From what I
can see amongst my VAXen it was only the QBUS machines
that
warranted that sort of power cable; the whole thing was interlocked so
that
if there was a power cable in there you couldn't do anything other than
disconnect. Even when you wanted to remove the PSU you had to make sure
there was nothing in the way of the interlocks.....
I've never gotten around to tracing out what that plunger under the
sheet-metal does, though- my current hypothesis is that it discharges
the HV caps.