Likewise - we strictly enforce the LiPo sack usage at BRCA 12th Nationals. I also ask people to make sure the pouches are closed. Those who were present during both our incidents are usually the ones making sure that is done - they've seen how much damage can be done to their other kit when an incident occurs!
Using sealed metal boxes there must be some vents in the box so that when the pressure builds ups the lid will not blow off - then you are back to square one. There must be some vents in the box to allow the expanding gases to escape, otherwise the lids will just come off. The vents need to be numerous and relatively large as there are a lot of gases given off.
Charging in a metal box is not the best plan. When these things do get into thermal runaway, they give off copious amounts of noxious gases. As the metal box has no access for sand to be tipped on the LiPo, the gases escape into the atmosphere. Not only do we have lithium in there, we also have the noxious fumes of the plastic box and the silicone wires. This goes on for quite a time. In an enclosed space like a sports hall it might mean having to get everyone out and then trying to clear the hall of the gases.
With the LiPo sack you can take immediate action to cut off the gases at source as quickly as possible. Once smothered, the source of any gas is stopped. The incident is well-contained and can be cleared up afterwards.
Please let's not scaremonger about clubs and insurance and events that we know nothing about. The precautions are simple and not costly. Just get three large buckets of kilne-dried sand, position them where everyone at the meeting knows their location and then dump the contents promptly on any incident that occurs. It might cost the driver a few quid in new kit, and the club a table to replace and some clearing up (sand gets everywhere!) but that's it in our experience - LiPo sacks and sand.
Linings for metal boxes - try a shop that sells wood-buring stoves. Micalite board and vemiculite bricks come in thicknesses from 25mm upwards and are usually rated above 800C. I am not sure how you cut them but will guess they are a bit like tiles - score and tap. They will be more than enough for a LiPo container. My Dad used to buy asbestos boards and cut them up for his fire safe, and he lived to be 94! Those were the days...
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