More on wiring a reverse-polarity alarmJan 1, 2003
To the editor: In the question and answer column in a recent issue ("Wiring a reverse polarity alarm," March/April 2000, Issue No. 104), I believe paragraphs two and three of Chuck Husick's answer to the question need further explanation. Taking paragraph three first, it is not true that one of the two conductors is arbitrarily called the hot, or black, wire. A typical domestic (or marina) power distribution system starts with a three-wire 120/240 volt output from a utility company transformer. The central or neutral wire is grounded at the transformer, and again at the customer's main service box. U.S. and international standards call for this wire to be colored white. The other two wires are typically black and red, and the voltage between either of these wires and the white or grounded neutral wire is 120 volts. The voltage between the black and the red wire is 240 volts, which is required for electric stoves, clothes dryers, etc. The white wire is always at or very close to ground potential. The black (or red) wire is at 120 volts when measured with an AC voltmeter.
It follows from the above that Husick's paragraph two can't be right. One wire (the white wire) is called a grounded conductor. Its potential does not decrease when the other (black or red) wire's potential is increasing. It is true that when the wires are connected to a load there is an alternating current along the white wire, but if it is properly grounded there should be no detectable voltage from the white wire to ground, although there could be some in a poor installation. As a complicating factor there are circumstances when a white grounded wire is a neutral wire, but that is beyond the scope of this discussion.
Correct polarity connection is extremely important primarily because all switchingcircuit breakers, fuses, appliance switches, etc.is in the black/red side of the circuit, so that when the switch is off there is no voltage at the appliance. If the polarity is reversed and the switch is OFF, then the appliance will not work, but its electrical parts will be at a high and lethal voltage.
All appliances should be manufactured with all wiringblack/red and whiteinsulated from the body or frame of the appliance. It is the green safety grounding wire which is connected to the body. Note the difference between it and the white grounded wire.
If the load circuitblack/red and whiteis properly insulated, then the current in each of the two wires will be the same, and the GFCI will not operate. However, if there is any leakage to ground, then the currents will not be the same and the GFCI will detect a difference as small as five milliamps, which is less than the lethal current of more than 15 milliamps.
Full information on all of the above is available in the National Electrical Code.
Brian W. Pollard, a retired electronic engineer, lives in Soamesville on Mt. Desert Island in Maine. He and his wife own a Hinckley Pilot and have sailed about 30,000 miles along the coasts of New England and Atlantic Canada.
Chuck Husick responds:
Your comments are totally correct. I'm afraid that I tried to oversimplify my explanation of the designation of the AC neutral.
In teaching Ocean Navigator's marine electrical seminar, I have found that persons not familiar with electrical systems become very confused by the concept of AC as opposed to simple DC systems.
By the way, I have found that the quality of the neutral at many docks can vary quite a bit from what we would like to see. Floating neutrals are less common now than they used to be, in part due to detection by tripping GFIs at the consumer end.