Lightning protection in explosive atmospheres: IEC 62305-3

Short answer

A lightning discharge can ignite an explosive atmosphere through the arc at the point of strike or through sparks at joints, pipes and terminals. For these structures Annex D of IEC 62305-3 calls for a preferred type B earthing arrangement, common equipotential bonding, SPDs outside the hazardous zone where possible, requirements by zone (0, 1, 2, 20, 21 and 22) and more frequent inspection than for an ordinary building.

Short answer. In an explosive atmosphere, lightning does more than cause damage: it can ignite the mixture through the arc at the point of strike, through a spark at a loose joint or through a gap between a pipe and its support. For that reason Annex D of IEC 62305-3 adds requirements to those in the rest of the standard: common equipotential bonding, a type B earthing arrangement where possible, SPDs outside the hazardous zone where possible, measures by zone and more frequent inspection. The standard itself notes that its specific requirements for these structures are still under consideration, and that Annex D is offered as interim information.

What the standard says about its scope

Clause 1 of IEC 62305-3:2010 states, in its Note 1, that specific requirements for structures that are dangerous to their surroundings because of an explosion hazard are under consideration, and that Annex D is given for use in the meantime. Annex D is normative, and its own note makes clear that it is based on arrangements proven in practice and that the authority having jurisdiction may give other requirements. Apply the annex, and check whether the authority at the site asks for anything additional.

Minimum protection level

Note 2 to clause 5.4 of IEC 62305-2:2010 says that if the authority having jurisdiction requires lightning protection for a structure with an explosion hazard, at least a class II lightning protection system (LPS) must be adopted. Class I is permitted in every case, especially if the environment or contents are exceptionally sensitive. The authority may accept class III if lightning is infrequent or the contents are of low sensitivity.

Basic requirements of Annex D

No melting or ejection of material away from the point of strike. The LPS is designed so that, in the event of a direct strike, effects occur only at that point, where sparking or impact damage may arise. If down-conductors cannot leave the hazardous area, they are installed without exceeding the autoignition temperature of the substance present.
Drawings with the areas marked. The designer must receive the plant drawings showing areas with solid explosives and the hazardous areas, classified under IEC 60079-10-1 (gases) and IEC 60079-10-2 (dusts).
Earthing. The type B arrangement (ring) is preferred. The metal body of a tank can serve as the ring. For structures with solid explosives or explosive mixtures, earth resistance should be as low as possible and no greater than 10 Ω.
Common equipotential bonding between the LPS and the installations in the hazardous area.

Hazardous zones and what each one requires

IEC 62305-3 uses the zones for gases and vapors (0, 1 and 2) and for combustible dusts (20, 21 and 22). Zone 0 or 20 is where the explosive atmosphere is present continuously or for long periods. Zone 1 or 21 is where it is likely to appear in normal operation. Zone 2 or 22 is where it is not likely in normal operation and, if it appears, lasts only briefly.

Zone	What Annex D adds
2 and 22	May need no additional measures. External metal installations (columns, reactors, vessels) with the minimum thickness in Table 3 need no air-termination devices or down-conductors and are earthed in accordance with clause 5.
1 and 21	The above, plus: if there are insulating pieces in the pipework, the operator defines the protection, for example isolating spark gaps suitable for an explosive atmosphere, and both these and the insulating pieces are installed outside the hazardous areas.
0 and 20	The requirements of zones 1 and 21, plus: electrical equipment inside tanks holding flammable liquids must be suitable for that use, and closed metal vessels with zone 0 or 20 inside must have the Table 3 thickness at the possible points of strike, on the condition that heating of the inner face is not dangerous. With thinner walls, air-termination devices are installed.

In general, the external parts of the LPS (air-termination devices and down-conductors) are kept at least 1 m from the hazardous area where possible. If they must pass through it, the conductors should preferably be continuous or have joints made in accordance with clause 5.5.3, and connections must be prevented from working loose.

Pipework, fueling stations and tanks

Pipework. Metal pipework above ground, outside the process units, is earthed at least every 30 m. Connections must avoid sparking as lightning current passes: welded lugs, bolts or threaded connections at flanges are acceptable. Clamps are accepted only if a test shows there is no ignition risk. Spark gaps in an equipotential bond require the agreement of the system operator.
Fueling stations. Metal pipework is earthed and connected to steel structures and rails if present, sometimes with isolating spark gaps suitable for the area, because of traction currents, stray currents and cathodic protection.
Storage tanks. A continuous metal container with a thickness of at least 5 mm of steel or 7 mm of aluminum, without spark gaps, protects itself and needs no additional measures, provided that heating of its inner face at the point of strike is not dangerous. Tanks covered with soil and their pipework need no air-termination devices. In a battery of tanks it is enough to earth each tank at one point, and the tanks are connected to each other. An isolated tank is earthed once if its largest horizontal dimension is up to 20 m, and twice if it exceeds that.
Floating-roof tanks. The floating roof is bonded to the shell with flexible bonding conductors, arranged so that they do not form a closed loop, and with multiple connections roughly every 1.5 m around the perimeter of the roof.

