Super cap risk assessment?

[tron9000]

With the price of super caps coming down since I left education (when if you said to anyone, do you have a 1F cap - they'd laugh at you), I've been getting more students using them in projects.

The worry that we have is the amount of fast, high energy that they are exposing themselves to. Take a 300F cap at 2.7Vhttp://onecall.farnell.com/bussmann-by-eaton/xv3550-2r7307-r/cap-super-300f-2-7v-snap-in/dp/2148519 (you all know the maths): E=0.5*C*V^2 = 1093.5J. That's a lot of energy and at £25, is well within their budget! I know you probably don't charge up to the full 2.7V, but still its scary that all it would take it for someone to carelessly place their metal watch strap or braclet across the terminals and BANG!

We' can't really stop students from ordering supercaps, but maybe limit their capacitance and max voltage, if they need it to finish their work, they go to have it! We do have measures in place for students using LiPO batteries, by placing them in battery bags. We come up with one design, so that they can be enclosed in a case with some terminals (see attachment), without having to employ charge balancers and divert far from the unit acting like a super capacitor. This should allow fast charging and slow down the discharging somewhat.

So far only a few students have been using them and in the future, and the popularity of using super-capacitors and energy harvesting circuit on the up, it's better to have some safety measures in place now than later, after the inevitable has happened.

Has anyone else had experience with super caps and safety concerns? am I being paranoid?

[Rerouter]

tell them to remove any metal jewelry before starting work on a project, there are many teaching references of what happens when the wrench goes across the 11KV phases,

Now as i am sure you are aware, the super caps only pose a risk when exposed to a short, so you could do the norm at tafes, and insulate the terminals, and install a suitably sized fuse one one leg, this removes most of the issues that come from shorts, or the class clown drawing arcs over bits of metal,

e.g. 10A, that is above the normal charging currents i would imagine, and 2.7 x 10A = 27W, enough for a nasty burn to skin from a suitable resistance, but not contact welding,

[Psi]

i used to have 9x  2600F 2.5V caps wired in series. Had them hooked up to some metal rods and i used to wave them around hitting each other for fun
They were cabled to the caps with a wire gauge that was selected to limit the current to the caps rated max of 600A
It sends lots of sparks 60cm or so.

Never got hurt, just a very minor burn once.

Afrotechmods has a cool video on the caps i used

[tron9000]

well that video kinda hits home what I'm on about!

2600F for $15 - that's a helluva a lot of bang for buck!! WANT!

I guess then the maximum energy stored will have to be limited, unless a written risk assessment is given! Bloody bureaucracy!

[amyk]

am I being paranoid?

Yes.

A lion battery can provide much more energy, plus contains flammable chemicals.

[tron9000]

Yes.

A lion battery can provide much more energy, plus contains flammable chemicals.

We do have measures in place for students using LiPO batteries, by placing them in battery bags.....

yes but LiPO's have additional circuitry to stop constant short circuits and over charging, we only put them in bags to protect them from piercing and, if the inevitable happens, puffing up and exploding. Plus a LiPO batteries ESR is higher than a super caps would ever be typically.

[CatalinaWOW]

This isn't really a new hazard.  Back in the days of tubes it wasn't that hard to salvage 10,000 microfarads of capacitors rated at 500 VDC, or the supply to charge them.  Roughly 1200 joules.  The kind of sparks that fly from a higher voltage discharge are different, but also spectacular. 

The key is to make sure that those involved are aware of the hazards, and behave appropriately.  Attempting to remove all risk from life is futile as long as we continue to supply gasoline and electric mains to the public.

[tron9000]

Well that may be, still if a student does get injured and our insurance gets involved, and we have no risk assement, we're shafted! Its really to appease the ambulance chasers.

[CatalinaWOW]

Okay, you are appeasing ambulance chasers (a futile task since they get paid for detecting risks, don't get paid if they don't find one.  It is like trying to get a four year old to stop asking why).

Your primary defense is still education and training.  I think defining a stored energy level as a risk threshold will get you into many difficulties.  A standard D battery has thousands of Joules stored.  Discharge rate capability would eliminate any camera with a flash and many other things you will have in your school and lab.  Requiring fuses and other protective devices is good, but unless you require that no materials come into the lab except through faculty hands, and the faculty takes the responsibility for installing these protective devices (and taking steps to assure that they cannot easily be bypassed), you are back to education and training of the students as your defense.

