Afternoon Paul
The pavement manhole covers in Edinburgh date from around 1910; and are still very much in use, although the vast majority have probably not been lifted/opened for over 50 years.
Back before the 1950s, the one by our then family home was opened about once per year, and the sealant replaced - a type of putty with a distinct yellow colour.
But that maintenance routine seemed to be abandoned after nationalisation of the electrical supply industry.
That I recollect, the cabling from that manhole in to our house was upgraded about 1950, when we had an immersion heater fitted. I don't know whether that was total replacement or an additional cable drawn in.
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Possibly someone else can confirm; but as far as I recollect, cast-iron tending to be crystalline, does not rust readily, particularly if it has had been primed properly..
In contrast, the other box looks relatively modern, 10 to 20 years old, probably made from mild steel, so much less likely to survive as long as the cast one.
The cast one, being on the line of the school wall, was/is probably the main junction box from the street run in to that school, which in itself appears to be from the early 1900s, so around the time that the LCC was starting to provide such services.
Unless the cabling has clearly deteriorated or needs upgrading/uprating, generally will survive for "centuries".
Not quite the same; but I recollect reading in the then 11970s Post Office Telephones magazine that some telegraph poles had just been taken out of service in Birmingham; and were about 120 years old, having been erected in the early days of telegraphy.
Edited by deleted (Fri 13-Feb-15 17:47:47)