if the grounding jumper is properly in place, then it's just as safe as a 4
wire.


This is NOT true.

The three wire system, even when properly installed to code is
dangerous becasue an OPEN circuit (which can happen) in the 3rd ground/
neutral wire) can end up energizing the metal chassis of the dryer
through the 120 volt clock motor or other 120 volt parts in the
dryer. If someone then touches say the properly grounded washing
machine and the dryer at the same time, they could get a dangerous
shock.

That being said, I have a three wire install in my house and it can be
made safe with the simple addition of an axtra ground wire connected
from the case of the dreyer to a good ground such as a nearby water
pipe. I have a ground wire connected between the dreyer and the
washer, therefore there can be no voltage across them.

Yes three wire system were code and can be grandfathered to code, but
they can still be dangerous and I would add the extra ground wire to
be safe. Rewiring for a full 4 prong plug can be expensive if a new
cable has to be pulled. Adding an external ground wire is cheap and
easy and effective.

Mark