number would be multiplied by 100 (RX100) to represent
2,500 ohms. My meter has been modified, so using a
cross reference table I see that this resistor is a 47,000
ohm resistor.
The question: “How do I obtain this cross reference
table?” The answer: “Make it.” Each meter is different, so
each meter requires its own chart.
To make the chart, choose the values for resistors
shown in Figure 9. Then, measure the resistors one at a
time, making sure to zero the meter between each
measurement. Write down the meter reading for each
resistor next to its value. When the table is filled out, it is
ready to be used as a cross reference chart.
Conclusion
This modified meter is no longer a regular ohmmeter;
it’s an “insulation tester.” With a
meter like this, I have found wires
where the insulation was partially
rubbed off; I found a leaky lightning
arrestor; I found wire to wire faults
inside walls; and I found many
instances of water damage. Any fault
or conduction that a fire alarm panel
detects with its ground fault
detection circuit I can now find with
this insulation tester.
My old inexpensive analog meter
has a use again, and I can easily find
wiring faults in a building that can’t
be found with a regular ohmmeter.
NV
REFERENCES
EC&M: Insulation
Resistance Testing:
How and Why?; John A. DeDad
http://ecmweb.com/mag/electric_
insulation_resistance_testing_3/
Megger: A Guide To Diagnostic
Insulation Testing Above 1 kV
www.biddlemegger.com/
biddle/5kV-DiagnosticTesting.pdf
Megger: A Stitch In Time
www.omnicontrols.com/
articles/Megger-A_Stich_in_Time.pdf
Princeton University:
Supplemental Instructions:
XP – DC Circuit –
Non-Linear Resistor
www.princeton.edu/~gschmidt/
Rider/Lab200/nonlinear_resist
ance/ nonlinearresistance01.htm
Hyperphysics:
Moving Coil Meters
http://hyperphysics.
phy-astr.gsu.edu/hbase/magnetic
/ movcoil.html
48
September 2010
