Coordinated SPDs: class I, II, III and Iimp current

Short answer. A coordinated system of SPDs (surge protective devices) places SPDs at the entrance of each lightning protection zone (LPZ), with energy coordinated between stages. IEC 62305-4 (clause 7 and Annexes C and D) sets where each test class goes: class I, with impulse current Iimp, at the structure entrance where direct lightning is possible; class II and class III near the equipment. It works if the effective protection level stays below the voltage the equipment can withstand.

What a coordinated system is

IEC 62305-1 (definition 3.54) calls a coordinated SPD system a set of SPDs properly selected, coordinated and installed to form a system that reduces failures of electrical and electronic systems. Clause 7 of Part 4 establishes that:

• With more than one inner zone (LPZ 1, LPZ 2 and subsequent), an SPD is installed at the line entrance of each zone.
• With only LPZ 1, at least one SPD is installed at the line entrance to LPZ 1.
• In both cases additional SPDs may be needed if the distance between the SPD and the equipment is large.
• Both power and signal lines are protected.

SPD testing is governed by IEC 61643-1 (power) and IEC 61643-21 (telecommunications and signalling).

The three test classes and where they go

The short-circuit current of a class III SPD follows from UOC, because the test uses a combination wave generator with a 2 Ω impedance. An SPD with class I and II characteristics can be used at the sub-distribution board.

The condition for neglecting SPD failure due to strikes to the structure (ND) or to the line (NL) is ND + NL ≤ 0.01. In general, the standard recommends class I where direct or partial currents are involved (strikes to the structure or to the lines) and class II or III for induced effects (nearby strikes). Many buildings do not need protection against direct strikes, and for them a well-designed class II system may be enough.

The Iimp current

Table D.1 gives the preferred values of Iimp for SPDs connected line to neutral, with the corresponding charge Q and specific energy W/R:

Iimp is associated with longer waveforms (such as 10/350 µs) than those of Imax. To choose it, the current at the point of installation must be estimated, which depends on the protection level and on how the current divides. Without a specific current-sharing calculation, the standard assumes that 50 % of the current goes to the building’s earthing system and the other 50 % returns through the equipotential bonding SPDs. With level I (200 kA) and a three-phase system with neutral, that gives 25 kA per SPD. If instead three metallic services enter and the Part 1 model is applied, the current in each bonding SPD of the three-phase system drops to 8.3 kA. The earthing arrangement also affects the sharing.

Effective protection level

Clause C.2.1 compares the protection level UP of the SPD, which is the residual voltage at its nominal current In, with the impulse withstand voltage UW of the equipment. But the voltage at the equipment is the effective protection level UP/F:

• For voltage-limiting SPDs: UP/F = UP + ΔU.
• For voltage-switching SPDs: UP/F = the larger of UP and ΔU.

ΔU is the inductive drop in the connecting conductors. At the structure entrance, 1 kV per metre of conductor is assumed. If the conductors are 0.5 m or shorter, UP/F = 1.2 × UP may be assumed. The shorter they are, the better.

The internal systems are protected if they are energy-coordinated with the upstream SPDs and one of three conditions is met:

UI is the overvoltage induced in the loop between the SPD and the equipment. If there is spatial or line shielding, it is usually negligible. If a failure could cost lives or leave the public without service, the standard calls for considering twice the voltage due to oscillation and requiring UP/F ≤ UW / 2. A long circuit can double the voltage (up to 2 × UP/F) if the equipment is left open-circuit.

Installation procedure

Annex C.3.5 summarises the procedure:

1. Install SPD 1 at the structure entrance, with the current requirements of C.2.2.
2. Determine the withstand voltage UW of the internal systems and the protection level UP1 of SPD 1, and verify C.2.1.
3. If it is not met, install SPD 2 closer to the equipment (sub-distribution board), energy-coordinated with SPD 1, and verify again.
4. If it is still not met, install SPD 3 next to the equipment, coordinated with the previous ones, and verify that UP/F3 ≤ UW.

Energy coordination between cascaded SPDs follows IEC 61643-12 or IEC 61643-22, and the manufacturer must supply the information needed to achieve it. SPDs must be installed with the shortest possible connecting conductors, with the cross-sections of Table 1 of Part 4 (16 mm² copper for class I, 6 mm² for class II, 1 mm² for class III). The closer to the entrance an SPD is, the more equipment it protects; the closer to the equipment, the more effective it is.

Spark gap and varistor

Part 1 (Annex D.6) treats SPDs with spark gaps and SPDs with metal-oxide varistors as different cases. In the spark gap, wear comes from erosion of the electrodes by the arc and from mechanical shock. In the varistor, damage comes from overload by absorbed energy and from flashover, and the main parameter is the charge. For the surge arresters of MV, HV and EHV switchyards, see surge arresters, and for photovoltaic systems, SPDs in photovoltaic systems.

Isolating interfaces

Clause 8 and definition 3.56 call isolating interfaces the devices capable of reducing conducted surges on lines entering the zone. They include isolation transformers with an earthed screen between windings, metal-free fibre-optic cables and optoisolators (optocouplers). Annex B adds double-insulated class II equipment without a PE conductor. Their withstand level must be coordinated with the overvoltage categories of IEC 60664-1, and if necessary they are protected with SPDs. A fibre cable without a metal armour, metal moisture barrier or steel wire between structures needs no protection measures.

Guides in the same series

The boundaries where the SPDs are installed are the lightning protection zones (LPZ); the set of measures is in SPM against LEMP; the bonding of the SPDs to the network is in equipotential bonding network and 3D earthing system. In an industrial plant, see also industrial lightning protection.

When to call us

The selection of classes, currents and protection levels depends on the lightning protection level, the earthing, the cables and the withstand voltage of each item of equipment. If you are designing or reviewing the surge protection of a plant, a control room or a substation, our engineering design and studies team can support you on the electrical side of the project. Write to us through the contact page.

Related guides

• IEC 62305: lightning protection, what each part covers
• SPM against LEMP: what they are and how they are managed
• Lightning protection zones: LPZ 0A, 0B, 1 and 2
• Equipotential bonding network and 3D earthing system
• Transient overvoltages in plants: causes and protection

References

• IEC 62305-4:2010, Protection against lightning, Part 4: Electrical and electronic systems within structures: clauses 7 and 8, Annex B.10 and B.13, Annex C (selection and installation of coordinated SPDs) and Annex D (Table D.1).
• IEC 62305-1:2010, definitions 3.54 to 3.56, Annex D.6 and Annex E.5.
• IEC 61643-1, IEC 61643-12, IEC 61643-21, IEC 61643-22, IEC 60364-5-53 and IEC 60664-1.

Frequently asked questions