Testing The Co And Pbx Ground Differential - Mitel SX-200 EL Technician's Handbook

Technician's Handbook
6. The resistance between the two grounds should be less than five
OHMS. If not, try alternate ground point(s) and repeat the test.

Testing the CO and PBX Ground Differential

If a PBX experiences trunk lock-ups, or trunk seize failures, perform the
CO/PBX Ground Differential Test.
This test determines whether the DC ground potential between the
selected building ground point and the CO ground point is within
acceptable limits by measuring the two currents: "Loop" current and
"Ring" current.
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
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Milliamperes.
Connect the meter between the Tip and Ring trunk leads.
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For a ground-start trunk, apply the building ground momentarily
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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
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loop current.
DC Loop Current = _______________________
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