Arc explosions illustrate the dangers of electricity

I wrote a blog post a while back about the difficulty or having cars with 42V instead of 12V batteries. I also pointed out the difficulties of distributing dc inside your house and to your house. It got picked up by EDN, and the comments were interesting. Someone challenged my assertion that 24V relays switches are less reliable. Sorry, I worked for GMC Truck and Coach as an auto engineer in the electrical group. Heck, just read any switch or relay datasheet and you can see you have to de-rate for dc and de-rate even more for higher-voltage dc. Someone pointed out the phone company uses 48V dc, and I had to explain that the 48V the POTS (plain-old telephone system) sends to your house is also high impedance, 600 ohms, so that make is much less arc-prone and easier to switch.

Arc-flash-person

Others challenged my observation that it is hard to distribute dc in your house due to the fire hazard from the arcs and the same problems with switches and relays. Well, even ac has arcs that are hard to quench. Bigger dc circuit breakers have magnets in them to pull the arc one side and make it longer so it can break. Really big breakers, both ac and dc, have compressed air that blows the arc out just like your kid with a birthday candle.

Arc-flash-box-person

So here is a nice video of an ac arc flash that should give you some idea of the difficulty of quenching an arc. Palo Verde had a horrible arc flash in 2008 that thankfully had no injuries. And here is a training video of an arc flash form the fine folks at e-Hazard.com

Here is another training video from Westex flameproof clothing:

And if you wondered if there was any glory left in the American worker, check out this high-voltage lineman working from a helicopter.

So that’s the trouble with dc. Since the voltage is not going through zero 120 times a second it is much harder to quench the arc. The operative word is “plasma”. That is what Fran Hoffart from Linear Tech taught me about li-ion batteries. He said that the burning lithium is certainly a problem. But the real mess is that a plasma ball forms, and that shorts out any other battery cells in the vicinity. An arc is plasma, and that is some nasty stuff. I mentioned to Fran that the iron phosphate chemistry lithium cells are supposed to be burn-proof. Fran looked at me with an expression that said “you can’t be that stupid” and replied “they all burn”. It is remarkable the difference you hear when talking to people who are making and selling the battery cells versus the people like Fran, that are making the chips to reliably charge the cells.

Arc-Flash-outside

I guess that is why that outlaw biker told me that the only thing that he was really scared of was electricity. I asked why and he said “Because it can kill you and you can’t see it.”

1 thought on “Arc explosions illustrate the dangers of electricity

  1. Richard King

    Paul,

    My understanding regarding the LiFePO chemistry is that not that it won’t burn, everything burns given the right conditions, but that it has a much lower tendency to go into thermal runaway mode that other Li+ battery chemistries are prone to.

    We’ve run experiments abusing LiFePO cells in various ways (shorting, external heat sources) and we’ve never achieved open flame or an explosion, When we subjected cells to heat from red-hot heating elements from 1″ it melted the case and boiled off the electrolyte (stinky..) but it would not ignite. The only time we ever saw significant physical deformation was when we over-discharged the cells which would “puff up” distorting their case but never outgas.

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