Technical information

Technician’s Handbook
64
Correcting Ground Path Problems
Safety ground absorbs the dangerous voltages that come in contact
with the PBX cabinet. The safety ground connects to the PBX through
the ground termination of the system’s three-prong power cord.
System ground provides a stable ground reference for the voltages the
the system uses. The system ground attaches to the PBX through a
separate ground wire (6 AWG recommended) that connects directly to
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 metallic strap that connects around the
water meter).
Problems Caused by Incorrect PBX Grounding
If the CO and PBX have different ground reference voltages, each can
fail to recognize the signals that the other sends. For example:
 Ground start trunks may not seize when the PBX grounds the ring
lead.
 The CO may not release trunks when the PBX removes its
termination.
Low-frequency AC ground differentials can disrupt the operation of 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 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 resis-
tance test, measure the AC voltage.
2. Start with your meter set for high AC voltages and adjust it down
until you get a reading.