Nice and cheap to run then...

Over £600 for the full 500 hours worth!

I'm guessing these are better turned up on a variac than just switch on from scratch!

This is a very nice find

The cold resistance of these is something silly like 0.7 ohms so that would be quite a bit of surge current!
It would be interesting to DOL start one when hooked up to the scope and see just how much it is but I don't think my main fuse and by extension the house wiring would appreciate it!

The impedace of the house wiring and upstream supply may well be on the same order as the cold lamp, so reduce the current vs. the expected 400-ish A. But this reduction in current will slow the warm up of the lamp respectively so make the current last longer in the early stages of the warm up. How fast should it warm up ? - With that we could estimate how the protection react & the heat pulse on the wiring

The very beginning of the warm up can be estimated if we know the volume of the filament, assuming that all the heat is dumped into its thermal capacity and none escapes (same assumption as used when evaluating wiring withstand of short circuits) - Which i think will be close enough at temps below few 100s C, which is the temp range where the peak inrush happens