For this
troubleshooting exercise, there are two problems of the type that can
easily occur when troubleshooting circuits outside the classroom. One
of the problems is easily noticible, and the other might remain
undetected (since it doesn't cause any immediate problem).
- Flip wall switches, etc., to see if there are any problems with the branch circuit you've been assigned to.
- Once you're through with the previous step, put the wall
switches in positions that will not cause the breaker or the GFCI to
trip.
- If a breaker has tripped, turn it back on.
- If the GFCI has tripped, reset it.
- The GFCI may or may not trip if there is a short between
ground and hot. (The breaker may trip before the GFCI has a chance to
trip.) If the GFCI does trip, you know there is a current path between
hot and ground.
- If there is a short between hot and neutral, the GFCI
will not trip. It only trips if it sees unequal currents in the hot and
neutral wires. The only way these currents can be unequal is if there
is a current path between hot and ground.
- Unplug the breaker box power cord (from the socket at the workbench). When you do this, always hang the power cord over the 2x4 beneath the breaker box, so that you can see the plug while you're working.
- For the branch circuit you are working on, disconnect the
ground and neutral wires at the breaker box from the ground and neutral
busses.
- Plug in the breaker box power cord.
- Turn on the wall switch that is causing the problem.
Whenever you are measuring voltages at the breaker box or at a metal wall box, be sure that you are not touching the box. This will protect you from shock if you accidentally touch a wire that is hot. Do not assume that neutral or ground wires are safe
to touch. If there is a problem with the wiring or if something is
plugged into a receptacle, these wires could become hot (able to give
you a shock). Keep your fingers on the plastic part of the meter probes, and be sure they can't slip and touch the metal tips.
- What voltage do you get between the disconnected neutral wire and the neutral bus? ____________
- What voltage do you get between the ground wire and the neutral bus? ___________
- Which wire is probably shorted to a hot wire somewhere, the neutral wire or the ground wire? ____________
- Unplug the breaker box power cord.
Use a megohmmeter set on low ohms for the next three steps:
- Resistance between the branch circuit's neutral and hot wires. __________
- Resistance between the branch circuit's ground and hot wires. ___________
- If the neutral wire is the one with the
problem, measure the resistance between neutral and hot for all
the receptacles of the branch circuit. If the ground wire is the one
with the problem, measure between it and hot on all branch receptacles.
Ohms for receptacle closest to breaker box: _____________ . Ohms for
other receptacle: ____________
For the remaining steps, use a regular meter.
- Check for continuity between the grounds of both receptacles. Is there continuity? ____________
- To check for continuity, set the meter to the ohms
setting that has the icon for soundwaves next to it. The meter will
beep whenever it sees low ohms between it's probes.
- Turn off the switch associated with the problem that trips the breaker &/or GFCI.
- At the breaker box, reconnect the branch circuit's neutral and ground wires to the neutral and ground busses.
- Plug in the breaker box power cord.
For the next step, do not be kneeling
on the floor, do not be touching anything connected to ground, and do
not make the measurements if your shoes are damp.
- For the receptacle closest to the
problem, measure voltages between your hand and the hot, neutral, and
ground terminals: Hot: __________, Neutral: ___________, Ground:
__________
- Be very careful not to have the meter probe
touch both the metal box and the screw for which you are measuring
voltage. Also, be sure your fingers are not touching the metal of the
meter probes nor the metal of the box or switch. Then, measure the
voltages at the two terminals of the switch related to the problem.
- Do not remove any switches or receptacles from their boxes.
- Tell the instructor what you consider to be the most likely causes of the two problems that exist.
- After the instructor has given you the
go-ahead, you may remove the switch &/or the receptacle to verify
what you determined.
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