SURGE PROTECTION DEVICES FOR ELECTRICAL INSTALLATIONS

This application note discusses the affects of surges and lightning induced
overvoltages on ac power systems. The note will introduce the source of
many common transients and surges and suggest the application of surge
protection devices in this area. Finally, typical examples are shown for
common installations

AC power protection is often overlooked when deciding a surge protection
strategy. When providing protection, users are keen to apply surge
protection devices, or SPDs, to process signal lines, transmitters, file
severs and other important equipment as the source of surges is more
obvious. However, a common source of surges is the AC power supply. It is
important to note that any cable entering an electronic device is also a easy
path for lightning current to enter and cause untold damage; the power
supply is no exception.

HOW SURGES ARISE
Surges on the mains supply can come from a number of sources. The
obvious route is via a lightning strike to the distribution system. In such a
case tens of thousands of amps may flow through the electrical system
and cause damage to sensitive electronics. Lightning may also induce
currents into signal cables entering a building. Once inside the building they
can easily couple into mains cabling. The opposite is also true; surge
current in the mains system can also be coupled into data cabling. The only
solution is to stop any surge current passing beyond the building entry.
Heavy current load switching can cause large surges on the supply either
through inductive effects or due to the transformerÕs limited regulation. Lift
motors and other large electrical machines can generate a significant
number of surges, especially when the motor is worn. In a domestic
situation, this is commonly seen when refrigerators or freezers switch on
or off via their thermostats.

Expected surge levels
Many standards give an outline of the magnitude of current to be expected in
various parts of a building. BS 6651:1999 splits the areas of a site into areas
A, B and C with area C receiving the largest surges. Category C is on the
supply side of an incoming power board, category B usually refers to the
distribution board and category A represents the load side of a socket outlet.
Each of these areas are split into exposure levels - low, medium and high.
With each of these examples, a peak voltage and peak current are given to
which the SPDs should be tested. The largest surge - category C and high
exposure - is given as 20kV and 10kA. In other words, the SPD should give
protection up to at least 10kA. ANSI/IEEE C62.41-1991 gives recommendations
on the protection level required. The document is very informative and
shows measured waveforms and statistical data based on real installations.