10_lightning_and_surge_protection_for_intrinsically_safe_circuits_ex-magazine_2011_low-10.pdf

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Installation requirements with regard to at-

mospheric discharges (lightning strikes)

The applicable IEC 60079-14 [7] standard re-

fers to the lightning protection measuresspecified in the standard series IEC 62305Part 1 4 [3, 4, 5, 6].The overall lightning protection concept in-cludes:> A risk analysis to determine the necessity

of lightning protection and to select therequired (technical and economic) light-ning protection measures

> Measures to protect structures from ma-terial damage and damage to personscaused by direct lightning strikes by in-

stalling a lightning protection system(LPS) consisting of air-termination sys-tems, down conductors, earth-terminationsystem, lightning equipotential bondingand separation distance.

> Protection measures against the effectsof lightning electromagnetic impulses(LEMP) by installing a LEMP protectionsystem (LPMS) for structures with electri-cal and electronic systems. An individualcombination of protection measures are

Page 44 |Ex-Magazine 2011

Lightning and surge protection

for intrinsically safe circuitsby Manfred Kienlein

Lightning currents and overvoltage in

potentially explosive atmospheres

When assessing the risk for potentially

explosive atmospheres, the following light-ning-related ignition sources in accordancewith EN 1127-1 must be observed:> Melting at the point of strike,> Heating of discharge paths,> Uncontrolled flashover in case the separation distance is not maintained,> nduced voltages in cables,> Lightning strikes into metallic cables en-

tering hazardous areas

In case of lightning-related risks (risk analy-

sis in accordance with IEC 62305-2 [3]) allequipment, protective systems and compo-nents of all categories must be protected byadequate lightning and surge protectionmeasures. It is vital that these are not dam-aged inside of zone 0 or 20 by lightningstrikes outside of these zones.

Legislation, Standards and Technology

Special explosion protection measures must be taken in all industrial sectors where gas,

vapour, fog or dust occurs during the processing or transport of flammable substances

which, in combination with a mixture of air, may present a hazardous explosive atmosphere.

In 2003, ATEX 137 (Directive 1999/92/EC) [1] was converted into several national laws (e.g. the

Betriebssicherheitsverordnung - German Operational Safety Regulations; VEXAT Austrian

Operational Safety Regulations). In accordance with these regulations, the user is obliged to

prepare an explosion protection document. This document is based on a risk assessmentthat assesses the potential hazards posed by the presence and expansion of potentially ex-

plosive atmospheres according to Ex-zones. Then possible ignition sources resulting from

operating requirements are identified and corresponding equipment is selected. This paper

deals with lightning-related ignition sources in compliance with EN 1127-1 [2].


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earthing and equipotential bonding, spa-tial shielding, cable routing and screening,coordinated protection by surge protect-ing devices (SPD).

The IEC 60079-14 [7] standard basically re-

quires that the effects of lightning strikes arereduced to a safe level. In view of the factthat the ATEX Directive 137 calls for systemsto be installed, mounted and operated ac-cording to the state of the art, the new light-ning protection standards must be applied forexplosion protection. Not only the effects of a direct lightningstrike, but also the electromagnetic effect ofthe lightning current on the installation of theelectrical system present a risk in potentiallyexplosive atmospheres. The steepness of thelightning current causes a high change ve-

locity of the magnetic field, which in turn in-duces a high voltage (ignitable energy) invarious cables (extraneous conductive parts)that form a loop. If an explosive atmosphereis present at the same time (for example onthe terminals in the enclosure of an intrinsi-

cally safe equipment) this ignition energymay at any time cause fire or explosion.For this reason, it is vital to implement a con-sistent and harmonized protection concept.This protection concept, which is also re-ferred to as lightning protection zones con-

cept, is described in the standard IEC 62305-4and forms the basis for the implementation ofa LEMP lightning protection system. Intrinsi-cally safe circuits are particularly at risk dueto the induction effect of the lightning cur-rent. Subclause 12.3 of the IEC 60079-14standard includes requirements for protect-ing intrinsically safe circuits against light-ning, however, IEC 62305-4 is state of the artfor coping with overvoltages in a system sub-ject to explosion hazards. To ensure safe operation of systems inhazardous areas it is decisive to harmonise

the requirements of EN 1127, IEC 60079-14and IEC 62305-4. This will be shown by thefollowing example. It is assumed that a light-ning protection system (LPS) with class ofLPS II based on the relevant lightning protec-tion level LPL II that was determined in a risk

assessment has been installed to protect theintrinsically safe system and the hazardousarea (zone 0, 1) from direct lightning strikes.The intrinsically safe measuring circuit is in-stalled in LPZ 0B (LPZ: lightning protectionzone; see Table 1).

The example shows a possible method toprotect intrinsically safe measuring circuitsfrom the direct and indirect effects of a light-ning strike.

Figure 1 shows a typical installation of anintrinsically safe measuring circuit consistingof a combination of an isolating barrier, an in-trinsically safe measuring circuit and a tem-perature transmitter (galvanically isolatedfrom the sensor).

