Comment by jonatron
I visited an exchange back in 2009, when Local loop unbundling (LLU) on ADSL was big, and fibre was limited to large business and datacentres. The huge generator was probably more interesting than the racks of concentrators. I'm not sure how much battery back-up power time the new PON systems have, I assume less than a generator backed system.
Was this the BT demo exchange somewhere in Essex ? Bishop Stortford maybe ?
I did that tour around the same time and it was fascinating ! right in the middle of the 21CN (ethernet core network) transition.