so may cause body resistance to shunt the circuit,
USE OF BASIC TEST EQUIPMENT
causing an erroneous reading.
12. Connect the ground lead of the meter first
Learning Objective: Recognize the various
when making voltage measurements. Work with
types of general-purpose test equipments
one hand whenever possible.
a s s o c i a t e d with aircraft electrical
maintenance, and identify tests the AE will
make using these equipment.
Test equipment, like any other tool the AE
In open circuits, current flow stops. The cause
uses, is susceptible to damage, misuse, and
may be a broken wire, defective switch, etc. To
detect open circuits, you perform a continuity test,
program is essential to ensure your shop or
which will tell if the circuit is complete or
division has the most reliable test equipment. For
refer to the latest edition of Aviation Maintenance
ohmmeter. Normally, you will make this test in
Ratings Fundamentals, NAVEDTRA 10342-3,
a circuit where the resistance is low, such as the
and NEETS, module 16. Also, when making tests
resistance of a copper conductor. If the resistance
on equipment, you should adhere to the following
is very high or infinite, the resistance between the
two points shows an open circuit.
1. Always connect an ammeter in series--
of a cable connecting two electronic units. You
never in parallel.
can see that both plugs are disconnected, and the
ohmmeter is connected in series with conductor
never in series.
A, which is under test. The power should be off.
3. Never connect an ohmmeter to an
When checking conductors A, B, and C, the
current from the ohmmeter flows through plug
4. On test meters, select the highest range
2 (female), conductor A, B, or C, to plug 1
first, then switch to lower ranges as needed.
(female). From plug 1, current passes through the
5. When you use an ohmmeter, select a scale
jumper to the chassis, which is grounded to the
that gives a near midscale reading, since midscale
aircraft structure. The aircraft structure serves as
is where the meter is most accurate.
the return path to the chassis of unit 2 and
completes the circuit through the series-connected
switch in a resistance position when not using the
ohmmeter. The ohmmeter shows a low resistance
because no break exists in conductors A, B, or
the internal battery. There is less chance of
C. However, checking conductor D (shown by
damaging the meter if the switch is on a high ac
reading in figure) reveals an open. The ohmmeter
volts setting, or in the OFF position. Meters that
shows maximum resistance because current
have an OFF position dampen the swing of the
cannot flow in an open circuit. With an open
needle by connecting the meter movement as a
circuit, the ohmmeter needle is all the way to the
generator. This prevents the needle from swinging
left since it is a series-type ohmmeter (reads right
wildly when moving the meter.
7. View the meter from directly in front to
If you can't use the aircraft structure as the
eliminate parallax error.
return path, use one of the other conductors
8. Observe polarity when measuring dc
(known to be good.) This technique will also
voltage or current.
reveal the open in the circuit.
9. Do not place meters in the presence of
strong magnetic fields.
GROUNDED CIRCUIT TEST
10. Never measure the resistance of a meter
or a circuit with a meter in it. The high current
Grounded circuits happen when some circuit
required for ohmmeter operation may damage
part makes contact either with the aircraft metallic
the meter. This also applies to circuits with
framework or with a metal object that is a ground.
low-filament-current tubes and to some types of
The most common cause of grounds is insulation
fraying from a wire, allowing the conductor to
11. When you are measuring high resistance,
come in contact with aircraft structure.
do not touch the test lead tips or the circuit. Doing