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Electronic systems protection
Introduction
Types of SPD
IEC/BS EN 62305 deals with the provision of SPDs to protect
against both the effects of indirect lightning strikes and
high-energy direct lightning strikes.
–– Direct lightning strikes are protected by lightning current or
equipotential bonding SPDs (Mains Type 1 SPDs & Signal/
Telecom SPDs to Test Category D)
–– Indirect lightning strikes and switching transients are
protected by transient overvoltage SPDs (Mains Type 2 and
Type 3 SPDs and Signal/Telecom SPDs to Test Category C)
Lightning current or equipotential bonding SPDs
Lightning current/equipotential bonding SPDs are designed to
prevent dangerous sparking caused by flashover.
Flashover is caused when the extremely high voltages
associated with a direct lightning strike breaks down
cable insulation. This can occur between the structural
LPS and electrical services and presents a potential fire
hazard and risk from electric shock.
Transient overvoltage SPDs
Transient overvoltage SPDs are designed to protect electrical/
electronic equipment from the secondary effects of indirect
lightning and against switching transients. SPDs should be
installed at sub-distribution boards and at equipment level for
critical equipment.
IEC/BS EN 62305 refers to the correct application of lightning
current and transient overvoltage SPDs as a coordinated set
where the service entrance lightning current SPD handles the
majority of surge energy and prevents flashover whilst the
downstream transient overvoltage SPDs ensure equipment
protection by sufficiently limiting the overvoltages.
For further information, please refer to the Furse Guide
to BS EN 62305 Protection Against Lightning.
IEC/BS EN 62305-2 Risk Management is used to evaluate
the required level of lightning protection measures necessary
to lower the risk of damage to a particular structure, its
contents and occupants to a defined tolerable level.
If the risk evaluation demands that a structural LPS is
required, then lightning current or equipotential bonding SPDs
are always required for any metallic electrical services entering
the structure.
These SPDs are necessary to divert the partial lightning
currents safely to earth and limit the transient overvoltage to
prevent possible flashover. They are therefore an integral part
of the structural LPS and typically form the first part of a
coordinated SPD set for effective protection of electronic
equipment.
If the risk evaluation shows that a structural LPS is not
required but there is an indirect risk, any electrical services
feeding the structure via an overhead line will require lightning
current SPDs typically installed at the service entrance, with
coordinated transient overvoltage SPDs downstream to
protect electronic equipment.
In order to provide effective protection, a transient
overvoltage protector/SPD must:
–– Be compatible with the system it is protecting
–– Survive repeated transients
–– Have a low ‘let-through’ voltage, for all combinations of
conductors (enhanced SPDs to IEC/BS EN 62305)
–– Not leave the user unprotected, at the end of its life
–– Be properly installed
Important
The primary purpose of lightning current or equipotential
bonding SPDs is to prevent dangerous sparking caused
by flashover to protect against the loss of human life.
In order to protect electronic equipment and ensure the
continual operation of systems, transient overvoltage SPDs
are required.
IEC/ BS EN 62305-4 specifically states that
‘a lightning protection system which only employs
equipotential bonding SPDs provides no effective
protection against failure of sensitive electrical or
electronic systems.’
Compatibility
The protector must not interfere with the system’s
normal operation:
–– Mains power supply SPDs should not disrupt the normal
power supply such as creating follow current that could
blow supply fuses, or cause high leakage currents to earth
–– SPDs for data communication, signal and telephone
lines should not impair or restrict the systems data or
signal transmission