Yesterday Mr. Bullock asked why there was usually a grounding wire
between the motor and the iron pump case, in an electrically pumped
piano.

When the humidity is low, any v-belt drive will generate considerable
static electricity.  When both ends of the drive are insulated from one
another (as is usually the case in a piano), charges will accumulate
at one or the other end, causing the two parts to have a considerably
different potential, or absolute voltage.

From time to time, this voltage difference may be high enough to cause
a static spark to jump between the motor and the pump or other nearby
metal.  This can produce an annoying ticking sound.  A bit of wire
connecting the motor shell and the pump casting cures this.

These sparks can be hazardous in an explosive or dust laden industrial
atmosphere, and must be prevented, by code.  Modern industrial belt
drives avoid this problem, by using special v-belts which include
a conductive layer, and cannot make static electricity in the first
place.

I see no reason to include the piano plate in this circuit.  Indeed,
this could produce a shock hazard, in case the motor developed an
internal ground fault.  Then, as discussed yesterday, a modern
three-wire grounding plug and cord would have to be used.

I personally see no harm in installing a modern three-wire connection,
if it makes you feel more comfortable.  But before doing this, you
should test both sides of the normal motor circuit to the motor shell,
with an accurate ohmmeter.  Any weakness in the motor's internal
insulation, a common problem on old motors, could cause a wasteful and
potentially heat producing current flow through the grounding wire.

Richard Vance