You have that backwards. A dry chemical extinguisher will coat the robot in dust and crap, CO2 leaves no residue making it easier to salvage bits.
I’d also call dry chemical much more common. C02 extinguishers are not typically found at your big box home store.
I’ve never actually seen a CO2 unit in a pit at an FRC event. I’m sure some teams have them, but dry chemical is pretty much what everyone has.
You might be badly burned if the frame is hot, but Ohm’s law plus physiology says you’re extremely unlikely to suffer any injury from the electricity itself. Wikipedia says the resistance of wet skin is about 1 kOhms. So a 12V battery can push, say, 12 mA through you. The chart on that page notes that 12 mA of AC would be noticeable after a few seconds, but not deadly. It’s important to note that DC is much less dangerous in this regard than AC. Unless you have wires connected directly to your heart[1], it takes a lot more DC to kill you than AC. If anyone is doubtful, I can hold a demonstration at Champs where I will bravely grab both terminals of a robot battery with either hand an suffer no ill effects.
Mind you there’s plenty of other dangers from the battery, but they’re all the fire, explosion, direct-short causing a welding arc sort.
TL;DR: The robot battery (and your car battery[2]) aren’t going to electrocute you unless you stab a positive and negative wire from them into your heart. In which case you have plenty of other problems besides.
[1]I have before. It was weird.
[2]Had a dead battery once in college and got a jump from a doctor. He gave me the same electrocution warning. And he should know, because he’s a doctor. I didn’t argue because I wanted to get home.
If there was a short with the frame, does this mean the frame isolation test during inspection was not done?
Or would this happen even if the frame isolation test passed?
That is a fair point, I should have thought a bit further before posting that last line. Looking back, I should have known a bit better[1]. Even so, I still stand that touching an electrically charged chassis is far from a good idea, even if it’s just for peace-of-mind.
[1]I do a lot of work with electrical systems. Also got shocked by a 240 ~3-6A wall socket (for the second time) a few years ago, less than a fun experience.
It is possible even if circuits are electrically isolated from the frame* before the match*. If the outer shielding of the wire (or a solder joint, or wire crimp) becomes damaged, it is very possible for it to contact the robot chassis. With the rigors of Stronghold, this is entirely within the realm of possibility.
Unless it changed this season, Michigan does not use the standard road case system that the rest of the country uses. There is no case 8, just a toolbox and a tote or two with the non-tool items that are usually in case 8 (paper towel glass cleaner, etc). I’m actually not sure where the fire extinguisher is stored with Michigan fields.
Disclaimer: I didn’t do any events in Michigan this year, so it may have changed.
Or maybe it happened after inspection? Wire coming loose during a match?
You make a good point. Though the extinguisher that came out eventually did look like the standard CO2 extinguisher packed with the regional fields.
There’s a lot of variables that can go into this, we can’t say for certain if it was or was not done. It’s possible it was done, but after the fact the insulation wore through. It’s possible the frame short was intermittent, and only occurred when a wire was pinched in a mechanism a specific way. Or the team was doing a hasty repair and pinched a wire, or drilled through the insulation of a wire, or disconnected a motor and had the leads just dangling… There are any number of ways for the frame to short out either before or after a frame isolation test is done. And most of the time the frame isolation test is not performed during reinspection.
The reason we do the frame isolation test is to ensure that it takes two faults to cause a problem, not just one. Two faults obviously can still happen, but it’s half as likely to happen as having one fault.
This is correct. (Though I am far from an authority on the topic, I’ve helped with a handful of event setup and teardowns.)
You are aware your car has an electrically charged chassis? In the States we bond one leg of of our power to ground. Touching a charged chassis really isn’t an issue. You don’t want to use your body to complete the electrical circuit though. :]
Before using your extingisher on an electrical fire, you want to remove the inigtion source. Powdered fire extinguisher are messy. Never my first choice when other options are present.
On the other hand, it’s not like it’s wise to be choosy beyond making sure the extinguisher is rated for your particular emergency.
“No, no, it’s not going to spread that much. Let’s wait and see if we can find the CO2 extinguisher…”
I cannot argue with that.
That was definitely a dry chemical extinguisher, the CO2 kind would have dissipated much faster.
From talking with some inspectors, that is the theory. The wire that caught fire went around a corner on a frame piece, and it is suspected that the insulation wore through because of robot movement.
In addition there were some other factors that may or may not have come into play. There may be a suggestion made on additional safety tests in inspection, or a change in wiring rules. I don’t want to be more specific in case I am wrong.
Thinking about this now, I realize that something like this could have happened to us. We had a almost unmeasurable frame short (later discovered to probably caused by a limit switch wire connected to a Talon SRX), which passed inspection because the resistance was higher than the required limit.
Last night and this morning we fried two Talon SRXes, and after a ton of troubleshooting we discovered that our LED ring power cable was being pinched and sometimes connecting positive to the frame. If the resistance was a little lower, I think the same thing could have happened to us. This appeared to be what destroyed the Talons.
First the test only requires the resistance across your frame to be above 3k ohms
Secondly from what I’ve learned from the team the short was caused by an old unused LED controller that became grounded to the already charged chassis, which led to the heat and fire
Thirdly the battery had been removed (or disconnected unsure) from the robot before being Taken off of the field after seen smoking on the field
And finally it was definitely a dry chemical extinguisher, the residue was swept and vacuumed up immediately following the incident, but multiple fire extinguishers were brought over to aid the fire, the first extinguisher had its safety pin break off when someone attempted to pull it rendering the extinguisher useless increasing the time it took to extinguish the flames.
I would like to congratulate the team however on successfully redoing most of their robot as well as getting their shooting back up and rinning! A great turn around for them after an incident like that. Go team!
2012 - member of 2834
2013 - electrical and mechanical function
2014 - driver, mechanical, and drivetrain
2015 - driver, drivetrain, mechanical knot tying
2016 - captain, mechanical, drivetrain
Look closely to the right of the robot in the video. A volunteer in a black shirt brought a CO2 extinguisher, and you can see him pull the pin, while another volunteer(white shirt, towards the back of the robot) was holding a dry chemical extinguisher. As others have said, the dry chemical was applied to the fire; you can see the guy in the white shirt aim and discharge his extinguisher.
This is probably why the dry chemical was used instead of CO2. After seeing the pin break, he decided it would be better to put it out with the powder than to let it burn to search for another CO2, if there was one. (Good idea.)
There was also an “Injury Risk” sign next to the field. That exercise ball seemed to be an effective fire blanket!
I just found another video on YouTube from the field, recorded by YouTube user zenith828: https://m.youtube.com/watch?v=6QyrZZ3zIX4
I was the initial inspector on this robot. We did the isolated frame check and it passed with no issue. Wiring looked pretty clean actually.