CEMP-E
TI 811-12
18 August 1998
b. Power lines serving the system, nearby electrical and electro-mechanical devices, and lightning
strikes are sources of transients.
c. Power line variations, due to transients from large starting loads or other disturbances, may
cause temporary low voltage conditions to exist. Power line conditioners or uninterruptible power
systems protect UMCS equipment from the effects of powerline variations.
d. Communication links except fiber optic cables, between the central station and island station,
between the island station and smart field panels, and between smart field panels and remote terminal
units or unitary controllers must have surge protection circuits installed at each end. Triple electrode gas
surge arrestors must be installed within 3 feet of the building cable entrance and connected to the
building grounding system.
e. Power circuits serving UMCS equipment must be surge protected.
f.
Control and sensor lines connected to UMCS equipment must be surge protected.
14. TRANSIENT PROTECTION DEVICES. Surge arresters provide low impedance paths to ground for
surge voltage and near-lightning strikes which exceed threshold voltages ranging from 6.8 volts to
100,000 volts. A variety of different devices are available to protect against lightning and other transients
in power supplies, data transmission lines, digital hardware, controllers, and instruments. Fuses and
circuit breakers will be used to limit current in power supplies from overcurrent and short circuits.
Transient protection devices will be used to protect electronic circuits. Types of transient protection are
enumerated below.
a. Spark gap surge protection devices, such as gas filled tubes, are generally used to handle
surges due to lightning or other transients. Gas filled tubes are available for a range of threshold voltages
to meet various applications, such as power or signal lines. Gas filled tubes are relatively slow to react
when compared to semiconductor devices, thus requiring that they be used in conjunction with other
faster acting protection devices, such as zener diodes. These faster acting devices protect the circuit
until the overvoltage is shunted to ground by the gas filled tube.
b. Solid state surge protection devices, such as varistors, silicon avalanche diodes, zener diodes
and double anode zeners are semiconductor devices that provide low voltage clamping for high speed
transients. Double anode zeners are also used across relay coils to eliminate coil generated
electromagnetic interference (EMI). Solid state surge protection devices are used in conjunction with
spark gap surge protection devices, to provide protection against overvoltage in excess of the solid state
device ratings.
c. Crowbars consist of an electronic circuit that rapidly senses an overvoltage and provides a low
impedance path to ground. The overvoltage setpoint of crowbar circuits is adjustable to suit the
application. One use of crowbars is to limit the voltage output of DC power supplies.
d. Optical isolators provide DC isolation between interconnecting wiring and input circuits by the
use of LEDs and photocells. These circuits are used primarily to isolate control and sensor wiring circuits
from the UMCS input circuits. Optical isolators prevent damaging transients from passing through them,
but are still subject to failure when large surges occur. Optical isolators typically provide up to 2500 volts
RMS isolation.
e. Inductor-capacitor-resistor passive filter networks are used in input/output circuits to attenuate
high frequencies associated with fast rise times in voltage transients.
15. GROUNDING. The ideal grounding system is one which provides a zero impedance path for
currents at all frequencies the system is expected to encounter. The most common type of grounding
system consists of a grounding circuit that is terminated by rods or pipes driven into the ground. Use of
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