FEATURE SURGE PROTECTION DON’T TAKE THE HIT
Simon Bircham, low voltage product manager at Eaton’s Bussmann business, explains the role that surge protection devices (SPDs) play in protecting sensitive electrical and electronic equipment from damage caused by overvoltages
E
lectrical transients (overvoltages, electrical power surges or spikes) are
fast, short duration events that cause energy peaks in electrical circuits. These can damage or destroy sensitive electrical and electronic equipment. High transient overvoltages are
typically caused by the energy released during lightning strikes. These transient overvoltage events can be extremely damaging to electrical and electronic equipment. Other overvoltage events can be caused by switching inductive, capacitive and resistive loads or interrupting short circuit faults resulting in high overvoltage transients. High power machines, power outages
and high energy power switching in other equipment can all cause overvoltage transients, as can storm damaged powerlines and, as mentioned, electromagnetic pulses from lightning. Around 80% of transients are generated
from internal sources - at least 20% come from external sources, including lightning strikes and power utility switching. The 20% of transients from external sources are considered the most damaging to any system and require careful consideration in any electrical system. The 80% from internal sources are often overlooked resulting in unnecessary circuit damage and unwanted power failures - all of which are easily avoidable with the use of SPDs.
PROTECTING AGAINST RISK Overvoltage events frequently cause electrical and electronic equipment disruption and damage, leading to expensive down-time. Flashovers from lightning can potentially start fires or cause dangerous electric shock risk. What can be done to protect against such risks? The answer is to correctly select and install surge protection devices (SPDs). These devices must deal with the
electrical energy involved in surges expected at their installation point. They regulate the voltage supplied to equipment by momentarily ‘switching’ from an open circuit mode into a low impedance mode and shunting the surge energy to earth and, in doing so, limiting the overvoltage to a safe level. When the surge event is over, the device returns to
26 MARCH 2014 | ELECTRICAL ENGINEERING
its open circuit mode, ready for the next event or, if it has reached the end of its life, it remains in open circuit mode safely until it is replaced. A visual indicator or remote signalling unit will clearly alert the operator when the SPD has reached the end of life. Coordinating the types of SPDs used is
crucial. Typically this involves the following device classifications: Type 1 - installed in the main electrical
switchboard when the building is equipped with a lightning protection system. These can discharge a very high lightning current that might be fed into the power distribution system. Type 2 - fitted in the main
distribution switchboard, these discharge indirect lightning strike current, which can cause induced or conducted overvoltages on the power distribution network. Type 3 - these are
installed close to equipment as a supplement to Type 2 devices to reduce sensitive equipment from transient overvoltages. Their current discharge capacity is very limited, so they should not be used alone. The need for such protection should be
obvious, yet while SPDs work very effectively, they are often ignored.
GETTING THE RIGHT INFORMATION A good reference and source of information for contractors is Section 534 of BS7671:2008 (17th Edition) First Amendment that includes a section on
Below: configurations of SPDs can be customised according to application
Above: surge protection is vital for productivity
surge protection - ‘Devices for protection against overvoltage - Selection of Surge Protection Devices’. This affects all people involved in the design, installation and verification of such installations. The section gives guidance on how to choose SPDs, fit them and coordinate them for building installations. Not included, however, are SPDs already incorporated into equipment connected to the installation concerned. Details are provided on protection against over current and the consequences of SPD failure, SPD installation in conjunction with RCDs and the connecting conductors, fault protection and insulation resistance measurement. It also covers transient overvoltages caused by direct/indirect lightning strikes near buildings having a lightning protection system, and it deals with the installation of SPDs where required by Section 443 of BS 7671:2008 (protection against overload current), or where specified by the designer. Section 534.2.3 gives guidance on SPD
selection regarding voltage protection level, with continuous operating voltage, temporary overvoltages, nominal current discharge, prospective fault current, and for the connection of more than one SPD in series. The length of conductor is important - for example, if the length between SPD and the equipment being protected is greater than 10m, oscillations could result in the voltage at the equipment terminals being twice that of the SPD’s voltage protection level. A risk assessment will determine whether surge protection is essential. Often this may be less to do with how
often overvoltages may occur than with how much damage could result. Considering the value of equipment and business data, SPDs provide a very cost effective insurance against loss of valuable equipment, data and disruption to business.
Eaton’s Bussmann Business
www.cooperbussmann.com T: 01509 882 600
Enter 216 / ELECTRICALENGINEERING
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