Proper grounding is essential for the reliable operation of PBX systems, ensuring safety and maintaining the integrity of communication signals. Inadequate or incorrect grounding can lead to a range of issues, from trunk failures to system malfunctions. This guide provides detailed steps for testing and correcting ground path problems in PBX systems, focusing on the two types of grounding: Safety Ground and System Ground.

Types of Ground in Telephone Systems

There are two critical types of ground used in telephone systems:

1. Safety Ground:

• Purpose: Absorbs dangerous voltages that may come into contact with the PBX cabinet.
• Connection: The safety ground connects to the PBX through the ground termination of the system’s three-prong power cord.

1. System Ground:

• Purpose: Provides a stable ground reference for the voltages the system uses.
• Connection: The system ground attaches to the PBX through a separate ground wire (recommended 6 AWG) that connects directly to a system cabinet ground-stud.

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

Problems Caused by Incorrect Grounding

Incorrect grounding can lead to several issues that may disrupt PBX operations:

• Signal Recognition Failures: If the Central Office (CO) and PBX have different ground reference voltages, they may fail to recognize each other’s signals, leading to issues like:
• Ground start trunks not seizing when the PBX grounds the ring lead.
• The CO failing to release trunks when the PBX removes its termination.
• Disrupted PBX Logic Circuits: Low-frequency AC ground differentials can interfere with PBX logic circuits, causing system failures or incorrect operations.
• Audio Interference: AC ground differentials at radio frequencies can cause audio interference and disrupt PBX logic circuits.

Ground Path Resistance Test

The Ground Path Resistance Test ensures that the PBX’s safety ground and system ground are properly connected and within acceptable resistance levels.

Steps:

1. Turn Off the PBX System Circuit Breaker:

• Ensure the system is safely powered down before testing.

1. Connect the Cabinet’s Chassis Ground:

• Connect the PBX system’s chassis ground to the electrical panel ground using a minimum 10 AWG wire, no longer than 15 meters.

1. Disconnect the System Ground Wire:

• Disconnect the ground wire from the system’s ground-stud on the PBX.

1. Set Your Meter:

• Set your meter to the OHMS setting at the highest scale.

1. Measure the Resistance:

• Measure the resistance between the PBX chassis safety ground and the system ground wire.
• Adjust your meter down until you get a reading.

1. Interpret the Results:

• 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, which can indicate grounding issues.

Steps:

1. Use the Same Setup as the Resistance Test:

• Connect your meter as you did for the Ground Path Resistance Test.

1. Measure AC Voltage:

• Set your meter for high AC voltages and adjust it down until you get a reading.

1. Interpret the Results:

• You should get a reading of 1 Vac or less.
• If your reading is greater than 1 Vac, check if the electrical panel ground connects to the building ground. If this connection is present, try an alternate ground point and measure the AC voltage again.

1. Analyze the Results:

• If the safety ground and system ground connect to the building ground, your measurement represents 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

This test helps diagnose trunk lock-ups or seize failures by determining whether the DC ground potential between the building ground and the CO ground is within acceptable limits.

Steps:

1. Disconnect the Building Ground Wire:

• Disconnect the building ground wire from the PBX ground-stud.

1. Disconnect a Trunk:

• Disconnect a loop-start or ground-start trunk from the PBX.

1. 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, momentarily apply the building ground to the ring side of the trunk to signal the CO to complete the loop and provide DC loop current.
• Allow the current to stabilize and record the Loop current.

1. 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.
• Allow the current to stabilize and record the Ring current.

1. Calculate the Ground Potential:

• Divide the Ring current by the Loop current.
• A result of 2.0 indicates the CO and PBX ground points are at the same potential.
• A result between 1.85 and 2.15 indicates an acceptable building ground.
• A result outside these limits suggests the need to locate an alternate building ground.

Conclusion

Correct grounding is crucial for the reliable operation of PBX systems. By following these ground path tests and taking corrective actions when necessary, you can ensure that your PBX system operates smoothly, free from the issues caused by incorrect grounding. Proper grounding not only protects the equipment from potential damage but also ensures that communication signals are transmitted clearly and reliably.