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37 Railway Road Blackburn South, VIC 3130 Ph: (03) 9894-2677 Fax: (03) 9894-3216 Email: [email protected] Web: www.erico.com ABN 70 078495646 A Practical Guide to Selecting Surge Protection Devices By: Rohit Narayan, BE Electrical, MIEAust, CP Eng., MBA Tech Mgmt ERICO Australia www.erico.com The use of surge protection devices, or SPD’s, in low vo ltage (240/415V) power systems, is a relatively new concept, compared to the use of other inherent components, like circuit breakers, fuses, links and isolators. The selection of appropriate surge protection devices is somewhat of a mystery in the industry, for this and other reasons. In Europe and the USA there are comprehensive standards, written on the testing, selection and application of SPD’s. Even then, there is no clear protocol or common understanding of what is a good surge protection scheme. There are various performance measurements, that are written down in the specification sheets or nameplates of SPD’s. This article attempts to discuss and explain the 4 key performance yardsticks that one can use, in making the appropriate selection of SPD’s. These yardsticks are well defined and discussed by IEC61643 standards and they are: • • • • Maximum Discharge Current, or Imax Nominal Discharge Current or In Voltage Protection Level, or Up Maximum Continuous Operating Voltage, or Uc as defined by IEC standards Let us now discuss what the key performance yardsticks tell us about the performance of the SPD in more detail. 1) Maximum Discharge Current, or Imax The Imax gives an indication of the amount of surge energy the SPD will be able to handle without getting damaged. The Imax , is the maximum single shot current, the SPD can handle. ABN 70 078495646 37 Railway Road Blackburn South, VIC 3130 Ph: (03) 9894-2677 Fax: (03) 9894-3216 Email: [email protected] Web: www.erico.com The current and the voltage of the wave shapes at which this single shot is applied, are defined by IEC standards. The graph in Figure 1 is taken out of AS1768 2003 Interim Standard and provides a depiction of what these wave shapes look like. Figure 1 Standard Voltage and Current Wave Shapes 2) Nominal Discharge Current or In In is an indication of how long the device will last in the power system. It is worth highlighting a common misunderstanding, that SPD’s are only good for one shot of a lightning surge. This is not true and there are many devices available that may withstand thousands if not tens of thousands of surges. The IEC standards require SPD’s tested for common power system applications, to withstand 15 impulses at In followed by 10%, 25%, 50%, 75% and 100% of Imax . In is a multiple shot rating of a SPD. Obviously a SPD, with a higher In rating, will withstand more surges and last longer. 37 Railway Road Blackburn South, VIC 3130 Ph: (03) 9894-2677 Fax: (03) 9894-3216 Email: [email protected] Web: www.erico.com ABN 70 078495646 3) Voltage Protection Level, or Up Typically, SPD’s are connected between the phase and the earth or the neutral in the case of MEN systems. The way a SPD works, is that remains open circuit at nominal voltages, but if the voltage exceeds its clamp voltage, the SPD will temporarily short out to ground. This enables the excess energy to be diverted to earth. This is perhaps the reason that SPD’s are also commonly referred as surge diverters. Figure 2 shows typical wiring of SPD’s. Supply Transformer Surge Diverter 63 Amps 16 mm2 SPD Neutral CB or Fuse Earth Various Loads Figure 2 – Typical Wiring of SPD’s 37 Railway Road Blackburn South, VIC 3130 Ph: (03) 9894-2677 Fax: (03) 9894-3216 Email: [email protected] Web: www.erico.com ABN 70 078495646 The Up characterises the performance of a SPD in limiting the voltage. The Up indicates how well the SPD, clamps an applied surge and hence a SPD with a lower Up, is a better device in terms of limiting the voltage across an equipment. The graph in Figure 3 below shows what the Up is with respect to the applied standard voltage wave shape and the clamp voltage. Peak Voltage is approximately 6000V Voltage Applied Voltage Voltage Protection Level, Up Clamp V Voltage After Clamping by SPD Time in micro seconds Figure 3 – Voltage Protection Level, Up 4) Maximum Continuous Operating Voltage, or Uc SPD’s are voltage-limiting devices and it is important to select a SPD that will not attempt to clamp slight over voltages at 50 Hz. Uc is a guide to how rugged the SPD is against over voltages. If the SPD attempts to clamps the voltage continuously, then this can either result in damage to the SPD or even a fire hazard if the SPD get hot. 37 Railway Road Blackburn South, VIC 3130 Ph: (03) 9894-2677 Fax: (03) 9894-3216 Email: [email protected] Web: www.erico.com ABN 70 078495646 Example: Comparing Performance Yardsticks of Two Products. This example demonstrates, that two SPD’s which have identical Imax ratings may have starkly different In, Up and Uc. The CRITEC TDS MT 277 is the better choice in this example. CRITEC TDS MT 277 CRITEC DSD1100 Imax = 100 kA In = 80 kA Up= 750 V at 3 kA(8/20µs) Up=1000V at 20 kA(8/20µs) Uc= 480V Imax = 100 kA In = 50 kA Up= 850 V at 3 kA(8/20µs) Up=1700V at 50 kA(8/20µs) Uc= 275V Figure 4 : Comparing Two SPD’s It is worth pointing out that there are several schemes that can be used for choosing installation locations of SPD’s. The choice of the scheme depends on cost, the sensitivity of the equipment being protected, the frequency of the occurrence of surges, the importance of the systems or the processes being protected. For example, a simple scheme would have a SPD with low Up and a high Imax and In at the Main Switchboard an no subsequent downstream protection. In a larger installation, there may be a need to install a SPD at the main switch board, as coarse primary protection and SPD’s on distribution boards as finer secondary protection. ABN 70 078495646 37 Railway Road Blackburn South, VIC 3130 Ph: (03) 9894-2677 Fax: (03) 9894-3216 Email: [email protected] Web: www.erico.com In critical applications, surge reduction filters or SRF ’s are used as finer protection. The yardsticks described above, that is, Imax, In, Uc and Up can still be used to define performance of surge reduction filters, SRF’s. But other than lowering the voltage to which the equipment is exposed, the SRF’s also reduces the voltage rise time or dv/dt. It is widely recognised that electronic equipment is at danger of being damaged, both, from large amplitudes and high rise time associated with power surges. While surge diverters or SPD’s take care of the amplitude factor only, the SRF’s take care of the amplitude and the dv/dt factors both. Figure 5 below, explains the performance of CRITEC TSG SRF. Figure 5 : Operation of CRITEC TSG SRF The available literature in standards and elsewhere on SPD’s is extensive. Hopefully this paper provides a simple and practical understanding of some key performance yardsticks that can be used to select SPD’s and SRF’s.