Feature Surge Protection
Hager’s Chris Howells takes a look at what factors to consider when conducting a risk assessment for surge protection - now a requirement of Amendment 1 of the 17th Edition
Risk reduction A
s the modern world becomes more reliant on electronic equipment, so the consequences of the failure of this equipment has become increasingly severe. Not only are there more systems, but also the size of the circuitry is smaller so smaller power surges will damage it. Transient overvoltages occur in elec- trical installations due to either light- ning or from switching.
Lightning
Lightning strikes can cause overvolt- ages via a direct strike on an installa- tion or its low voltage lines, or by a strike in the vicinity of low voltage overhead lines or to the ground close to underground cables.
If the installation has a lightning rod or is adjacent to tall structures such as tall trees or near a hilltop in lightning prone areas, then you should install surge pro- tection using an SPD Type 1 device. It is important to also take into account the probability of a direct or nearby lightning strike occurring. The AQ criteria method can help in deciding whether protection is required against a direct or indirect lightning strike. This takes into account the probable number of light- ning strikes per year.
For the UK, the map in figure 1 shows the probable number of thun- derstorms per year in a given loca- tion. If the risk assessment deems that the probability of a direct light- ning strike is low, then protection against transient overvoltages is not needed if the equipment has with- stand values according to table 44 in BS 7671 (see table below). However, where higher levels of equipment reliability or higher risks (e.g.
Impulse withstand category I (low impulse voltage)
II (normal impulse voltage) III (high impulse voltage)
fire) are expected, then additional pro- tection against overvoltage by SPDs may be needed, irrespective of the AQ value. As an alternative, specifiers can use the risk assessment method described in Regulation 44342.4. This method is based on a technique detailed in the IEC Technical Report 61661 - Assessment of risk damage due to lightning. It considers the con- sequences of damage to electronic equipment brought about by over- voltages. For certain circumstances, it includes a calculation to indicate if SPDs are required.
Switching transients Transient overvoltages due to switch- ing are generated in inductances, such as those of electric motors and dis- charge lighting ballasts, when there is an abrupt change in current caused by opening or closing contacts. Overvoltages due to switching can occur hundreds of times a day, but are generally smaller. However, switching overvoltages can affect modern elec- tronic equipment. While the magni- tude of a switching overvoltage may not cause the immediate failure of electronic equipment, its frequency can lead to its degradation so that fail- ure will eventually occur in a switch or circuit breaker.
Evidence suggests that overvoltages such as these have affected LED light- ing circuits, underfloor heating, PCs, building management systems, alarms and CCTV systems. The results of one study found that 28% of the failures of electronic equipment were due to tran- sient overvoltages.
Another factor to consider is that such overvoltages can trip RCDs. Protection against lower transients
Example of equipment in category1 Sensitive electronic equipment connected to the fixed installation
Domestic appliances and portable power tools connected to the fixed installation
Equipment intended to be installed in a part of
the fixed installation where a high degree of availability of overvoltages is expected, such as distribution boards, circuit breakers and wiring systems
IV (very high impulse voltage) Equipment intended to be installed at or near
the intake to the installation, such as the energy meter
Notes: 1. Table 44.4 of BS 7671 gives a fuller list of equipment falling into each category 2. This table applies only for installations of rated voltage (Uo) 230V. For installations of other rated voltages, see Table 44.3 of BS 7671
12 6.0kV
Required minimum impulse withstand voltage2 1.5kV
2.5kW 4.0kV
Above: a map showing the
probable number of thunderstorms per year in a
given location
will typically require a class II SPD device with additional class III protec- tion for sensitive equipment.
Consider cascading
Cascading is the term used to describe the method of combining several levels of SPDs into one installation. While it may be decided that a high current carrying capacity device is needed to divert the bulk of a transient overvoltage, the residual voltage of these individual devices are still two high for sensitive equipment. The next step is to limit this voltage to 800V. This can be achieved with Class II devices which have a lower current carrying capacity, but in combination or cascade will limit the voltage. Cascading increases the current diverting capacity of a SPD system while maintaining a low voltage to protect valuable equipment.
Summary
This electronic age in which we live is reliant on the information and data con- tained on PCs, servers and other sys- tems. At the very least an installer and specifier should talk to the customer about the sensitivity and value of the equipment that requires protection. It’s worth noting that it is not just the physical value of the equipment but the value of the data included on the equip- ment, so it makes sense in most cases to make a relatively small investment in surge protection, from the point of view of business continuity or just the incon- venience of losing equipment.
Hager
www.hager.co.uk T: 01952 677 899
Enter 202 SEPTEMBER 2012 Electrical Engineering
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