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3. ELECTRICAL PROTECTION: SYSTEM 85 AND DEFINITY GENERIC 2
WITH TRADITIONAL MODULES
This chapter identifies the electrical hazards that can harm personnel, the switch, terminals, and building
wiring; instructs you in how to evaluate the basic wiring design for real or potential electrical hazards;
and describes the protective devices and methods used to minimize those hazards.
If this chapter does not provide all the information that you need or if your wiring design presents some
atypical requirements, see Related Resources in About This Document for a list of documents on
electrical hazards and protection.
TYPES OF ELECTRICAL HAZARDS
Surges from four major electrical hazards can cause electrical or fire damage to personnel, the switch,
terminals, building wiring, equipment, or the building itself. These hazards are lightning, power cross,
power induction, and ground-potential rise, and they are conducted through the distribution cables.
Lightning causes a high-voltage surge by striking on or near the distribution cables. Power lines that fall
across the distribution cables are a power cross, which also causes a high-voltage surge. Power lines
that run long distances parallel and close to telephone lines create power induction. And an increase in
the voltage at the grounding point because of a power fault or because of lightning current flowing into
the ground in the nearby area causes ground-potential rise.
PROTECTING AGAINST HAZARD EXPOSURE
The areas that you must protect from electrical hazards are as follows:
●
AT&T distribution cables
●
Cabling that enters the facility from the telephone company
●
Customer building entrance facilities (BEF)
Cabling is considered exposed to lightning if any of these conditions exist:
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It is aerial
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It is above ground in an area where thunder is heard more than five times a year
●
The surrounding terrain has a history of lightning damage
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It is buried without a grounded metallic shield or the grounding is poor
●
It is buried in a span larger than 140 ft in soil with resistivity greater than 100 ohms per meter
●
It is outside the "cone of protection" of buildings or other conducting structures
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