Primary Protection - Avaya 6424D+M User Manual

3-4
ELECTRICAL PROTECTION: SYSTEM 85 AND DEFINITY GENERIC 2 WITH TRADITIONAL MODULES

PRIMARY PROTECTION

Primary protection is provided by a fusible link and an overvoltage device. The following are
descriptions of these elements of primary protectors.
Overvoltage Devices
There are three types of overvoltage devices: carbon blocks, gas tubes, and solid-state devices.
Carbon Blocks
A carbon block is a device holding two or more small carbon electrodes gapped at a distance that allows
a voltage power surge to arc across the gap to ground. On average, carbon blocks operate when a
voltage of 500V to 700V occurs. Use carbon blocks to protect against hazardous voltages caused by
lightning, power crosses, power induction, and ground-potential rise.
Gas Tubes
A gas tube is a device containing a special gas that allows an electrical-voltage surge to arc through the
gas to ground. On average, gas tubes operate when a voltage of about 300V occurs. Use gas tubes to
protect against surges caused by lightning, power crosses, power induction, and ground-potential rise.
Solid-State Devices
A solid-state device uses a semiconductor in one of two ways to protect against high-voltage surges. It
either shunts the high voltage to ground or prevents it from exceeding a certain level. A solid-state
device usually operates at lower voltages and is much faster than carbon blocks and gas tubes. Use
solid-state devices to protect against surges caused by lightning, power crosses, power induction, and
ground-potential rise.
Sneak-Current Device: Heat Coll
A heat coil is a device that grounds a conductor when a hazardous current of more than 0.35A overheats
it. Heat coils protect against continuous faults and may only be used at the BEF. They are used in
combination with overvoltage devices and are installed in series behind any overvoltage device that is
used.