Following are a few random thoughts on electrical safety in pianos:

I have seen old wiring, both in pianos and old houses, that falls
into the extremes of both good and bad.  I think that variation in
environmental conditions, including high temperature caused by
overloading, and the presence or absence of contaminants can cause
similar materials to age very differently.  Also there may have been
variations among original materials from different manufacturers and
from different batches.

I believe in always checking the electrical insulation with a megohm-
meter, especially with anything that is going to be left connected.  An
electrical resistance tester (Megger) will test for leakage at about 500
volts, which is better than you can do with a multi-meter, even the
pretty good ones.

I would replace wiring that showed any measurable leakage or that looked
like it had been physically damaged or disturbed (e.g., badly installed
new terminations.)  I think that the old motors that were wound with
fabric insulated wire should be expected to leak a little.  Make sure that
they are clean and not over-oiled, that the start switch works correctly,
that the contacts are good, and that splices and connections are not
compromised.

Make sure that the on-off switch disconnects the hot side of the line, so
that the motor is really off when it is "off".  If the motor case is
grounded or connected to other metallic parts of the machine, this is
especially important.  Use a polarized plug, and a GFCI if you are
especially concerned.  If you can't keep it from tripping the leakage may
be at a dangerous level.

Use very small fuses (available from motor shops) that will just let the
motor start without blowing, and put MOVs (metal-oxide varistor) across
the line (after the fuses and before the switch) and between hot and
ground and neutral and ground to protect against surges.

Do not use any materials for insulating wires or other electrical
components that were not specifically designed for that purpose.  Do not
use rubber hose, duct tape, or any glue or coating that is not
specifically designed to be an insulator.  The original materials were
generally fine when new.  Now they may not be, and modern materials that
look the same may not be either.

Spencer Chase