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SURGES AND
SURGE PROTECTION
Common and difference mode surges
Cables consist of more than one conductor. During a surge,
all conductors
will tend to move together in potential relative to local
ground. This is
referred to as common mode. However, a difference in voltage
can also develop between the conductors. This is referred
to as "difference mode" (also known as "transverse"
or "series" mode).
Both surges can damage electronic equipment. Common mode
surges tend to be bigger, but equipment tends to be more
vulnerable to difference mode. However,
Telematic surge protectors limit both types of surge.
How surges damage equipment
Before a surge can damage electronic equipment, several
conditions need
to be fulfilled:
- With respect to a local voltage reference point, the
voltage shift
is the same for both conductors.
- Local voltage reference point
- Pair of signal conductors
- The voltage difference occurs between the conductors,
i.e. one conductor shifts in voltage with respect to
the other
Voltage/current relationship
Sufficient voltage must be present between two vulnerable
points on the
equipment to cause significant current to flow. The vulnerable
points are
usually signal or power supply inputs or outputs, and
the equipment's zero
voltage reference point which is commonly the casing or
chassis connected to the mains supply earth. The voltage
above which significant current starts to flow is often
called the breakdown voltage (or potential).
Time/energy relationship
The current must flow for sufficient time to deposit enough
energy within
electronic components to cause damage - commonly the melting
down of
some part of the device.
Surge protection devices (SPDs) how they work
Surge protection devices limit the transient voltage to
a level which
is safe for the equipment they protect by conducting the
large surge
current safely to ground through the earth conductor system.
Current
flows past, rather than through, the protected equipment
and the SPD
thereby diverts the surge. The SPD limits both common
and difference mode voltages to the equipment.The voltage
which the equipment receives during a surge is called
the "limiting" or "let-through" voltage.
One way of regarding a surge protection device is as an
earth connection
which is only present during a surge.
What equipment needs protecting?
In principle, wherever a cable enters an equipotential
zone, equipment connected to that cable is exposed to
possibly damaging surges. The degree of risk depends on
factors such as:
a) Cable length.
b) Frequency of occurrence of lightning.
c) Exposure of the site to lightning and the degree of
isolation.
d) Whether cables run above ground or underground.
It is essential to protect ALL cables which introduce
a significant
risk, as will be seen later.
Consider again our system on its earth plane, but now
with all incoming
cables feeding equipment through suitable SPDs. Because
of
the low impedance earth plane, this will still be close
to an ideal system.
Note 1: Small, self-contained, isolated pieces of equipment,
e.g. multimeters,
transistor radios and cassette players, do not, in general,
need protecting,
because they do not have a ground connection.
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