Technical information

Troubleshooting and Repair

3. You should get a reading of 1 Vac or less.

If your reading is greater than 1 Vac, check to see if your electrical panel

ground connects to the building ground. Depending on local utility

regulations, the connection usually exists between the electrical panel

ground and a cold water pipe entering the building. If this connection is

present, try an alternate ground point(s) and measure the AC voltage again.

If the system ground and safety ground both connect to the building

ground, your measurement is the metallic loop from the PBX chassis, to

the electrical panel ground, to the cold water pipe, and back through the

system ground wire.

If the safety ground and system ground do not connect at the electrical

panel, your measurement indicates the AC voltage differential between

protective earth and the building ground.

Ground Path Resistance Test

1. Turn off the main PBX system circuit breaker.

2. At the PBX system, connect the cabinet’s chassis ground to the

electrical panel ground. The wire should be a minimum 10 AWG and

not longer than 15 meters.

3. At the PBX system, disconnect the ground wire from the system’s

ground-stud.

4. Set your meter to OHMS at the highest scale.

5. Measure the resistance between the PBX chassis safety ground and

the ground wire that provides the system ground. Adjust your meter

down until you get a reading.

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.