ian mckenzie wrote:I wonder if the three-light-rule was developed when lights and batteries were not so reliable. Two seems reasonable now, except when soloing.
I'd tend to agree with that, especially if spare cells are available on the caver, (or within a team) for one or other light.
There used to be a distinct possibility of a reserve light blowing a bulb when turned on (or having had a failed filament from banging around while off) and some lights had a fairly limited maximum bulb life.
I'm trying to remember the last time In recent years I've heard of anyone I cave with having to use even a secondary light, and I'm not sure I can remember any cases where their main light was a decent and vaguely modern one.*
Personally I just have one light on the helmet, albeit one which is virtually two independent lights in one casing.
I find side-mounting anything to the helmet to be a pain in tight caves, and given that I really don't expect to need a secondary light, the chances are a helmet-mounted one would get trashed by wear and tear before I got around to using it.
An adequate-to-move-with light on the knife cord around my neck is my primary backup, and more use than a helmet-mounted one for the rare occasions I have to swap battery packs on the main light for particularly long trips.
On long trips and/or ones where I expect to be on my own (solo trips, or moving up/down deep alpine caves at my best pace) I'll usually wear a zebralight around my neck
(*Actually, I can think of one case, but that was where the primary light had a battery-headset connection fault which was known about in advance (and which was fairly trivial to fix for someone mechanically-minded). I'm not sure that really counts.)