charles was thinking very hard :
I had to replace lead-sheathed stuff when I rewired here (1978ish). The
insulation had persihed in places so the sheath was live,

The lead sheath was supposed to be earthed. It was supposed to be used
with metal terminal boxes, with a clamp onto the sheath to maintain the
earthing. A common dodge to connect to the sheath, was a copper pyro
clip + brass nut and bolt.