Measures for a metal tank or vessel

Is it a continuous metal container, without spark gaps, of steel 5 mm or thicker (aluminum 7 mm)?
- Yes: Is heating of its inner face at the point of strike dangerous?
  - Yes: Additional measures are required: assess air-termination devices and the LPS design
  - No: It protects itself: earth it in accordance with clause 5
- No: Is it covered with soil, as is its pipework?
  - Yes: No air-termination devices needed: instruments and electrical equipment must be suitable for the area
  - No: Design the tank LPS with air-termination devices, down-conductors and earthing per Annex D

Surge protection

For structures with solid explosives, Annex D requires SPDs as part of the LPS wherever explosive material is present, and preferably outside those locations. If an SPD sits inside a location with exposed explosives or explosive dust, it must be explosion-proof. In areas hazardous because of gases or dusts, SPDs are placed outside the hazardous zone where possible, and any that remain inside must be approved for the zone. SPD selection is covered in transient overvoltages in the plant.

Inspection and maintenance

Annex D.6 calls for an inspection and maintenance plan from the end of installation, personnel familiar with the requirements for hazardous areas, and periodic inspection or continuous supervision by qualified personnel. It also calls for:

An electrical test of the LPS every 12 months (with a tolerance of 2 months), using instruments that comply with IEC 61557-4.
A DC resistance of no more than 0.2 Ω for any object bonded to the LPS.
Measurement of earth resistance, using the three-point method where possible.
Inspection of the SPDs at intervals of no more than 12 months or when the electrical tests are done, and after any suspected strike.
Immediate documentation of any damage attributable to lightning, and retention of the histories for trend analysis.

Table E.2 of Annex E.7 sets, for these structures, a visual inspection every six months and an electrical test every year. In an ordinary level I or II building, that standard allows visual inspections to be spaced up to one year apart.

What not to do

Do not use a clamp on a hazardous-zone pipe without a test that rules out ignition.
Do not install spark gaps or insulating pieces inside the hazardous zone.
Do not assume a thin tank is protected just because it is metal: protection depends on thickness and on heating of the inner face.
Do not create a bond with other structures without agreement from the system operator.

Guides in the same series

The basic air-termination, down-conductor and earthing system is the one in LPS classes I to IV. Inspections and tests are explained together with those of the other systems in LPS inspection and maintenance, and the tanks and pipework of a complete plant are covered in industrial lightning protection.

When to call us

Lightning protection of a hazardous area is decided together with the area classification, the earthing and the surge protection of the same project. If you are reviewing or extending a plant with tanks, pipework or classified areas, our engineering design and studies team can support you on the electrical side: earthing, equipotential bonding, protection and control. Write to us through the contact page.

References

IEC 62305-3:2010, Protection against lightning, Part 3: Physical damage to structures and life hazard: clause 1 (Note 1), Table 3, Annex D (structures with an explosion hazard) and Annex E.7 (Table E.2).
IEC 62305-2:2010, Part 2: Risk management: clause 5.4, Note 2.
IEC 60079-10-1 and IEC 60079-10-2: classification of areas with gases and with combustible dusts, cited by Annex D.
IEC 61557-4: equipment for measuring the resistance of earth connections and equipotential bonding.

Frequently asked questions

What class of lightning protection is used for structures with an explosion hazard?

IEC 62305-2 states that where the authority having jurisdiction requires lightning protection for a structure with an explosion hazard, at least a class II LPS should be adopted. Class I is acceptable in every case, and the authority may accept class III where lightning activity is infrequent or the contents are not sensitive.

Does a metal tank need air-termination devices of its own?

Not always. IEC 62305-3 considers that a continuous metal container with a thickness of at least 5 mm of steel or 7 mm of aluminum, without spark gaps, protects itself, provided that heating of its inner face at the point of strike is not dangerous. Buried tanks do not need air-termination devices either, but their instruments and electrical equipment must be suitable for the area.

How often is lightning protection inspected in a plant with a hazardous area?

Annex E.7 of IEC 62305-3 calls for a visual inspection every six months and an electrical test once a year. Annex D adds an inspection and maintenance plan, qualified personnel, and inspection of the SPDs at intervals of no more than twelve months, as well as after any suspected strike.