Plug-in testers are a safe, inexpensive way to troubleshoot
120-volt household circuits.By simply plugging in the tester,
you get the results of a variety of tests, indicated by a
series of three lights on the front of the unit. A key on the
tester tells you what each particular combination of lights
means.
Plug-in testers will only work with 120-volt receptacles. If
a receptacle is incorrectly wired with 240 volts, the unit will
be destroyed when it's plugged in. Most plug-in testers perform
several standard tests; the most common are described
below.
Correct Wiring
There's not
a lot to say here — obviously, this is the reading you
want — but I'd like to stress a point that I've made in
other JLC articles. When wiring a receptacle, never use the
push-in type connections that come on the backs of some
receptacles. These are not reliable; they may work loose over
time. Always use the screws; otherwise, a "Correct Wiring"
indication one month may result in a dangerous situation the
next month.
Loop the stripped end of the wire around the screw and
tighten it snugly. Make sure you loop the wire so that it gets
tighter, not looser, as you tighten the screw.
Reversed Polarity
I've
been asked why this matters. "What difference does it make if
you plug in a lamp and the current goes in the neutral leg and
comes out the hot leg? The light bulb still comes on, doesn't
it?"
Not long ago, a little girl in my home state put her tongue
into the round metal bulb holder of a decorative electric
candle — the kind you see in windows at Christmas. Even
though the switch was off, the girl was electrocuted because
the fixture had been wired backwards and the round cylinder
that holds the bulb was hot.
On a recent service call, I was working on a pump in an
underground concrete enclosure. The owner had switched off the
power; I measured 0 volts from hot to neutral. But when I
touched the neutral, I got a nasty shock that caused my elbow
to smash into the concrete wall. Remeasuring, I read 120 volts
from neutral to earth. On troubleshooting, I found that the
receptacle the pump was plugged into was wired in reverse, so
the neutral was hot. I went home with a sore, bruised elbow,
but the situation could have been lethal.
If a branch circuit is wired backward, the same situation
exists every time an appliance or light fixture is plugged in.
Unfortunately, this is more common than you might think. In
some cases, I have seen entire houses wired in reverse. The
only remedy is to rewire.
Advanced Plug-In Tester
Industrial Commercial Electronics has taken the
plug-in tester concept to a new level of
sophistication with its SureTest line of
“branch circuit analyzers.” Besides the
usual checks that common plug-in testers do, the
SureTest ST-1D (the model we tested) will check for
a bootleg ground, read line voltage, check for
voltage drop, measure the load on the circuit
between the receptacle and the panel, and measure
the impedance of the building’s grounding
path. Considering the cost — less than $300
— the tool is great for anyone who needs to
quickly and safely check a circuit.Say, for instance, that
you’re adding on a home office for your
client, and that the office will be stuffed with
expensive, state-of-the-art electronic equipment
— computer, fax machine, copier, etc. Despite
assurances from the electrician, and despite the
presence of plug-in surge protector strips at every
receptacle, your client still wants evidence that
the equipment is protected against power surges
(either from the utility or lightning), and that
neither noise in the lines nor power fluctuations
will garble data. Unfortunately, there is no way
for you to guarantee protection against lightning
— the voltage from a near or direct hit is
massive (see
Foolproof
Surge Protection, 2/94); and it’s up to
the power company to deliver continuous,
good-quality electricity. But with the SureTest,
you can be reasonably certain of the power quality
within the house itself.
So you pull out the SureTest and plug it in a
receptacle. The green lights tell you the
receptacle is wired correctly; the digital display
confirms that you’ve got 120 volts present
from the utility. (Wild fluctuations in the reading
would indicate that the utility is delivering
poor-quality power.) You push the “display
advance” button, and you get the voltage drop
on that receptacle under 15-amp load (given as a
percentage of 120 volts). Push the button again and
you get voltage drop under 20-amp load.
For either size circuit, any drop under 5% is
okay; above that, you may have a problem. Besides
causing equipment problems, excessive voltage drop
causes heat buildup in the wire and at connections,
and can be a fire hazard.
Let’s say you get a reading above 5%. Maybe
this is because you’ve pulled the wiring for
the addition off a lightly loaded preexisting
bedroom circuit, but the extra length of wire is
creating too much resistance. So you rewire the
circuit as a “homerun” back to the
panel. Or maybe there’s a bad connection at
the receptacle that’s causing the resistance.
Either way, you locate the problem, fix it, and
test again.
The next test is for excessive voltage between
ground and neutral — an indication of how
much noise is in the lines from the operation of
other appliances on the circuit. For computer
operation, a few volts is okay; for dedicated lines
for faxes or copiers, no more than a few millivolts
should be present. With this test, it’s best
to leave the SureTest plugged in overnight, or even
for a few days. It will hold the peak reading that
develops. That way, you can figure out how the
intermittent operation of other appliances on the
line — a hairdryer, a television, or a vacuum
cleaner — might affect the circuit.
Push the advance button again and you get a
reading of the load on that circuit, in amps, back
to the panel (it’s best to take this reading
from the last receptacle on the circuit). This is a
good way to check a dedicated circuit; if you get
any reading at all, it may mean that the circuit
shares a neutral. Again, the SureTest allows you to
test for load over an extended period; just leave
the device plugged in and it will record the peak
reading.
A final test measures the resistance of the ground
path in ohms — a very important test when it
comes to sensitive electronic equipment. A low
reading, preferably below .25 ohms, helps assure
that excessive voltages that may develop in the
line (from lightning or a power spike, for example)
will be safely returned to ground at the panel
without destroying equipment.
For more information on the SureTest ST-1D or
other less expensive models, contact ICE (2421
Harlem Rd., Buffalo, NY 14225; 800/442-3462).
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Open Ground
In this case,
there is no ground connection at the receptacle, perhaps
because it's a two-wire circuit, or because the ground wire has
come loose, or because the installer cut the ground wire too
short to make the connection. (Sometimes this is done on
purpose because an untrained installer doesn't know what to do
with the wire.)
An open ground gives the illusion of safety when actually
there is grave danger. People see the three-prong receptacle
and assume there's a proper ground. If a tool or appliance
plugged into that receptacle develops a hot-to-ground fault,
the user can get shocked and possibly electrocuted.