High voltage
ohmmeters
Most ohmmeters of the design shown in the
previous section utilize a battery of relatively low
voltage, usually nine volts or less. This is perfectly
adequate for measuring resistances under several mega-ohms (MΩ),
but when extremely high resistances need to be measured, a 9
volt battery is insufficient for generating enough current
to actuate an electromechanical meter movement.
Also, as discussed in an earlier chapter,
resistance is not always a stable (linear) quantity. This is
especially true of non-metals. Recall the graph of current
over voltage for a small air gap (less than an inch):
While this is an extreme example of
nonlinear conduction, other substances exhibit similar
insulating/conducting properties when exposed to high
voltages. Obviously, an ohmmeter using a low-voltage battery
as a source of power cannot measure resistance at the
ionization potential of a gas, or at the breakdown voltage
of an insulator. If such resistance values need to be
measured, nothing but a high-voltage ohmmeter will suffice.
The most direct method of high-voltage
resistance measurement involves simply substituting a higher
voltage battery in the same basic design of ohmmeter
investigated earlier:
Knowing, however, that the resistance of
some materials tends to change with applied voltage, it
would be advantageous to be able to adjust the voltage of
this ohmmeter to obtain resistance measurements under
different conditions:
Unfortunately, this would create a
calibration problem for the meter. If the meter movement
deflects full-scale with a certain amount of current through
it, the full-scale range of the meter in ohms would change
as the source voltage changed. Imagine connecting a stable
resistance across the test leads of this ohmmeter while
varying the source voltage: as the voltage is increased,
there will be more current through the meter movement, hence
a greater amount of deflection. What we really need is a
meter movement that will produce a consistent, stable
deflection for any stable resistance value measured,
regardless of the applied voltage.
Accomplishing this design goal requires a
special meter movement, one that is peculiar to
megohmmeters, or meggers, as these instruments
are known.
The numbered, rectangular blocks in the
above illustration are cross-sectional representations of
wire coils. These three coils all move with the needle
mechanism. There is no spring mechanism to return the needle
to a set position. When the movement is unpowered, the
needle will randomly "float." The coils are electrically
connected like this:
With infinite resistance between the test
leads (open circuit), there will be no current through coil
1, only through coils 2 and 3. When energized, these coils
try to center themselves in the gap between the two magnet
poles, driving the needle fully to the right of the scale
where it points to "infinity."
Any current through coil 1 (through a
measured resistance connected between the test leads) tends
to drive the needle to the left of scale, back to zero. The
internal resistor values of the meter movement are
calibrated so that when the test leads are shorted together,
the needle deflects exactly to the 0 Ω position.
Because any variations in battery voltage
will affect the torque generated by both sets of
coils (coils 2 and 3, which drive the needle to the right,
and coil 1, which drives the needle to the left), those
variations will have no effect of the calibration of the
movement. In other words, the accuracy of this ohmmeter
movement is unaffected by battery voltage: a given amount of
measured resistance will produce a certain needle
deflection, no matter how much or little battery voltage is
present.
The only effect that a variation in voltage
will have on meter indication is the degree to which the
measured resistance changes with applied voltage. So, if we
were to use a megger to measure the resistance of a
gas-discharge lamp, it would read very high resistance
(needle to the far right of the scale) for low voltages and
low resistance (needle moves to the left of the scale) for
high voltages. This is precisely what we expect from a good
high-voltage ohmmeter: to provide accurate indication of
subject resistance under different circumstances.
For maximum safety, most meggers are
equipped with hand-crank generators for producing the high
DC voltage (up to 1000 volts). If the operator of the meter
receives a shock from the high voltage, the condition will
be self-correcting, as he or she will naturally stop
cranking the generator! Sometimes a "slip clutch" is used to
stabilize generator speed under different cranking
conditions, so as to provide a fairly stable voltage whether
it is cranked fast or slow. Multiple voltage output levels
from the generator are available by the setting of a
selector switch.
A simple hand-crank megger is shown in this
photograph:
Some meggers are battery-powered to provide
greater precision in output voltage. For safety reasons
these meggers are activated by a momentary-contact
pushbutton switch, so the switch cannot be left in the "on"
position and pose a significant shock hazard to the meter
operator.
Real meggers are equipped with three
connection terminals, labeled Line, Earth, and
Guard. The schematic is quite similar to the
simplified version shown earlier:
Resistance is measured between the Line and
Earth terminals, where current will travel through coil 1.
The "Guard" terminal is provided for special testing
situations where one resistance must be isolated from
another. Take for instance this scenario where the
insulation resistance is to be tested in a two-wire cable:
To measure insulation resistance from a
conductor to the outside of the cable, we need to connect
the "Line" lead of the megger to one of the conductors and
connect the "Earth" lead of the megger to a wire wrapped
around the sheath of the cable:
In this configuration the megger should read
the resistance between one conductor and the outside sheath.
Or will it? If we draw a schematic diagram showing all
insulation resistances as resistor symbols, what we have
looks like this:
Rather than just measure the resistance of
the second conductor to the sheath (Rc2-s), what
we'll actually measure is that resistance in parallel with
the series combination of conductor-to-conductor resistance
(Rc1-c2) and the first conductor to the sheath (Rc1-s).
If we don't care about this fact, we can proceed with the
test as configured. If we desire to measure only the
resistance between the second conductor and the sheath (Rc2-s),
then we need to use the megger's "Guard" terminal:
Now the circuit schematic looks like this:
Connecting the "Guard" terminal to the first
conductor places the two conductors at almost equal
potential. With little or no voltage between them, the
insulation resistance is nearly infinite, and thus there
will be no current between the two conductors.
Consequently, the megger's resistance indication will be
based exclusively on the current through the second
conductor's insulation, through the cable sheath, and to the
wire wrapped around, not the current leaking through the
first conductor's insulation.
Meggers are field instruments: that is, they
are designed to be portable and operated by a technician on
the job site with as much ease as a regular ohmmeter. They
are very useful for checking high-resistance "short"
failures between wires caused by wet or degraded insulation.
Because they utilize such high voltages, they are not as
affected by stray voltages (voltages less than 1 volt
produced by electrochemical reactions between conductors, or
"induced" by neighboring magnetic fields) as ordinary
ohmmeters.
For a more thorough test of wire insulation,
another high-voltage ohmmeter commonly called a hi-pot
tester is used. These specialized instruments produce
voltages in excess of 1 kV, and may be used for testing the
insulating effectiveness of oil, ceramic insulators, and
even the integrity of other high-voltage instruments.
Because they are capable of producing such high voltages,
they must be operated with the utmost care, and only by
trained personnel.
It should be noted that hi-pot testers and
even meggers (in certain conditions) are capable of
damaging wire insulation if incorrectly used. Once an
insulating material has been subjected to breakdown
by the application of an excessive voltage, its ability to
electrically insulate will be compromised. Again, these
instruments are to be used only by trained personnel. |
