How do grow-room fires start?
http://www.torontohemp.com/thcfaq.htm

How do grow-room fires start? (this isn't really a frequently asked question, but more one that I believe is too infrequently answered)

There can be hazards involved with indoor gardening. The risk of fire, water damage, electrical shock and chemical poisoning must be considered and addressed by those who choose to "grow their own." Often, the least understood and most dangerous hazards are also the easiest to avoid.

Gardeners should use organic fertilizers and pest control whenever possible and avoid contact with any toxic chemicals, both through the growing process and in the finished product. Gardens should be arranged in such a way that any spills or floods will be safely contained. Electrical safety should always be kept in mind. Electrical equipment and fittings should be installed and maintained properly. Also, water and electrical equipment can be a very dangerous mixture, and growers should be careful to avoid being burned by (or staring at) hot bulbs.

Perhaps the most common (and avoidable) cause of fire in a garden is "arcing", which occurs when current flows through an improper electrical connection, sparking from one contact to another. This can heat up the fixture, eventually causing it to break, explode or melt and allowing the arcing or super-heated pieces to ignite nearby flammable material. Gardeners must ensure that any High Intensity Discharge (H.I.D. - e.g. High Pressure Sodium or Metal Halide) lamp (bulb) being installed is screwed in properly, making good contact with the little metal tab in the light fixture's socket. This may involve bending the little tab out towards the bulb before screwing the bulb in (with the unit unplugged and using an insulated tool of course), because the tab can be pushed in by the last bulb that was used, and bulb bases are not always identical - some may screw deeper into the socket than others.

Generally there will be some period of warning before a catastrophic failure. Any equipment that produces an unusual buzzing, flickering, smell or heat should be disconnected and serviced immediately (H.I.D. ballasts do normally emit a quiet and steady buzz, they do get warm, and the lamp may flicker a bit while the unit is warming up).