BUILD A LOW VOLTAGE
Ground faults are a curse to fire
alarm systems. Even a small amount
of current leaking to ground
somewhere in the building can
cause an unscheduled fire drill.
Worse yet, a second ground fault
somewhere else in the building can
short out the whole system. That’s
why all fire alarm systems have
ground fault detection circuits, so the
attached building wiring can be fixed
before there is a fire.
3 Volt Battery Ohm Meter -
Measures a Very High Resistance
3 Volts from the Internal Battery
of the Ohm Meter Plus the 36 Volts
of the Added Batteries -
Measures a Lower Resistance
It was perplexing. I was trying to repair a fire alarm system in a school. The air outside was warm and humid, but inside it was air-conditioned. A light on the fire alarm panel indicated “ground fault” somewhere in
the wiring in the building, but my digital ohmmeter
claimed there wasn’t any continuity to ground. How was I
going to find this conductive path if my ohmmeter
couldn’t detect it?
After trying several means of detection, I decided to
give my ohmmeter a voltage boost. A couple of 12 volt
batteries from my service stock in series with the negative
lead of my ohmmeter did that. Now, using about 27 volts
of oomph to drive the current of my ohmmeter, it
indicated the continuity and I could follow the wiring to
find the ground fault. It turned out to be water condensed
on a fire alarm pull station in the gym. The humidity from
the warm air leaking from the outside through the wall
was condensing on the cool air-conditioned switch on the
inside. So, why didn’t my ohmmeter show any continuity
until it got that extra voltage boost from the batteries?
I was dealing with a voltage based non-linear
resistance; a resistance that changes value as voltage
changes so current doesn’t seem to follow Ohm’s Law.
The resistance of the water condensation was almost
infinite for the three volt internal battery of my ohmmeter,
but the resistance dropped down to a few thousand ohms
when the voltage was increased by the addition of the
extra batteries (see Figure 1).
Typical Non-Linear Resistance Curve
■ FIGURE 1. This chart shows how a non-linear resistance
goes down as voltage goes up. The voltage shown here
would be the battery voltage of the ohmmeter.
What I had come up with was not a new invention. It
was a test instrument used by electricians called an
insulation tester which is an ohmmeter with a high