composed of both coils and the pointer, comes
Discharge equipment having a consider-
to rest at a position in which the two forces
able capacitance before and after making
balance. This position depends upon the value of
megger tests to avoid the danger of
RX, which controls the current in coil a. Because
receiving a shock. You can do this by
changes in voltage affect both coils in the same
grounding or short circuiting the terminals
proportion, the position of the moving element
o f the equipment under test.
is independent of the voltage. If you short the test
OSCILLOSCOPE
leads together, the pointer will come to rest at zero
because the current in coil a is relatively large.
An oscilloscope is a versatile instrument used
Since R3 limits the current, the instrument is not
damaged under these circumstances. Figure 2-6,
in viewing wave shapes of voltages or currents.
view B, shows the external appearance of one type
It is capable of giving information concerning
frequency values, phase differences, and voltage
of megger.
amplitudes. It can also trace signals through
electronic circuits to localize sources of distortion
Most meggers you will use have a 500-volt
and to isolate troubles to particular stages.
When you crank the generator faster than its rated
The majority of oscilloscopes used for test and
other measurements contain a basic presentation
speed, the clutch slips. This prevents the generator
unit (screen) and accessory units, such as
speed and output voltage from exceeding rated
values. A 1,000-volt generator is available for
amplifiers, synchronizing circuits, time-delay
extended ranges. When you want to measure an
circuits, and sweep oscillators. These circuits are
extremely high resistance (10,000 megohms or
used to display a stationary pattern on the
cathode-ray tube (CRT). The sensitivity of an
more), a high voltage is needed to cause enough
oscilloscope should be adequate for the smallest
current flow to operate the meter movement.
signals to have sufficient amplitude for screen
When you use a megger, observe the following
display.
precautions:
NEETS discusses the CRT, the oscilloscope,
and their operation in detail.
When making a megger test, make sure the
TIME-DOMAIN REFLECTOMETRY
equipment is de-energized.
Time-domain reflectometry (TDR) is a
Observe all rules for safety in preparing
measurement concept widely used in the analysis
equipment for test and in testing, especially
of wideband systems. The art of determining the
when testing installed high-voltage apparatus.
characteristics of electrical lines by observation
of reflected waveforms is not new. For many
Use well-insulated test leads, especially
years power-transmission engineers have located
when using high-range meggers. After con-
discontinuities in power-transmission systems by
necting the leads to the instrument and before
sending out a pulse and monitoring the reflections.
connecting them to the component under
Discontinuity is any abnormal resistance or
test, operate the megger to make sure there is
impedance that interferes with normal signal flow.
no leakage between leads. The reading should
TDR is particularly useful in analyzing coaxial
be infinity. Make certain the leads are
cables, such as those in fuel or oxygen quantity
not broken and the connections are good by
indicating systems. The amplitude of the reflected
touching the leads together while turning the
signal corresponds directly to the impedance of
crank slowly. The reading should be zero.
the discontinuity. You can find the distance to the
discontinuity by measuring the time required
When
for the pulse to travel down the line to the
using high-voltage meggers, take
proper precautions against electric shock.
reflecting impedance and back to the monitoring
oscilloscope.
There is enough capacitance in most electrical
TDR analysis consists of inserting an energy
equipment to store up sufficient energy from
the megger to give a very disagreeable and
step or pulse into a system and the subsequent
even dangerous electric shock. The megger has
observation, at the insertion point, of the energy
reflected by the system. Several arrangements are
a high protective resistance, and its open circuit
possible, but the following procedure is used with
voltage is not as dangerous, but always be careful.
