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Guide toSurge Protection Devices

IntroductionThe whole nature of how electrical equipment is used in homes and at work has evolved; witheveryday activities relying on electronic equipment. Products such as computers, printers,flat screen televisions, industrial control equipment such as PLC’s, alarms, microwaves andwashing machines are common place. These can all be vulnerable to transient overvoltages,which can significantly reduce the equipment’s lifespan through degradation and damage.This guide expands upon some of the requirements found in the 17th Edition of the IET Wiring Regulations and otherstandards, related to the protection of electrical equipment from electrical surges. It considers protection against voltagetransients on the electrical installation only. Consideration should also be given to the protection against transientovervoltages transmitted by data transmission systems. BS EN 50174 refers.Note: this guide does not ensure compliance with BS 7671 or indeed guarantee that equipment is protected against electricalsurges. The electrical specifier should use their own judgment, consulting BS 7671 and the BS EN 62305 series (protectionagainst lightning) to determine the need and correct selection of surge protection devices.

ContentsRisks associated with electrical surgesPage 4Terminology and selection criteriaPage 8Selection of suitable devicesPage 9ConnectionPage 12Cascading devicesPage 14Inspection & TestingPage 16SPD Quick selection guidePage 17

Risks of electrical surgesSurge protective devices (SPD) assist in the protectionof valuable electrical and electronic equipment againsttransients, originating from lightning and also from switchingsources.These transients can cause damage ranging from thepremature ageing of equipment, logic failures and down time,to the complete destruction of equipment within the entireelectrical installation. Products such as LCD screens, dataservers and industrial equipment such PLC’s are critical tobusiness activity. Protecting this equipment may now be anecessity.The choice of a surge protective device depends upon: The exposure of the building to lightning transients The sensitivity and value of the equipment that requiresprotection (it is recommended that the contractor shoulddiscuss the installations requirements with the customer) The location and therefore the exposure level of theinstallation The equipment used within the installation and whether thisequipment could generate switching transientsThe Hager SPD range of solutions may offer protection toprevent damage to this sensitive equipment by diverting thedamaging transient over-voltages. In the majority of cases thiswill eliminate equipment failures and reduce downtime.4Guide to Surge Protection Devices

BS 7671 and the AQ criteriamethodLightning discharges could contain currents of 200,000Awhich if struck at or near power transmission lines wouldgenerate a significant voltage transient. This voltage transientcould cause significant damage to both domestic andcommercial electronic equipment.Average number of days ofthunder during the wholeyear (1971 to 2000)Average value (days) 1412 to 14The UK regional map illustrates the likely lightning activitycaused by the number of thunderstorm days across thecountry.10 to 128 to 106 to 84 to 6 4Protection against over-voltages is the subject of section 443of BS 7671. Here the AQ criteria method is introduced whichis based on the likelihood of the equipment being subjectedto over-voltages caused by lightning strikes, taking account ofthe probable number of lightning strikes per year.For electrical installations in the UK, the map shows that theprobable number of thunderstorm days per year in any givenlocation is less than 25, and therefore condition AQ1 applies.Map courtesy of the Met OfficeGuide to Surge Protection Devices5

Where this is the case and for installations being supplied byoverhead lines, Regulation 443.2.2 indicates that provided theimpulse withstand voltage of the equipment is not less thanthe values given in Table 44.3 (see Table 1 for installationsrated at 230 V to Earth), no additional protection by a SPD isrequired. However, where higher levels of equipment reliabilityor higher risks (e.g. fire) are expected, additional protection byan SPD against over-voltage may be required.Similarly, for an installation having no overhead lines, noadditional protection against overvoltages is required if theequipment meets the minimum voltage withstand values intable 44.3.There are some words of caution in the notes to this sectionwhere it is recognised that transient over-voltages transmittedby the supply distribution system are not significantlyattenuated. So an induced voltage some distance awaycould easily manifest itself at the electrical installation andcause potential harm to the equipment within. It is also worthconsidering that the AQ data is for thunderstorm days NOTlightning strikes. One storm will usually contain many lightningflashes which could lead to an over-voltage on the installationcausing damage to equipment.6“where higher levels of equipmentreliability or higher risks (e.g.fire) are expected, additionalprotection by an SPD againstover-voltage may be required.”Guide to Surge Protection Devices

Impulse withstandcategoryExample of equipment in category (note 1)I(low impulse voltage)Sensitive electronic equipment connected to the fixed installation.1.5 kVDomestic appliances and portable power tools connected to the fixed installation.2.5 kVIII(high impulse voltage)Equipment intended to be installed in a part of the fixed installation where a highdegree of availability of overvoltages is expected, such as distribution boards,circuit-breakers and wiring systems.4.0 kVIV(very high impulsevoltage)Equipment intended to be installed at or near the intake to the installation, suchas the energy meter.6.0 kVII(normal impulsevoltage)Required minimum impulsewithstand voltage (note 2)Table 1: Required minimum impulse withstand voltage for equipment where installation rated voltage is 230V to Earth (based on tables 44.344.4 of BS7671)Notes:1. Table 44.4 of BS 7671 gives a fuller list of examples 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.Guide to Surge Protection Devices7

