Download HOW TO USE A VOM

HOW TO USE A VOLT-OHM MULTI-METER (VOM)
With the increased use of IP/SIP delivery of analog lines to the subscriber’s site, then converting it to a
simulated analog line, more instances of line failures/interference are occurring. This results in
technicians needing to test the “emulated” analog lines to determine whether they meet Bellcore
specifications. One way of doing this is with a Volt-Ohm Multi-meter (VOM) to determine ringing
voltage, talk voltage and line current.
This will describe how to use the VOM. A subsequent tip will address how to test the line and what
specifications are involved.
There a many variations of VOM available on the market, ranging in price from under $10 (U.S.) to
around $100 (US). An inexpensive one should suffice for most testing. Before purchasing, or using a
VOM, check the specifications to see that it works in the electrical range that needs to be metered.
Some meters are for “electrician” use only and may not provide low-voltage readings.
Below are pictures of two versions:
BEFORE
testing, READ THE MANUAL for your VOM to use the proper settings to avoid injury to
yourself, or the meter.
All VOM are battery powered. When you turn them on, you should see activity on the digital display. If
the batter is low, you may get an indicator, or just no display. In either case, replace the battery before
testing.
Referring to the first picture, notice that there are three (3) jacks for the test probes (wires). The center
jack (COM) is always populated, since it is COMmon to both testing options. The black probe wire is
usually inserted here, as shown, but this is more a “standard use” than electrical necessity, since either
probe will work.
The second probe (Red) will be inserted into either the Volts or Ohms jacks, depending on what type of
testing is to be done. Since some other units have different labels/jacks, consult the documentation
for that device to verify the correct positioning of the probe wires for your testing.
The second picture shows a meter with both probes permanently attached, so no movement of the
wires is necessary. The type of testing is handled by internal circuitry.
Voltage testing is done “across” the line, such as on an electrical outlet. The black probe would go into
the Neutral (wide) slot and the red probe in the Hot (narrow) slot. If testing Alternating Current (AC)
voltage, normally showing a “~” on the meter, the probe insertion would not make any difference. With
Direct Current (DC) voltage, the black probe would be the negative (-) side and the red probe the
positive (+) side. Reversing the probes on a DC circuit will result in the opposite polarity, i.e. a positive
voltage will show as negative and a negative voltage will show as positive.
Current testing needs to be done in “series” with the circuit. Since you are measuring current “flow”,
the probes are inserted to put the meter into the circuit. This would be similar to cutting the wire on
one side of the circuit, then connecting each one of the cut ends to a separate probe.
Ohms (resistance) is typically done on a specific electrical component, with the power disconnected.
Ohms measurements use the internal battery of the meter to power the measurement. The probes
would be applied to either side of the component being tested.
Electrical Testing
Prior to installation, use an outlet tester or voltmeter to check the receptacle for earth ground.
Please refer to the following figure for applying probes for testing.
AC voltage is tested across the plug by inserting the appropriate probe into the outlet (red into PhaseHot, and black into Negative). The result should provide the expected 110Volts AC (VAC) +/- 10%.
Ground is tested from Neutral to Ground and then Phase (Hot) to ground. The reading from Neutral to
Ground should ideally be “0”, but is acceptable at less than 1 Volt. The Phase (Hot) to Ground should be
the same as the prior AC voltage reading.
The Safety Ground should have a DC resistance of .1 ohms or less and .1 or less voltage potential. It is
also a good practice to verify that your equipment ground and the electrical ground are the same by
checking for voltage (AC) between the two. Put one probe in the electrical outlet ground and the other
on the equipment ground. The reading should be “0” but no more than 2.5VAC.
A rare phenomenon can create a voltage potential between the earth grounds of two or more buildings.
If products installed in separate buildings are interconnected, the voltage potential may cause a
hazardous condition. Verify that any shielded cables are only grounded at one end to avoid this
condition and interference on the circuits.
James Warner
AVAYA
Backbone Engineer
With special thanks to Ron Sager for prior documentation and Rick Budinger for technical review.
May 2011