Mike's PBX Cookbook

PBX Ground Path Tests

Correcting Ground Path Problems

There are two types of ground used in telephone systems:

  1. Safety Ground - absorbs the dangerous voltages that may come into contact with the PBX cabinet. The safety ground connects to the PBX through the ground termination of the systems three-prong power cord.

  2. System Ground - provides a stable ground reference for the voltages the system uses. The system ground attaches to the PBX through a separate ground wire (6 AWG recommended) that connects directly to the a system cabinet ground-stud.

In most buildings, the metallic cold water system provides the ground source. It must provide a metallic connection all the way back to the building entry point (including a metal strap that connects around the water meter).

Problems caused by incorrect grounding

If the CO and PBX have different ground reference voltages, each can fail to recognize the signals that the other sends. For example:

Low-frequency AC ground differentials can disrupt operation of the PBX logic circuits and cause incorrect operation or system failures.

AC ground differentials at radio frequencies can cause audio interference and possibly disrupt PBX logic circuits.

Ground Path Resistance Test

  1. Turn off the PBX system circuit breaker.
  2. At the PBX system, connect the cabinets 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 systems 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 points and repeat the test.

Ground Path AC Voltage Test

This test measures the presence of AC voltage in the metallic loop.

  1. With your meter connected in the same way as you did for the resistance test, measure AC voltage.
  2. Start with your meter set for high AC voltages and adjust it down until you get a reading.
  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 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 ground and the building ground.

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 a 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:
  4. Measure the Ring current:
  5. Calculate the CO/PBX ground potential by dividing the Ring current value by the Loop current value.

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