In a fully lawyered up society, even teaching people about dangerous things like capacitors and batteries is potentially injurious to your pocketbook.  Someone will create a theory that your student (five years and 5000 miles from your classroom) would not have done the injurious thing he did if it were not for the ideas you exposed him to.  There is also the direct hazard that every time you show a roomful of people the possible ill consequences of misusing a device, some percentage of them will be thinking "Cool, I want to try that!".  I know I did.  It is how I know that those high voltage capacitor banks were easy and cheap to make.  Other standard stunts of the time - shorting a power supply based on 5U4 (dual diode) tubes and watching the plates glow red or even melt and similar not overly safe activities.

[Marco]

How much energy do electrolytics store as pressure before they blow?

[tron9000]

How much energy do electrolytics store as pressure before they blow?

Can they store energy as pressure? I think once it gets to that point it stopped being a cap anymore!

[smjcuk]

I'd hand out a lab safety fact sheet and get them to sign to say they have read it. That's what we had to do when I was at university and that was enough to say that we were provided education against the risks.

Anecdotal evidence only but I've never been bitten by a big capacitor even when I went through the slightly insane phase of buying up lots of them and charging them all up for fun. What always gets me is misreading resistor values leading to overheating components and burned fingers and the odd exploding electrolytic (the small ones are far more dangerous as they go projectile easily) so perhaps a general guide on how not to end in A&E would be a good start.

[Psi]

I would be more worried about high voltage caps than high capacity ones.
You will recover from a burn but not from being dead

[tron9000]

I would be more worried about high voltage caps than high capacity ones.
You will recover from a burn but not from being dead

Ha! Yeh we don't dish those out and they come under our high voltage risk assessment!

[eas]

How old are these students?

Lawyers apparently aren't the only people who come up with twisted reasoning about responsibility, it seems Engineers have their own pathology. Equating naked supercaps and the student projects into which they are incorporated with a finished, safety tested product like a camera flash isn't a very helpful comparison. Comparing a supercap to a lithium ion battery isn't much better. Yes, a lion battery holds more energy, and can deliver it quickly enough to have issues of its own. Shall we compare a stick of butter to a similar weight of nitroglycerine and conclude that the nitroglycerine isn't worth care because it has less chemical energy than a common foodstuff?

They are students, you are an educator. Liability issues aside, I don't see how it isn't your responsibility to help educate them about the risks. Don't pass off that responsibility by acting like its something invented by ambulance chasers, even if you are just doing it to fit in with some of the people on this forum.

Part of the teaching opportunity is to help them understand how and why a supercap can be dangerous, how those dangers are similar and different to dangers posed by other electrical components, and ways of managing those risks seem like important fundamentals to teach.

Pragmatically, you recognize a risk; you are in a position to mitigate that risk; mitigation is congruent with your role as an educator, whether by calling, or just paycheck (you aren't just some passerby who notices that someone bought supercapacitors). What is the personally responsible thing to do in this situation? But you don't need me asking you that, your gut already told you.

As for how to address the concern. I don't know your educational setting. Dramatic demonstrations get peoples attention. Another possibility, a quiz where people have to calculate the implications of accidental supercap discharge. How much would it heat a metal ring, chain or bracelet?

Perhaps a little story problem: Timmy trades a pack of baseball cards for five 300F supercaps and brings them home. When he gets home, his mother sees his new possessions says, "be careful there little Timmy, those supercaps can hold a lot of energy." Timmy says, "c'mon mom, they are only 2.7v, and they hold less energy than the pound of butter you sent me to the store for yesterday." Timmy's mother calmly opens a drawer, pulls out a loaded pistol, says "this is going to hurt me more than it hurts you," and shoots little Timmy in the leg. As he is writhing on the floor, Timmy's mother stands over him and calmly says, "The gunpowder that propelled that bullet through into your leg contained less energy than a stick of that butter. Still hurts, doesn't it?" Was Timmy's mother's analogy correct? Show your work. Bonus points for suggesting a better way for mom to have taught Timmy this important lesson.

[CatalinaWOW]

The camera flash example was chosen because the disposable ones (cardboard case) are readily disassembled and repurposed.  Widely done.  With the cardboard wrapper removed they expose the high voltage to direct contact so no safety testing applies.

