Wow Mark as I was reading your account of what happened, I just knew what was coming next. I had a very similar incident a few years ago while building a racing skull for a friend. I had been cutting some carbon on the table and there were a few "hairs" left lying there. Later I used an electric skill saw to cut something on the table and it sucked these hairs into the cooling system. What followed was a fireball around the comutator as the brushes swallowed it all up. Carbon fibre needs a completely new set of rules when being used. Glad it all ended well for you, the price of a battery is nothing compared to a house!
Regards Dene Collett www.denecollett.com -----Original Message----- From: KRnet [mailto:krnet-bounces at list.krnet.org] On Behalf Of Mark Langford Sent: 19 April, 2014 3:02 PM To: KRnet Subject: KR> flash fire on the workbench NetHeads, I had a wake-up call last night. I was checking a scrap piece of carbon fiber to see if it was big enough to cover the belly board I'm making for N891JF. After I'd decided it would do, I tossed it aside onto a pile of stuff that's slowly been gathering on the workbench. About ten seconds later I got this really big whiff of "the house is on fire", looked up and there was smoke everywhere, and a 6" diameter blaze on the workbench about three feet in front of me! The odd thing is that the fire was on top of the carbon fiber that I'd just had in my hand. I grabbed it and moved it to the clean spot on the bench beat the fire out with some leather gloves that were also on the bench. It didn't take long to figure out what happened....there's a little sealed lead acid (SLA) 4.5 Ahr UPS battery on the bench that I keep around for stuff like checking light bulbs, powering the flap motor to check direction and travel, etc. I'd thrown the carbon fiber right on top of that little battery and shorted the terminals. The fire was a combination of some coating that comes on the carbon fiber, and the battery case burning from the heat. What's left of the negative terminal was melted into a puddle. If I'd left the room after tossing that carbon fiber on the pile, I'd have had a battery explosion shortly after, not to mention a raging fire on my wooden bench, right under my bedroom! I do have a smoke detector that's hardwired in to the rest of the house, but narrowly missing a fire in the middle of my workshop (complete with oxygen and acetylene bottles) is not a pleasant thought. Just a head's up...and a reminder to keep the positive terminal of your aircraft battery insulated so nothing can short it to ground. That would be even worse. I have a similar 8 Ahr battery as a backup battery installed in N891JF. This particular one is probably 10 years old, and it although I may charge it once a year, it's at 12.83 Volts now, even after the direct short! See enclosed photo. You can see the damaged carbon fiber off to the right. Something to consider.... Mark Langford ML at N56ML.com website at http://www.N56ML.com --------------------------------------------------------
