Technical information

Technician’s Handbook
66
Divide the measured “Ring” current by the measured “Loop” current to
determine the relation of the building ground potential to the CO ground
potential. The result is 2.0 if the CO and PBX ground points are the same
potential.
A result between 1.85 and 2.15 indicates an acceptable building ground.
A result outside these limits means that you must locate an alternate
building ground.
1. Disconnect the building ground wire from the PBX ground-stud.
2. Disconnect a loop-start or ground-start trunk from the PBX.
3. Measure the Loop current.
 Set the meter to Milliamperes = DC and Range = 200
Milliamperes.
 Connect the meter between the Tip and Ring trunk leads.
 For a ground-start trunk, apply the building ground momentarily
to the ring side of the trunk. This application of ground will signal
the CO to complete the loop and provide DC loop current.
 Allow sufficient time for the current to stabilize and record the loop
current.
 DC Loop Current = _______________________
4. Measure the Ring current:
 Set the meter to Milliamperes = DC and Range = 200
Milliamperes.
 Connect the meter between the Ring trunk lead and the building’s
open-ended ground wire. Be sure that you disconnect the
building ground wire from the PBX ground-stud.
5. Allow sufficient time for the current to stabilize and record the Ring
current.
 DC Ring current = ________________________
6. Calculate the CO/PBX ground potential by dividing the Ring current
value by the Loop current value.