A gas discharge tube (GDT) is a sealed
glass-enclosed surge protector device containing a special gas mixture trapped
between two electrodes, which conducts electric current after becoming ionized
by a high voltage spike. Also referred
to as "spikes," electrical surges are sudden, brief rises in voltage
and/or current to a connected load.
GDTs can
conduct more current for their size than other components .They can handle a
few very large transients or a greater number of smaller transients. The
typical failure mode occurs when the triggering voltage rises so high that the
device becomes ineffective, although lightning surges can occasionally cause a
dead short.
When the voltage
is at a certain level, the gas acts as a poor conductor. When the voltage
surges above that level, the electrical power is strong enough to ionize
the gas, making it a very effective conductor. It passes on
current to the ground line until the voltage reaches normal levels, and then
becomes a poor conductor again. During this period, this gas acts as a negative
resistance i.e. resistivity decreases as current increases.
This methods
has a parallel circuit design i.e. the extra voltage is fed away from
the standard path to another circuit. A few surge protector have a series
circuit design
i.e. the extra electricity isn't shunted to another line, but instead is slowed
on its way through the main line. Basically, these suppressors detect when
there is high voltage and then store the electricity, releasing it gradually.
This method offers better protection
because it reacts more quickly and doesn't dump electricity in the ground line,
possibly disrupting the building's electrical system.
As a backup,
some surge protectors also have a built-in
fuse. A fuse is a
resistor that can easily conduct current as long as the current is below a
certain level. If the current increases above the acceptable level, the heat
caused by the resistance burns the fuse, thereby cutting off the circuit. If
the gas discharge
arrestor doesn't
stop the power surge, the extra current will burn the fuse, saving the
connected machine. This fuse only works once, as it is destroyed in the
process.
Some surge
protectors have a line-conditioning system for filtering out "line
noise," smaller fluctuations in electrical current. Basic surge protectors
with line-conditioning use a fairly simple system. On its way to the power
strip outlet, the main wire passes through a toroidal choke
coil. The choke
is a just ring of magnetic material, wrapped with wire i.e. a basic electromagnet.
The ups and downs of the passing current in the main wire charge the
electromagnet, causing it to emit electromagnetic forces that smooth out the
small increases and decreases in current. This conditioned current is more
stable, and so easier to handle.