Lightning and Surge Protection of Intrinsically Safe Systems

During the processing or transport of flammable substances, mixtures are often created that can form an explosive atmosphere. The construction and operation of plants in which such substances may occur require a high level of safety measures. This is also required by Directive 1999/92/EC [1] and is addressed to the operator or employer. According to this directive, the operator is obliged to assess the explosion risk of their plant and must ensure that all minimum requirements are met. This also includes classifying hazardous areas into zones, marking these areas, and documenting all protective measures in an explosion protection document.

In Germany, the European Directive 1999/92/EC [1] is implemented into national law through the Industrial Safety Ordinance (BetrSichV) [2] and the Hazardous Substances Ordinance (GefStoffV) [3]. The Technical Rules for Operational Safety (TRBS) and for Hazardous Substances (TRGS) specify the respective ordinances. They provide operators with guidance on risk assessment and recommend protective measures for the respective ignition sources.

The following article considers the ignition source lightning according to TRGS 723 [4].

An electrical circuit is considered intrinsically safe if current and voltage are limited to such an extent that a spark or thermal effects cannot ignite an explosive atmosphere. Unlike other types of protection (e.g. increased safety), this applies not only to individual equipment but to the entire circuit. This applies both to normal operation and to foreseeable fault conditions (e.g. lightning strike). Therefore, the entire intrinsically safe circuit must be protected against energy intrusion from external sources. The installer or operator is responsible for proving intrinsic safety and correct system installation.

The standards DIN EN 60079 Part 11 [5], DIN EN IEC 60079 Part 14 [6] and Part 25 [7] regulate both product requirements and specific installation requirements. For example, intrinsically safe systems require cables and wiring to be protected against mechanical damage, corrosion, chemical and thermal influences.

In addition to mechanical requirements, the ingress of external energy into the intrinsically safe measurement circuit must be prevented. Energy coupling can occur galvanically, inductively or capacitively.

Direct Lightning Strike (S1)

• galvanic coupling
• inductive / capacitive coupling

Indirect Lightning Strike (S2)

• conductive lightning partial currents
• inductive / capacitive coupling

The purpose of a risk assessment is to identify ignition sources and evaluate their effectiveness in hazardous areas. A possible ignition source is a direct lightning strike or the inductive and capacitive effects of this high impulse current on the intrinsically safe circuit.

TRGS 723 [4] and DIN EN IEC 62305-3, Annex 7 [9], Section 5.8 describe the ignition source “lightning strike”. The following aspects are considered:

• A lightning strike is an atmospheric discharge between cloud and earth, generating extremely high temperatures at the strike point. High currents lead to heating and potential shifts along the discharge path. Lightning currents also have electromagnetic effects.
• A lightning strike can ignite an explosive atmosphere not only through a direct hit but also through effects at a distance.
• If the lightning channel passes through an explosive atmosphere (e.g. a gas cloud), immediate ignition occurs. High currents may also cause heating of plant components or spark discharges.

The lightning channel penetrates Zone 1 and can cause immediate ignition if an explosive atmosphere is present.

• Potential differences to adjacent equipment may cause spark discharges or flashovers.
• In metallic structures near the discharge path, currents and potential shifts occur due to capacitive and inductive coupling.

If the separation distance is not maintained, uncontrolled flashovers may occur from the down conductor to internal installations. Lightning current discharge generates strong electromagnetic interference (surges) in metallic installations.

• The effects of distant lightning strikes in supply lines (cables and pipelines) must also be considered if they affect hazardous areas.

Damage sources S3 and S4 according to DIN EN IEC 62305-2 [8] describe direct and indirect lightning effects on all incoming metallic lines, which may introduce high ignition energy into hazardous areas.

• The risk assessment must evaluate lightning-related ignition hazards in hazardous areas as a whole.

Thus, hazards arise not only from direct strikes but also from electromagnetic effects and induced voltages. If an explosive atmosphere is present simultaneously (e.g. at terminal connections of intrinsically safe equipment), the released ignition energy may cause fire or explosion.

A comprehensive and coordinated protection concept is therefore essential. The lightning protection zone (LPZ) concept described in DIN EN IEC 62305-4 [8] forms the basis for lightning protection systems in Ex areas. Cables and wiring are particularly affected by inductive effects.

Intrinsically safe circuits are common in Ex areas. Although DIN EN IEC 60079-14 [6] specifies lightning protection requirements, DIN EN IEC 62305-4 [8] represents the state of the art for managing surges in explosive atmospheres.

It must be assessed whether lightning-related hazards can occur in intrinsically safe systems according to TRGS 723 [4]. This is a complex task and must be evaluated by experts in lightning protection or EMC specialists with knowledge of Ex systems.

Additional support is provided by the risk analysis according to DIN EN IEC 62305-2 [8] and Annex NA of DIN EN IEC 62305-1 [8].

If hazards are identified, all equipment, protective systems and components must be protected by suitable lightning and surge protection measures. It is particularly important that lightning strikes outside Ex zones 0/1 or 20/21 must not affect these areas.

DIN EN IEC 60079-14 [6] refers directly to DIN EN IEC 62305 Parts 1–4 [8] for lightning hazards.

The overall design concept includes:

• Determination of strike points using the rolling sphere method (R = 30 m according to TRGS 723 [4])
• External lightning protection system (LPL II) with air terminals, down conductors, earthing, equipotential bonding and separation distance
• Documentation of lightning protection zones and Ex zones
• Evaluation of electromagnetic coupling into loop circuits
• Implementation of LEMP protection measures (LPMS), including earthing, shielding, routing and coordinated SPD protection

SPDs must meet manufacturer installation requirements as well as EC type examination certificate conditions. Intrinsically safe circuits have special requirements.

According to DIN EN IEC 60079-25 [7], circuits must be either isolated from earth or connected at a single point to the equipotential bonding system. A circuit is considered floating if it withstands at least 500 V insulation test to earth.

Only SPDs with 500 V earth insulation capability should be used. If SPDs are installed in Zone 0 (e.g. tank sensors), they must also be certified accordingly:

II (1)2 G Ex ia [ia Ga] IIC T4 ... T6 Gb

All interconnection conditions according to DIN EN IEC 60079-14 [6] must be observed.

SPDs installed on tank lines must meet:

• 10 impulses of 10 kA (8/20 µs) without damage

In practice, SPDs should also be tested with:

• 10/350 µs waveform (direct lightning current)
• 8/20 µs waveform (induced surge currents)

Additional requirements:

• Installation in metallic enclosure with earthing ≥ 4 mm² Cu
• Cable length ≤ 1 m or metallic conduit with double-sided earthing

Lightning-related hazards in chemical and petrochemical plants are often not assessed holistically, although required by TRGS 723 since July 2019. The lightning protection zone concept significantly reduces risks when applied during design and implementation.

SPDs must meet explosion protection, coordination, and process requirements. Additional technical solutions are described in DIN EN IEC 62305-3 Annex 7 [9].