On Wed, 25 Jul 2001, Douglas Shook wrote:
:Hi all,
:
:What Randall explained is exactly right. I thought that an
:example might help to amplify what he explained.
:
:Forget 220VAC for a moment. If you have a 110VAC circuit, that
:110VAC potential is between the hot wire and ground. Each
:circuit needs a hot (black) and neutral (white) to complete the
:circuit. It is against code to fuse or switch the neutral side as
:the device (let's say a motor) would still have potential to
:ground (be hot) if switched off.
One can still find old installations that have switched or fused neutral.
They're mostly converted DC applications (where both sides were commonly
fused), or done by people who were used to doing DC stuff. This should be
fixed.
:If, however, we take the same scenario with a grounded circuit (a
:hot black, a neutral white, and a green ground connected to the
:case), when the hot wire in the motor shorted to the case of the
:motor, it would blow the fuse on the hot side as the current
:would be shorted directly to ground. The fuse would blow, the
:motor would stop and the case would be safe to touch.
:
:That third wire, the green ground wire, pretty much guarantees
:that electrical motors, washing machines, etc., will not be hot
:(the metal surfaces will not be hot) to touch as they are always
:grounded (there cannot be any potential between the floor you are
:standing on and the case of the device you are touching if it is
:grounded).
Well, there shouldn't be any difference from the safety ground and earth. I
lived in a house that had a circuit where the safety ground wasn't tied to
the rest of the house's ground circuit. It had about 20V between the safety
ground pin and earth. We got that fixed quick.
:
:Ground circuits are not intended normally to carry current. Only
:to carry current during a short, and to maintain ~0 VAC potential
:between the metal surfaces of appliances and the ground you are
:standing on. Ground can be run down metal conduit on two wire
:circuits.
:
:For 220VAC installations, the same scenario holds true, except
:220VAC potential is achieved between two hot wires, so there is
:no neutral wire. In this case, the white wire is used for
:grounding and to provide for 110VAC leg if needed.
Lots of former three-wire 220V installations that used to be allowed are now
against code. This used to be common for electric stoves and dryers, which
now must have a four wire installation. I think it's still allowed for some
installations, though.
David
--
dscheidt@tumbolia.com
Bipedalism is only a fad.
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