us much higher frequencyes, but they are still considered a problem specifically because they can be brought into the blasting area. Tables in section 2 (page 11) list minimum recommended safe distances for a variety of radio sources. Table 6 (page 15) shows that for “public use cellular telephones above 800 MHz”, the minimum safe distance is somewhere between 8 and 18 feet.

Page 6 begins a review of radio pickup circuits, i.e. configurations of the wiring in a blasting circuit that make them more likely to pick up RFI. The most common hazards appear to be having wires with a length around 1/4 of the radio wavelength, and/or having the wires in a circuit separated from each other and/or the ground. It’s surprising to me that they don’t use twisted-pair wires, which are a common thing in computer wiring to minimize susceptibility to induced currents.

Good blasting safety practice would mean careful construction of the blasting circuits to minimize RFI susceptibility, safe storage of blasting caps in Faraday-cage packaging, consideration of known nearby transmitting antennas, rigorous exclusion of mobile transmitters from the blasting site, and keeping the general public (and any transmitting devices they may have) far away. In the end, it seems like telling passing motorists to turn off their phones is just one more low-cost slice of the swiss cheese safety model. Those signs will never get everybody to turn their phones off, but the compliance of even a few is helpful.

#6

A better explanation than mine. Some caps I’ve seen used have a shunt across the wires right at the cap itself, Others do not. The shunt would help ensure the current does not reach the initiator.

#7

So why are long wires, the sort that might pick up these problem radio waves (acting as antennas), used at all? A blasting cap connected at the blast site to a battery, triggered by a (cheap, expendable) receiver that only responds to a code would solve the issue, no? What am I missing? Why not just light a nice long fuse?

#8

Many many years ago, the president of Pakistan was in a convoy that drove over a bridge; a minute later, the bridge exploded. Speculation was the bomb was tied to a cellphone, but the Pakistani security used a cellphone jammer than travelled with the limo - once it was out of range, the cellphone notifying of a missed call set off the bomb too late.

I talked to the people who dealt with explosives for an open pit mine. He mentioned that they had tested the latest iteration of electric blasting caps (this was about 25 years ago). Even draping the leads over an industrial walkie-talkie, it would not set off the cap - which is what the manufacturer told them.

To do an industrial blast, there would be dozens of holes drilled and filled with explosives. They would have to go off in sequence. the first blow a hole that creates an opening into a wider empty hole, then each subsequent series of blasts makes the rock break into that opening. There are a sequence of caps that would trigger slightly later from the pulse. It was wired with one wire for simplicity - when hooking it up, there was nothing but wires - no power connected, no battery, until it was ready to be blasted and the connection to a battery was made at a safe distance - rather than relying that the electronics will not produce a pulse when hooked up to a live battery until you are safely out of range. Same logic, hook up the battery very last, after the switch. (At times they used to use the plunger like to old cartoons)

Later caps were even simpler. they were set off by a blasting cord, their lead was an exploding cord. These caps were hooked to a single cord that clipped to all the caps leads in sequence. Where on the cord the caps were connected determined the order they went off. (milliseconds apart, that all that was necessary). The blasting cord was set off by one electric cap on the cord with a long wire, same safety reasons.

#9

The safety of a radio receiver based system would be massively less than the current system. Moreover, you would need a receiver at every blasting hole, of which in typical applications is lots.
Consider that the moment the receiver is attached to the detonator the system is effectively armed. You are reliant on all the upstream processes to work perfectly to protect you from an inadvertent firing. Every time they connected the receiver to a detonator, the moment they threw the on-switch, technicians would probably be saying a silent prayer that this one isn’t the one in a million faulty device that will purée them.

The traditional wired approach has the blasting technicians connecting and trailing wires behind them as they retreat from the blast site. They only connect the wires to the energy source when they are themselves with the controller.

There is a hybrid approach. Connect your blasting wire runs to a remotely controlled receiver, and then beat a path back to safety. These devices are however not single shot inexpensive things. A few thousand apiece. Blasting technicians are at least not faced with connecting a controller directly to each detonator.

Blasting often needs precise control of firing. Charges are initiated in a staggered pattern. This is especially true for demolition, where a structure will be weakened in just the right place before another set of staggered charges run up the structure fracturing it in just the right order. By mining operations will also require coordinated timing.Sophisticated electronic controllers help now, but timed det cord would get you there otherwise. (As better explained above.)

#10

Solve the issue of stray RF by employing RF as a vital link in the process you say?

