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Guide to BS EN/IEC 62305 BS EN/IEC 62305-4
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
BS EN/IEC 62305-4 emphasises the use of coordinated
SPDs for the protection of equipment within their
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
in their environment 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 equipment in their
environment. 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.
Enhanced SPDs
Whilst outright damage to equipment is not desirable,
the need to minimize downtime as a result of loss of
operation or malfunction of equipment can also be
critical. This is particularly important for industries that
serve the public, be they hospitals, financial institutions,
manufacturing plants or commercial businesses, where
the inability to provide their service due to the loss of
operation of equipment would result in significant
health and safety and/or financial consequences.
Standard SPDs may only protect against common mode
surges (between live conductors and earth), providing
effective protection against outright damage but not
against downtime due to system disruption.
BS EN 62305 therefore considers the use of enhanced
SPDs (SPD*) that further reduce the risk of damage and
malfunction to critical equipment where continuous
operation is required. Installers will therefore need to
be much more aware of the application and
installation requirements of SPDs than perhaps they
may have been previously.
Superior or enhanced SPDs provide lower (better)
let-through voltage protection against surges in both
common mode and differential mode (between live
conductors) and therefore also provide additional
protection over bonding and shielding measures.
Such enhanced SPDs can even offer up to mains Type
1+2+3 or data/telecom Test Cat D+C+B protection
within one unit. As terminal equipment, e.g.
computers, tends to be more vulnerable to differential
mode surges, this additional protection can be a vital
consideration.
Appropriate SPDs
should be fitted
wherever services
cross from one LPZ
to another