The isolating barrier is located in themeasuring and control cabinet in the controlroom building (safe area). The temperature

transmitter with the sensor is directly mount-ed to the tank containing flammable liquid.The sensor is located in Ex-zone 0, the trans-mitter itself is mounted in Ex-zone 1 and itsmetal enclosure is directly and safely con-nected to the metal tank on a permanent

LPZ OA Zone where the threat is due to the direct lightning flash and the full lightningelectromagnetic field. The internal systems may be subjected to full or parti-cal lighting surge current.

LPZ OB Zone protected against direct lightning flashes but where the threat is thefull lightning electromagnetic field. The internal systems may be subjected toto partial lightning surge currents.

LPZ 1 Zone where the surge current is limited by current sharing and and by SPDsat the boundary. Spatial shielding my attenuate the lightning electromagneticfield..

LPZ 2, ..., n Zone where the surge current may be further limited by current sharing andby additional SPDs at the boundary. Additional spatial shielding may be usedto further attenuate the lightning electromagnetic field.

Table 1: Definition of lightning protection zones (LPZ) in accordance with IEC 62305-1 [3]

Ex-Magazine 2011 |Page 45


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basis. The screened intrinsically safe cable(approximately 200 m long) connects thesetwo pieces of equipment. The control roomand the tank are incorporated in an in-termeshed earth-termination system (meshsize of approximately 20 x 20 m).

The following lightning hazards may de-stroy or interfere with the measuring circuit(Figure 1) and present a risk of explosion forthe system:

> Direct lightning strike into the air-termina-tion system of the measuring and controlcicuit

> Lightning strike near the measuring andcontrol circuit

> Direct lightning strike into the air-termina-tion system of the measuring and controlbuilding

> Lightning strike near the measuring andcontrol building

> Direct lightning strike into the tank> Lightning strike near the tank

To provide protection against all lightning-re-lated probabilities of damage to electricalequipment (in the control room and in thehazardous area), two surge protective devic-es (SPDs) must be integrated in the intrinsi-cally safe circuit, that is one SPD to protectthe isolating barrier in the control room and

one to protect the transmitter on the tank.The SPD on the tank equally prevents dan-gerous spark-over from the tank to the sen-sor line and additionally provide protectionagainst explosion.

Selection criteria for surge protective devic-

es installed in an intrinsically safe measur-

ing circuit

Certain selection criteria must be fulfilledto ensure the protective effect of the select-

ed SPD. Particularly intrinsically safe meas-uring circuits have their special features.These must be observed when selecting theprotective devices since they may negativelyaffect the intrinsically safe explosion protec-tion system.

Isolation from earth and dielectric strength

of the intrinsically safe equipment

In accordance with the IEC 60079-25 [8]standard intrinsically safe circuits may beisolated from earth or connected to theequipotential bonding system at one point

only. An intrinsically safe circuit is isolated

from earth, if it withstands a dielectric test inaccordance with IEC 60079-11 [10] with atleast 500 V against earth. If this is not thecase, it is to be assumed that the circuit isconnected to earth.Since multiple earthing of the measuring cir-

cuit is not allowed in all cases, a documentedtest on the effects of multiple earthing mustbe carried out for the measuring circuit withSPDs or SPDs (intrinsically safe equipment)that are approved for this special purposeand fulfil the requirements of isolation fromearth are to be used. These SPDs do nothave to be disconnected from the intrinsical-ly safe circuit during the dielectric test. Themanufacturer of the intrinsically safe SPDsmust prove that they are isolated from earth.These SPDs are only capable of reliably pro-tecting intrinsically safe equipment with a di-

electric strength > 500 V against earth. If

Figure 1: Application example of an intrinsically safe circuit

Lightning and surge protection for intrinsically safe circuits



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isolating barriers with a dielectric strength< 500 V (for example Zener barriers) areused, other SPDs must be selected andadapted to the special requirements of thebarrier.

Equipment category and type of protectionThe entire intrinsically safe circuit has atype of protection ia. In our example bothSPDs must have this type of protection (seeEC-type examination certificate). Since a sensor circuit entering zone 0 isconnected to a SPD on the tank (Figure 1),the SPD must be additionally approved forthis type of application. According to the EC-type examination certificate the SPD of typeDPI MD EX 24 M 2 must have at least the fol-lowing approval: II 2 (1) G Ex ia IIC T4 ...T6(Table 2).

Maximum permissible values for L0and C0 Before an intrinsically safe measuring cir-cuit is put into operation, its intrinsic safetyshall be verified. The isolating barrier, trans-ducer, cables and SPDs must fulfil the inter-connection requirements. If required, the in-ductances and capacitances of the SPDsmust also be considered.

According to the EC- type examinationcertificate, the internal capacitances and in-ductances of the BXT ML4 BD EX 24 (Figure

2) and DPI MD EX 24 M 2 surge protectivedevices from DEHN + SHNE (Figure 3) arenegligible and do not have to be consideredfor the connection requirements.