#11

I think it’s a policy built along the lines of, “Better safe than sorry.”

#12

Another point to make. As I understand (not a blast expert) the “plunger” beloved of cartoons is an electromechanical device. Plunging the handle pushes a magnet through a coil or something, generating a pulse of current. Hence, there is no battery to connect, no risk that “gee, did it not go off because the battery is dead, or is there a discontinuity in the circuit somewhere?” No risk that it might go off prematurely while connecting the battery. No suddenly discovering you’re all out of fresh batteries, need to make a run into town while there’s a hillside of explosives waiting to throw rocks everywhere. The only current source is a forceful push on that handle.

#13

Big Clive does an explanation: https://www.youtube.com/watch?v=9as5wfHV6rk&t=0s

Cody does a teardown: How Does An Old Blasting Machine/Generator Work? - YouTube

Just using batteries is a little fraught. You will need a lot of cells in the battery to get the needed voltage. Modern systems can use nice tiny voltage converters. It was not that long ago that such things didn’t exist. At least not in a form that anyone would want to use in exchange for a robust and simple device. And something that needs very deliberate action to trigger a blast. Not just flipping a switch.

#14

Deliberate action indeed…

#15

It would be easy to make the receivers insensitive to anything but a particular coded message, which would eliminate the problem of them going off when you didn’t want them to. The trouble is that some of them may not go off at all due to interference, which is a safety problem in and of itself.

It wouldn’t be that hard to build a hard-wired, coded system, though. The receivers would only enable themselves once they received a coded unlock message. Only then would they allow the current pulse to go through and trigger the cap. Wouldn’t cost more than a few tens of cents per cap. If you wanted to get really clever, you could have a system where every cap was scanned into the system while being deployed, and once hooked up, each would report in if it were successfully put on the network. Only once you verified that every cap was accounted for would you trigger them. For a few cents more per cap, you could add a shunt load that allows a trial run at full power, ensuring there are no high-resistance connections or the like.

#16

In a related question - how big an explosion does a standard blasting cap make? How close can you be and not get hurt?

Don’t need answer fast.

#17

My experience is very limited due to a desire for self preservation, but I do know that they are apparently available in various powers. I believe #8 is a common “size” as those are what I’ve seen used.

There are both electrically initiated caps and fuse initiated. I’d want to be at least 10’ from a cap exploded in the open air. Twenty feet might be better.

#18

The demonstration I saw - they had a thick steel pipe about 4" diameter, with hinged cover locked on at the end, other end closed. Hole drilled in cap to allow the blasting wire cap in. the cap was about the thickness of a pencil and say, 2 inches long, the wire was the same as your typical 5v supply for a small electric device. The demonstrator put the cap inside an empty aluminum coke can. In a decent sized room, our ears were ringing after the blast. He unlocked the cover and fished out little twisted bits of coke can.

I’m guessing if you held it in your hand, you (or rather, bystanders) would be looking for your hand and a bit above the wrist, or wiping it off themselves. The other problem is the metal shrapnel from the cap.

As for the exploding cord, it was about the thickness of a hiking boot shoelace - the instructor mentioned the story that allegedly someone “took some home” for playing with, and their daughter found it and used it for a skipping rope. Because she wrapped it around her fingers, it blew both sets of fingers off at the knuckles. a strong blow (like a hammer, or stomping on it, or whipping it against a stone) might set it off.

The caps and such can go off, but the standard truck bomb material - fertilizer and diesel fuel - won’t go off no matter how hard you hammer it or whatever - it takes a shock like being against a blasting cap explosion to set it off.

The moral I came away with is leave this stuff for the trained experts.

#19

As an electrical engineer, I have to strongly disagree with this.

It is fairly easy (assuming that you have the right skills) to make a receiver whose intention is to be insensitive to anything but a particular coded message. However, one of the first things you learn in EE school is how easy it is to make a radio receiver. All you need is something conductive for your antenna, and something that conducts better in one direction than the other. That pretty much describes almost every single electronic device made today. One of the more difficult parts of designing electronics is making sure that the various bits of your device DON’T act as a radio receiver.

Worst case, the output transistor of your fancy-shmancy receiver accidentally switches on due to coupled RF noise getting into the base of the transistor, completely bypassing the digital circuitry and software that is designed to only switch on the output when the proper code is received. At this point your “easy to make” receiver causes exactly the earth-shattering ka-boom that it was intended to prevent.

