Grounding The System - Comdial G0408 Manual

Digital Telephone System
2.3.2

Grounding The System

If spare conductors exist in the cables between the station and the 66M-xx connector blocks, it is
good practice to connect them to an earth ground. Doing this may help prevent them from
inducing radio frequency and/or AC interference into the system. It is also good practice to
disconnect any unused station jacks from the connector block and ground that wiring to an earth
ground as well.
Transient voltage spikes, if induced onto CO or CENTREX lines, can travel through the cable
and into the common equipment. The telephone company offers basic protection against this
condition but it is usually designed to protect the central office circuits. While it will also
provide some protection to the common equipment, you should not rely upon it for total
protection. To help ensure that external over-voltage surges do not damage the system, you
should install and properly ground primary protection devices, such as gas discharge tubes or
similar devices, on all lines. While the line boards have internal secondary surge protection on
all line ports, in order for this protection to be effective, you MUST connect the common
equipment cabinet to a reliable, effective earth ground.
Proper DSU grounding is necessary for trouble-free operation and personnel safety. The DSU
has the following three types of grounds:
• Service Ground—a neutral power line wire that is connected to the ground bus in the
premises' AC power panel,
• System Ground—a non-current carrying power line wire that is connected to the ground bus
in the premises' AC power panel,
• Frame Ground—a low impedance conductor that places the common equipment cabinet at
reference ground potential. The frame ground provides the greatest safety by limiting
electrical potential between non-current carrying parts of the system. The common
equipment cabinet provides a ground stud on its cabinet for access to its frame ground.
Effective grounding requires that you connect the frame ground to a good earth ground. A good
earth ground is one such as the ground bus in the premises' AC power panel or a public metallic
cold water pipe at a point immediately at its entrance to the premises and ahead of any meters,
pumps, or insulating sections that have been added for vibration reduction. Avoid using the
premises' structural steel frame as it may not be at earth ground potential. Make the ground
connection with #10 or #12 insulated, solid copper grounding wire. Keep the ground wire
separate from the three-wire AC line cord ground, do not splice it, and keep it as short as
possible.
The impedance of the wiring between the common equipment cabinet and the earth ground must
not exceed 0.25 ohms and the impedance between the earth ground and the power company's
reference standard ground must not exceed 4 ohms. Use an acceptable low impedance measuring
device to measure the impedance of these paths. The #10 or #12 wire size will minimize the
wiring impedance; however, if the impedance between earth ground and the power company's
standard reference ground exceeds 5 ohms, contact the local power company. The ground path
must always be of sufficient current-carrying capacity to prevent a build up of voltages that may
result in circuit noise, hazard to personnel, or equipment damage.
2 – 10 Installing The DSU
IMI66–107