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Technical reference
IEC/BS EN 62305-4 - Electrical & electronic systems within structures
Surge Protection Measures (SPM)
Some areas of a structure, such as a screened room, are
naturally better protected from lightning than others and it is
possible to extend the more protected zones by careful
design of the LPS, earth bonding of metallic services such as
water and gas, and cabling techniques. However it is the
correct installation of coordinated Surge Protective Devices
(SPDs) that protect equipment from damage as well as
ensuring continuity of its operation - critical for eliminating
downtime. These measures in total are referred to as Surge
Protection Measures (SPM) (formerly LEMP Protection
Measures System (LPMS)).
When applying bonding, shielding and SPDs, technical
excellence must be balanced with economic necessity. For
new builds, bonding and screening measures can be integrally
designed to form part of the complete SPM. However, for
an existing structure, retrofitting a set of coordinated SPDs is
likely to be the easiest and most cost-effective solution.
Coordinated SPDs
IEC/BS EN 62305-4 emphasizes the use of coordinated
SPDs for the protection of equipment within its environment.
This simply means a series of SPDs whose locations and
LEMP handling attributes are coordinated in such a way as
to protect the equipment, by reducing the LEMP effects to
a safe level. So there may be a heavy duty lightning
current SPD at the service entrance to handle the majority
of the surge energy (partial lightning current from an
LPS and/or overhead lines) with the respective transient
overvoltage controlled to safe levels by coordinated plus
downstream overvoltage SPDs to protect terminal
equipment including potential damage by switching sources,
e.g. large inductive motors. Appropriate SPDs should be
fitted wherever services cross from one LPZ to another.
Coordinated SPDs have to effectively operate together as
a cascaded system to protect. For example the lightning
current SPD at the service entrance should handle the
majority of surge energy, sufficiently relieving the downstream
overvoltage SPDs to control the overvoltage.
Poor coordination could mean that the overvoltage SPDs
are subject to too much surge energy putting both itself and
potentially equipment at risk from damage.
Furthermore, voltage protection levels or let-through
voltages of installed SPDs must be coordinated with the
insulating withstand voltage of the parts of the
installation and the immunity withstand voltage of
electronic equipment.