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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.
BS EN/IEC 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.
BS EN/IEC 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.
Table 2: General indication of system impairments, of which manufacturers of transient overvoltage protectors should provide details
Protectors for mains supplies Protectors for data lines
Parallel protectors In-line
protectors
Low frequency
protectors
Network
protectors
Radio frequency
protectors
Nominal operating voltage
Maximum operating voltage
Leakage current
Nominal current rating
Max continuous current rating
In-line impedance
Shunt capacitance
Bandwidth
Voltage standing wave ratio
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
BS EN 62305)
not leave the user unprotected, at the end of its
life, and
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. BS EN/IEC 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.”