Hitachi SM 6145, Установка, эксплуатация и обслуживание

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STS USASecondary Surge Suppressors USSP)
Copyright SM 6145, Rev. 23, March 2019 2-1
2. PRINCIPLES OF OPERATION
The principal function of the USSP is to suppress the small "overshoot" voltages that
escape the primary arresters. These voltages are potentially destructive to sensitive
components such as transistors. When a high surge voltage is impressed on equipment
lines (power or signal transmission), the primary Line-to-Ground arresters shunt the
surge to ground while the primary Line-to-Line arrestors hold the Line-to-Line voltage to
reasonable levels. However, in the presence of surges with fast rise times (measured in
thousands of volts or amperes per microsecond), the time required to achieve complete
breakdown of the primary arresters (if used alone) is sufficient to allow some overshoot
voltage into the equipment lines.
USSP models use a Zener/TVSS diode or varistor as transient voltage suppressors to
shunt this residual surge voltage before it can reach equipment components. The surge
current first passes through the inductor coils, which limit or "choke" the current to a
level which will not destroy the Zener/TVSS diode. The coils also assist in the firing of
the arrester. When the surge voltage reaches the reverse-bias Zener/TVSS value, the
Zener/TVSS limits the surge voltage.
USSP track circuit models use back-to-back Zener/TVSS devices to shunt overshoot
voltages in both low voltage AC and DC track circuits. The AC line models use a Metal
Oxide Varistor to limit the surge voltages equally in both polarities.
The high voltage arresters are always wired in a Line-to-Ground configuration to shunt
the surge energy to an intentional ground path rather than through the equipment to an
unintentional ground.