Category: Tyco Electronics Transient Voltage Surge Suppression TVSS
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Bowthorpe EMP
Transient Voltage Surge Suppression - TVSS
Lightning Protection
By far the most prominent cause of damaging transients is nearby or
even direct lightning strikes. Lightning discharge currents can be as
high as 200,000A in the UK with the average discharge current being
28,000A. Such large currents would vaporise any electronic equipment or
cabling in the case of a direct strike to a low voltage supply.
Lightning will tend to strike the most prominent feature, which is
invariably the roof of the building!
The need for an integrated approach at International Standards level for
the protection against lightning has produced through IEC technical
committee 81: Lightning protection a 5 part Standard IEC62305
(BSEN62305)
- Part 1: General principles
- Part 2: Risk management
- Part 3 Physical damage to structures and life hazard
- Part 4: Electrical and electronic systems within structures
- Part 5: Services. IEC62305-4 now replaces IEC61312 1-4
The decision as to whether to install a structural protection system is
essentially based on risk assessment which establishes the prominence of
a building given its geographical location.
The purpose of a structural protection system is to prevent damage to
the fabric of the building, prevent risk of fire and safeguard personnel
from the dangers of inadvertent electrical shocks. From the perspective
of structural protection for the building, a lightning strike to the
surrounding area is of no consequence.
However, from the point of view of surge protection of equipment within
the building, nearby lightning strikes are as important as direct
strikes. The reason for this is that structural protection is dealing
with the primary effects of a lightning strike, whereas surge protection
is dealing with the secondary effects of lightning.
Risk Assessment within IEC62305
The application of Risk Assessment procedure is complex as
manyfactors have to be taken into account to reach a “tolerable level”.
These factors include but are not limited to isoceraunic level,
collection area of the structure and associated power lines, aerials
etc. Risk of Loss of Human Life, Risk of Loss of Service to the Public,
Risk of Loss of Cultural Heritage, Risk of Loss of Economic Value.
For a substantial building located in an exposed geographical position
the Risk Assessment and determination of a “tolerable level” can only be
achieved following site survey by a competent specialist. In the UK it
is recommended that such surveys are conducted by a member of the
Association of Technical Lightning & Access Specialists (ATLAS).
Following an analysis of all the relevant factors a recommendation for
the building Lightning Protection System (LPS) can be provided.
The provision of an LPS system for a given building needs to be
augmented with adequate surge protection for the power supplies and
electrical subsystems. This fact is recognised in IEC62305-4 where the
further need for coordinated surge protection devices (SPD) protection
is referred to. Surge Protection Devices are defined in a suite of IEC
standards IEC61643.
Considering the power supplies within a building 3 classes of SPD may be
applicable according to the specific location. These classifications
are:
Class 1. Where a high risk of a direct lightning strike exists. For
example where the LV supply is via an exposed overhead line.
Class 2. This type of SPD may be used where there is no risk of a direct
lightning strike. i.e. Where the incoming supply is via an underground
cable.
Class 3. For protection of electrical subsystems “downstream” from the
main distribution board and usually applied at the supply outlet for
electrical apparatus.
In the UK where LV supplies are via a cable from a pole mounted
11kV/415V transformer protected by surge arresters on the 11kV side then
a class 2 SPD at the distribution board would be adequate.
Reference to IEC62305 and IEC61643 will give recommendations for the
protection of communication systems within a given building. In the
application of all SPDs it is most important that the devices used are
compatible and coordinated.
Insurance Company Attitudes to Surge Protection – Commercial
Considerations
With the ever increasing amount of information technology being used
for businesses, the costs of equipment repair and replacement have
become a major concern for many insurance companies. In some cases,
finances have been provided to replace computer systems worth some
£100,000s that have been destroyed by lightning or other surge
phenomena. Downtime and other consequential losses often cost far more
to replace.
Many other cases occur in domestic and small business applications
insurance. Many companies now recognise surge protection as being a
solution to this problem. It is generally accepted that a total
protection system can be fitted for a fraction of the equipment
replacement value. The coordinated approach to the lightning protection
of buildings together with the homologation of the standards has been
welcomed by Architects, Consultants, Building & Other Contractors
specialising in the Design, Installation, Testing and Maintenance of
buildings.
International Surge Protection Standard IEC61643 (BSEN 61643)
This standard provides the user with all aspects of lightning and
surge protection devices. SPDs that comply with this standard will
provide an effective means of eliminating costly damage due to transient
overvoltage conditions. Whilst lightning is the most obvious source of
transient overvoltages, it should be borne in mind that other sources
can be equally damaging.
For example the switching of heavy inductive loads associated within an
industrial environment can result in relatively low energy transient
overvoltages. The correct selection of an SPD according to IEC61643 and
IEC62305 is of paramount importance. A high energy capability SPD will
not necessarily give the best protection level for electronic equipment
and it is therefore most important that the selection of the SPD is
compatible with the equipment to be protected and the point of
installation.
Some Suggestions for Specifications for TVSS
Three Phase Mains - All Mode Protection. Main incoming electrical
supplies, distribution, and sub-distribution boards/panels shall be
protected against transient over voltages as a result of nearby
lightning strikes or surges derived from the switching of inductive or
capacitive loads. Surge arresters shall have the capability for
automatic recovery after a surge, and have the facility for ease of
replacement should the need arise.
Protection should be in accordance with the recommendations of IEC62305
and IEC61643. The peak transient “let through” voltage shall not be
greater than the immunity level of the system to be protected. The surge
protection itself shall have a surge current handling capability
suitable for the geographical location and risk assessment, but normally
in the range of 10 - 150kA.
Dataline Protection - All network communication, CCTV/video, process
control and telemetry cables entering and leaving a building, and all
communication cables that carry signals on wire conductors shall be
protected against transient over voltages as a result of nearby
lightning strikes or surges derived from nearby power systems,
electrical machinery or electro-static discharge.
Surge arresters shall have the capability for automatic recovery after a
surge. Protection should be in accordance with the recommendations of
IEC62305 and IEC61643.
Telecom Protection - All communication cables entering a building
including dial-up PSTN (or “external”) voice lines and leased lines or
where PABX extensions are routed between buildings, ISDN lines, and all
communication cables that carry signals on wire conductors shall be
protected against transient over voltages as a result of nearby
lightning strikes or surges derived from nearby power systems,
electrical machinery or electro-static discharge.
Surge arresters shall have the capability for automatic recovery after a
surge. Protection should be in accordance with the recommendations of
IEC62305 and IEC61643.
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Tyco Electronics UK Limited
Energy Division
Freebournes Road
Witham
CM8 3AH
Tel : 0870 870 7500
Fax : 0879 240 5287
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