Maximum values for voltage Uiand Strom Ii According to its technical data for use inexplosive atmospheres, the intrinsically safecircuit to be protected has a maximum supplyvoltage Ui max (29.4 V d.c.) and a maximumshort-circuit current Ii max (130 mA). The rat-ed voltage Ucof the SPD must be higher

Figure 2: SPDwithpermanent monitoring(DRC MCM) in anintrinsically safemeasuring circuit

Symbol Description

II Apparatus group: for use in any area other than mining

2 (1) Equipment category: Installation in Ex zone 1, the device to be protectedmay be installed in Ex zone 0

G For use in explosive gas atmospheres

Ex Electrical equipment built in compliance with the European standard

ia Type of protection intrinsic safety: No ignition even if two faults arepresent

IIC Subgroup: Also for use with extremely flammable gases such as hydro-gen and acetylene

T4...T6 T4: Ambient temperature range -40 C to +80 C

T5: Ambient temperature range -40 C to + 70CT6: Ambient temperature range -40 C to +55 C

Table 2: Symbols used for intrinsically safe SPD Type DPI MD EX 24 M 2



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than the maximum open-circuit voltage of thepower supply system. The nominal current ofthe SPD must be at least as high as the maxi-mum current Iimax of the isolating barrier tobe expected in the event of a fault. The certif-icate becomes invalid, if these interconnec-tion conditions are not observed when di-mensioning the SPD.

Coordination SPD with SPD and SPD with

terminal equipment Another important criterion is the coordi-nation of the relevant SPDs among one an-other and of the SPD with terminal equip-ment. If the coordination requirements in ac-

Figure 3: DEHNpipe for protecting transmitters and

lines from zone 0

cordance with IEC 62305-4 and IEC 61643-21[9] are not fulfilled, the devices may be dam-aged even if SPDs are installed, putting theinstallation into a critical state. The safest so-lution is to use devices from a single manu-facturer. For this application, not only the co-

ordination requirement for the inducedovervoltages (8/20 s impulse), but also a co-ordination test for the lightning impulse (10 /350 s impulse) are particularly important.The SPD installed on the tank is located inLPZ 0B and must therefore be capable of car-rying partial lightning currents (Table 1). If themanufacturer is not able to provide a coordi-nation test for both SPDs installed, all cablesand pieces of equipment must be routed andinstalled in LPZ 1 in accordance with thelightning protection zones concept. This mayresult in considerable additional installation

work. The following additional requirements inaccordance with IEC 60079-14 must be ful-filled and proven in particular if a SPD is in-stalled on the tank (lines from zone 0):> Use of SPDs with a minimum discharge

capacity of 10 impulses with 10 kA eachwithout failure or interfering with the pro-tective effect.

> Installation of the SPDs in a shielded me-tallic enclosure and earthing via a copperconductor of at least 4 mm.

> Installation of the cables between theSPD and the equipment in a metal tubeearthed on both ends, or use of screenedcables lines with a maximum length of1 m.

In the application example described above(Figure 3) all these requirements are alreadyfulfilled by using a surge arrester for field de-vices of type DPI MD EX 24 M 2. This elimi-nates significant follow-up costs for the in-stallation.

Lightning and surge protection for intrinsically safe circuits

Abbreviations

SPD

surge protective device

LPZ

lightning protection zone

LPS

lightning protection system

LEMP

lightning electromagnetic pulse

LPMS

lightning protection measures system

LPL

Lightning protection level



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References

[1] DIRECTIVE 1999/92/EC of the European Parliament and of the Council of 16 December 1999 on minimum

requirements for improving the safety and health protection of workers potentially at risk from

explosive atmospheres (15th individual Directive within the meaning of Article 16(1) of

Directive 89/391/EEC)

[2] EN 1127-1:2008-02 Explosive atmospheres

Explosion prevention and protection - Part 1: Basic concepts and methodology

[3] IEC 62305-1; 2006; Protection against lightning Part 1: General principles[4] IEC 62305-2; 2006; Protection against lightning Part 2: Risk management

[5] IEC 62305:3; 2006; Protection against lightning

Part 3: Physical damage to structures and life hazard

[6] IEC 62305:4; 2006; Protection against lightning

Part 4: Electrical and electronic systems within structures

[7] IEC 60079 14; Explosive atmospheres Part 14: Electrical installations design, selection and erection

[8] IEC 60079 25; Explosive atmospheres Part 25: Intrinsically safe electrical systems

[9] IEC 61643-21; 2000; Surge protective devices connected to telecommunications and signalling

networks Performance requirements and testing methods

[10] IEC 60079-11:2008:04; Explosive atmospheres Part 11: Equipment protection by intrinsic safety i


Summary

The relevant standards describe the dan-ger to chemical and petrochemical systemsposed by a lightning discharge and the re-sulting electromagnetic interference. During

the implementation of the lightning protectionzones concept the risks of sparking causedby a direct lightning strike or discharge ofconducted interference energies can be re-duced to an acceptable level even at theplanning and design stage of these systems.The SPDs must both fulfil explosion protec-tion requirements, coordination criteria andthe requirements resulting from the operatingparameters of the measuring and control cir-cuits.

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