Every circuit trace is a radio antenna, every semiconductor is an AM radio demodulator, and every circuit board is a tank circuit (for those who don’t know the lingo, a tank circuit is a circuit containing both capacitance and inductance, which under the right circumstances the circuit can resonate and cause significant instability).

It’s fairly easy to design a circuit that will only intentionally trigger its output when it receives a particular code, but making the whole thing immune to induced RF isn’t something that I would describe as “easy”.

#20

That’s all fair enough, but it’s something that RF engineers do all the time. It’s black magic as far as I’m concerned, but they’re able to build pacemakers and other critical devices that are largely immune to interference.

There are some obvious tricks one could use, like optical isolation–keep the RF and the driver side in separate, shielded sections, coupled only with an LED/photodiode pair. Just one of many features one might add for safety.

So yes, “easy” was perhaps a bit glib, but it is at least possible, which is untrue of ordinary “dumb” wiring, which is definitely sensitive to a wide range of completely uncontrolled RF input, including things like geomagnetic storms.

#21

It’s possible to do that, sure, but would it be a good design? Seems like a lot of ‘moving parts’ in an application that really needs to be as simple as possible - not only to avoid failure, but to avoid malfunction (that is, something not just going wrong, but going wrong in a way that is unintentionally functional in an undesirable way)

#22

As I said, I think the RF-triggered version is a bad idea due to the greater possibility of them failing to trigger.

For the hard-wired, coded version–sure, why not? This kind of stuff can be made very reliable in large quantities. If it were up to me, I’d put a barcode on each one that gets scanned as it’s being placed. They’d have two-way comms with the source and would each be able to check in. You’d have a high degree of certainty that every one was accounted for and in a working state. I’m no explosives expert, but that sounds like a pretty useful feature.

Maybe it’s not actually that common, but it seems pretty dumb that at the moment, a stray high-power RF transmitter can set off charges that are in the middle of being placed. Also, I think complexity is ok when it can be used to self-check, even if it has a higher baseline failure rate. I’d rather have 1% of units fail in a detectable and safe fashion than 0.1% fail undetectably.

#23

I asked the same question some years ago:

#24

Hence the magnetic induction plunger, and nothing but wire to the blast site. KISS - Keep It Simple…

Note too that blasts typically are set off in sequence. The first two or three create a cavity by blasting the rock outward, the rest now blow in sequence seconds to hundredths of a second apart to break the rock into the hole that was created. (Otherwise, you’d have to load all the explosive far down underneath, and blow upward.) This also conveniently blasts the rock into a loose pile, rather than having a big rock cliff or pit that just jumped up and down a few inches but is fractured but still in its original place. Think of a blast as a super strong sledge hammer.

it pushes at the rock. The rock will fracture. But it won’t go anywhere unless it has somewhere to go. A drilled hole will mostly push outward from the central axis of the hole - if the cap is at the bottom of the hole (it should be) the shock wave also travels upward - think of it like an expanding cone starting from the bottom.

So basically, you want one “trigger” and it is connected to all the blasting caps, which are sequenced to go off in turn. (Because some caps can delay a few milliseconds longer than others, and they are arranged by delay into each blast hole.) You sure as heck don’t want multiple radio receivers, as the first one may knock out the rest. The blaster’s worst case is when they have to go check if one or more holes did not go off completely … or worse, there’s a live blasting cap buried in there along with a hole full of explosive. After the blast, wait a specified number of minutes then go in, inspect, and wash out any missed holes.

One engineer described a test to remove large boulders. The made a shaped charge, filling a small funnel with explosive and putting the blasting cap in through the spout. This results in a basically spherical shock wave that grows as it travels through the explosive, effectively creating a (mostly) directional hammer on the rock it was placed against. (But you don’t want to be within 100 feet or more) The test boulder was intact after the blast. They tapped it with a backhoe and it crumbled into tiny chunks along the fracture lines from the blast. (AIUI, The special forces use miniature versions of this sort of thing to blow doors off hinges, etc.)

You can imagine if that was flat ground instead, it would be a struggle to dig out the rock. You want the explosive to do the digging for you and throw the rock into a pile. Arranging the blast waves to do that is part of the art of being a blaster.

As I said, my educated guess from the demo is if you hold one in your hand, you will need a new hook. It shredded an aluminum can into pieces. Plus the cap is a metal container (at least, the ones I saw) so if you are within 10 or 20 feet, better hope shrapnel bits miss you. You could shoot your eye out.