Terminology & selection criteriaSurge protection devices are classified according to theirstandard into different typesIimp – Impulse current of 10/350 μs waveform associated withType 1 spd’s Type 1 - SPD which can discharge partial lightning currentwith a typical waveform 10/350 μs. Usually employs sparkgap technology.In – Surge current of 8/20 μs waveform associated with Type2 spd’s Type 2 - SPD which can prevent the spread of overvoltages in the electrical installations and protects equipmentconnected to it. It usually employs metal oxide varistor (MOV)technology and is characterized by an 8/20 μs current wave.Up - The residual voltage that is measured across the terminalof the SPD when In is appliedUc - The maximum voltage which may be continuouslyapplied to the SPD without it conducting Type 3 – These SPDs have a low discharge capacity. Theymust therefore only be installed as a supplement to Type 2SPD and in the vicinity of sensitive loads. Type 3 SPD’s arecharacterised by a combination of voltage waves (1.2/50 μs)and current waves (8/20 μs).8Guide to Surge Protection Devices

Selection of suitable devicesBS 7671 section 534 gives the requirements for correctselection of devices against overvoltages.Regulation 534.2.1 prescribes that where required by Section443 or otherwise specified, SPD’s shall be installed:(i)(ii)near the origin of an installation, orin the main distribution assembly nearest the originof an installationThe notes to this regulation give further guidance, stating thata Type 1 or a Type 2 SPD may be used at the origin whilstType 2 and Type 3 are also suited for locations close to theprotected equipment.In summary, a Type 1 SPD is used at the origin of theinstallation, a Type 2 SPD is used at distribution boards and aType 3 SPD is used near terminal equipment.Surge protection needs to be selected such that their voltageprotection level (Up) is lower than the impulse withstandcapability of the equipment to be protected. 534.2.3.1.1suggests that this value should be referred to category II ofTable 44.3. This for a 230/400V installation suggests thatthe value should not exceed 2.5kV. However 534.2.3.1.2suggests that to protect sensitive and critical equipment, thenconsideration should be given to reduce this value to thatrequired for category 1 equipment (ie 1.5kV).Type 1 SPD’s are often referred to as equipotential bondingSPD’s and are fitted at the origin. A lightning protectionsystem employing these devices only, offer no effectiveprotection against failure of sensitive electrical and electronicsystems. In order to achieve this, additional coordinateddevices will have to be employed.Guide to Surge Protection Devices9

534.2.3.4 also gives guidance as to the selection of anappropriate device.The specifier should ascertain from BS 7671 whichconnection type is preferable (CT 1 or CT 2). Hagermanufacture devices with connection type CT2.Type 1 devices need to be selected such that the valueof Iimp is not less than that which shall be calculated inaccordance with BS EN 62305-4. However if this cannot becalculated then this value shall be not less than 12.5kA. Also,due to the connection method, the value of Iimp between theneutral conductor and the protective conductor shall be notless than 50kA for three phase systems and 25kA for singlephase where the value cannot be calculated.“The specifier should ascertainfrom BS 7671 which connectiontype is preferable. Hagermanufacture devices withconnection type CT2.”For Type 2 devices the value of In shall be not less than 5kAand the value between the neutral and protective conductorshall be not less than 20kA for three phase systems and 10kAfor single phase. Larger values may be required as classifiedin BS EN 61643-11.10Guide to Surge Protection Devices

Guide to Surge Protection Devices11

ConnectionTo gain maximum protection from the SPD, the connectingconductors should be kept as short as possible. This isto minimise any additive voltage drops on the connectingcables.OCPDaThe connecting conductors of the SPD shall have a crosssectional area of not less than 4mm2 copper and the totalconnecting lead length (a b) should preferably not exceed0.5m but shall in no case exceed 1m.SensitiveEquipmentSPDbWhere Type 1 devices have been installed the cross sectionarea shall be not less than 16mm2 copper. Manufacturer’sinstructions shall be followed.Some devices have dual line & neutral terminals. Thisconnection method reduces any additional voltage drop inthe connecting cables thereby obtaining the best possibleUp to the installation. There are conditions to this connectionmethod however. With Hager devices it is suitable forinstallations having a maximum demand up to 125A.12Main earthing terminal orconnecting conductor barOCPDSPD overcurrent protection device surge protection deviceGuide to Surge Protection Devices