On Tue, 14 Sep 2004 20:49:27 GMT, email@example.com (Scott Lurndal)
|>For the record, let me say that I agree that if the motor is defective
|>internally then it should be repaired or replaced. I jumped past that in my
|>original reply. However, adding a ground in no way presents a danger or a
|So answer me this. If he adds this ground, and the motor frame is also connected
|to the neutral (not at all uncommon in older motors), what will be the
|effect of this connection on the stove, dryer, and other metal appliances
|in the home which are grounded?
We should ask you for the answer since it is your hypothetical. But
I'll take the bait and reel *you* in.
Let's assume for a moment that the neutral is indeed connected to the
motor frame. I'm not sure this is as prevalent as you and others
suggest, but I'll play along.
First the two-wire case:
If Murphy's Law is on holiday and the extraordinary happens and the
non-polarized plug is actually plugged in correctly, then the motor
frame is connected to Earth via the neutral (grounded conductor). The
neutral is a current-carrying conductor; therefore there will be a
voltage drop along its length.
Assume for example that this is a 20 A circuit wired with 12 AWG and
it is 100 feet to the service panel. (We'll ignore the few feet of
flex pigtail from the saw to receptacle) To keep the numbers easy,
let's also assume that the motor draws 10A @ 120VAC. (1.2KW or ~ 1.6
From the copper wire tables in any edition of "Reference Data For
Radio Engineers" we find that 100' of 12 AWG at 20 deg C has a
resistance of 0.1588 Ohm. Dusting off Ohm's law, we calculate that
the voltage drop along this length of wire is 10(A) * .1588(ohm) 1.588 VAC.
Now if this is a hazardous voltage then I better go attach a ground
rod to my Tony Stewart model Mag-Lite flashlight, because it has not
one, but three, 1.5 V cells in it and it's orange besides.
When the motor isn't running, there is zero current in the wire, the
voltage drop is zero and the frame is effectively grounded via the
neutral. Does this meet current code, no. Is this a good thing, no;
but with the givens, it is *not* a shock hazard.
Now to your question. With everything as before, let's hook up the
three wire cord with "grounding conductor" (a terrible term as it is
too easily confused with "grounded conductor", hence my personal
penchant for calling it a "safety ground.")
Circuit wise, we now have two 12 AWG wires in parallel connected from
the motor frame back to the service panel. The resistance is half, as
is the voltage drop. And there is the benefit of redundancy in case
the neutral wire should open, which is one reason the safety ground is
now a code requirement. BTW, use of a GFI will expose this connection
in less than a heartbeat.
Now (absent a GFI) you want to know what happens to all of the other
appliances connected to this circuit branch. Note: I said *this*
branch; nothing happens to anything not connected to this branch.
Other loads don't give a rat's ass what is going on in the three wires
running to the table saw any more than they care what is going on in
your neighbor's house.
Other loads on *this* branch with metal frames or enclosures that are
grounded via the third conductor will be elevated above ground by some
factor. If they are downstream (further away from the service panel)
from the table saw, and don't have any fault currents of their own
they will be at the same potential as the saw, ~0.8 VAC in this
example. If they are upstream then they will be proportionally closer
to ground potential as they get closer to the panel.
BTW, do you realize that in recent memory, stoves and dryers had the
neutral tied to the frame and were "grounded" that way. And they
typically had unbalanced currents in the two hot legs since lights,
motors, etc were operated line-to-neutral. Thus, the chassis was
elevated above ground by the voltage drop in the neutral, miniscule as
it was. Did this worry you?