I agree that there are widely disparate treatments of risk.  I also agree that as an educator you have a responsibility to educate students on risk.   But I would also suggest that while doing the teachers answers version of your little story problem you explain the relationship between intentionally firing a weapon into a body part (whose energy content is less than a pound of butter) with the damage potential and energy content and damage potential of the supercaps.

Perhaps instead of throwing analogies back and forth it would be more beneficial to do a formal risk assessment.  Identify all of the imaginable negative outcomes.  Identify the magnitude of the potential damage that occurs for each outcome.  Identify the probability of each negative outcomes occurrence.  Compare these with your organizations accepted standards for risk tolerance.  There are formal strategies for doing these risk assessments that allow quantifying (with low resolution) the hazard involvement.  The conversations involved in assigning probabilities and consequences are nearly as valuable as the results.

[CatalinaWOW]

Just to get this started, and using damage and likelihood categories similar to US military usages.

Damage categories - 1.  Loss of life.  2.  Loss of limbs or significant hospitalization time.  3.  Injury or severe property damage.  4.  Minor property damage.  5.  Cleanup and nerve calming required.

Likelihood categories - 1. Extremely improbable <1 chance in a million.  2.  Highly unlikely < 1 chance in 100, 000.  3.  Unlikely < 1 chance in 10,000.  4.  Uncommon < 1 chance in 1000 and 5. Common - chances greater than 1000.

Likelihoods should be evaluated over the entire school system affected and over the lifetime of the curriculum, probably 10 to 20 years.

Now for some possible hazards.

1.  Spontaneous and instant release of all energy in cap.  Extremely improbably, but could be loss of life or loss of limb event.  Might choose to split this one into subgroups, with the spontaneous event occurring in storage (more likely, lower consequence), causing loss of life (lowest likelihood, but highest consequence) and so on.

2.  Accidental short across terminals.  You can argue about how to categorize this, but the likelihood will be common, with consequence probably in the 3 to 5 range.

3.  Failure cause by overcharge - Again common.  I have no idea what the failure mode is, so can only speculate on the damage category.

4.  Failure cause by physical damage to case -  Probably unlikely or even lower probability.  Again failure mode is speculative to me.  Another possibility for breaking down into subgroups for dropping, puncturing with a nail, squeezing in vice and so on.

5.  And on and on until your imagination runs out.  In reality the list gets extended by real life.  The space shuttle program did enormously extensive versions of this, and neither of the catastrophic failures were properly identified.

This analysis can also guide your risk mitigation program.  If you are sufficiently worried by the shorting scenario you can require armored insulated boxes around the caps, with fuses between the capacitor terminals and externally accessible terminals.

[Psi]

1.  Spontaneous and instant release of all energy in cap.  Extremely improbably, but could be loss of life or loss of limb event. 

Excluding a 50+ supercap series array, (which is unlikely in a school), I cant see even a 3000F Maxwell boostcap ever killing anyone or causing the loss of a limb.
Serious burns are possible and perhaps worst case you might lose a finger from the burn.
Eye damage would be possible but is easy to eliminated by always wearing safety glasses

I've never heard of supercaps actually exploding. They just dump all their energy as heat.

The max number of caps allowed in serious would be a good rule to consider
Above 12V you start to get extreme sparks flying

[tron9000]

How old are these students?

16 to 50

They are students, you are an educator. Liability issues aside, I don't see how it isn't your responsibility to help educate them about the risks. Don't pass off that responsibility by acting like its something invented by ambulance chasers, even if you are just doing it to fit in with some of the people on this forum.

ok maybe I didn't mention I am just a lab technician with a technical support roll on top. I do educate those individuals who come to me asking for assistance. I can sure you though I do take my responsibility seriously: this is a job I enjoy, it pays well, I have dependants who need me to be in work - yet well all do roll our eyes when we HAVE to have bits of paper that state the bleeding obvious!

This is not trying to fit in, this is always how I've felt, and I understand it is important to uphold those regs and keep the students safe. It goes without saying if I don't keep them safe, I get done for negligence, and will lose my job. This isn't shirking any responsibility, this is getting some input from a wider, experienced community.

anyway thanks for the advice, will take it on board guys.