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Significance of IEC/BS EN 62305-4
Previously transient overvoltage or surge protection was
included as an advisory annex in the BS 6651 standard, with
a separate risk assessment. As a result protection was
often fitted after equipment damage was suffered, often
through obligation to insurance companies. However, the
single risk assessment in IEC/BS EN 62305 dictates whether
structural and/or LEMP protection is required hence
structural lightning protection cannot now be considered in
isolation from transient overvoltage protection - known as
Surge Protective Devices (SPDs) within this new standard.
This in itself is a significant deviation from that of BS 6651.
Indeed, as per IEC/BS EN 62305-3, an LPS system can no
longer be fitted without lightning current or equipotential
bonding SPDs to incoming metallic services that have ‘live
cores’ such as power and telecoms cables which cannot be
directly bonded to earth. Such SPDs are required to protect
against the risk of loss of human life (including permanent
injury) by preventing dangerous sparking that could present
fire or electric shock hazards.
Technical reference
IEC/BS EN 62305-4 - Electrical & electronic systems within structures
Electronic systems now pervade almost every aspect
of our lives, from the work environment, through to filling
the car with petrol and even shopping at the local
supermarket. As a society, we are now heavily reliant on
the continuous and efficient running of such systems.
The use of computers, electronic process controls and
telecommunications has exploded during the last two
decades. Not only are there more systems in existence, the
physical size of the electronics involved has reduced
considerably (smaller size means less energy required to
damage circuits).
IEC/BS EN 62305 accepts that we now live in the electronic
age, making LEMP (Lightning Electromagnetic Impulse)
protection for electronic and electrical systems integral to the
standard through part 4. LEMP is the term given to the overall
electromagnetic effects of lightning, including conducted
surges (transient overvoltages and currents) and radiated
electromagnetic field effects.
LEMP damage is so prevalent such that it is identified as one
of the specific types (D3) to be protected against and that
LEMP damage can occur from ALL strike points to the
structure or connected services - direct or indirect - for further
reference to the types of damage caused by lightning see
Table 5 on page 16/5. This extended approach also takes into
account the danger of fire or explosion associated with
services connected to the structure, e.g. power, telecoms
and other metallic lines.
Lightning is not the only threat
Transient overvoltages caused by electrical switching events
are very common and can be a source of considerable
interference. Current flowing through a conductor creates a
magnetic field in which energy is stored. When the current is
interrupted or switched off, the energy in the magnetic field is
suddenly released. In an attempt to dissipate itself it becomes
a high voltage transient.
The more stored energy, the larger the resulting transient.
Higher currents and longer lengths of conductor both
contribute to more energy stored and also released!
This is why inductive loads such as motors, transformers
and electrical drives are all common causes of switching
transients.
Motors create switching events