The magnetic field of lightning induces voltage in the loops formed by control, protection and communication cables. IEC 62305-4 reduces it in two ways: spatial shielding of the zone, with meshes narrower than 5 m, and shielding and routing of the cables, with earthed metal ducts and small-area loops. The shielding factor SF measures, in decibels, how much a mesh attenuates the field.
Short answer. When lightning strikes, the magnetic field it generates passes through the loops formed by control, protection and communication cables and the equipment they connect, and induces a voltage in them that can damage electronics. IEC 62305-4 lowers that voltage by two routes that can be combined: spatial shielding, which attenuates the field inside the zone, and shielding and routing of the lines, which reduces the field reaching each cable and the loop area.
Why routing matters
The voltage induced in a loop is proportional to how fast the magnetic field changes and to the loop area. For a rectangular loop of width b and length l in a zone with a homogeneous field, IEC 62305-4 (Annex A.5, Equation A.34) gives the maximum open-circuit voltage as:
UOC/MAX = μ0 × b × l × H1/MAX / T1
where μ0 is 4π × 10⁻⁷ Vs/(Am), H1/MAX is the maximum field in the zone and T1 is the front time of the field. Halving the width or the length of the loop halves the voltage. That is why routing counts as much as shielding.
The standard illustrates four situations in its Figure A.5: an unprotected system, with a large loop between the signal wiring and the power wiring; a spatial shield that reduces the field inside the zone; shielded lines, which reduce the influence of the field on them; and routing that reduces the loop area.
Spatial shielding of a zone
A spatial shield defines a protected zone that can cover the whole structure, a part of it, a single room or only the equipment enclosure. It can be continuous or grid-type, or formed by the natural components of the structure. The standard recommends it when it is more practical to protect a zone than several items of equipment, and warns that it must be provided for in the initial planning: later retrofitting is more expensive and difficult.
In practice, large-volume shields are formed with the reinforcing steel of roofs, walls and floors, the metal frame, and metal roofs and facades (Figure A.3). To be effective, the mesh width should typically be less than 5 m. Ideally the bars are joined by welding or clamping at every crossing; since that is not possible in large structures, a practical approach is to connect roughly every 1 m. If the zone is formed by a normal LPS of Part 3, with meshes greater than 5 m, the shielding effect can be neglected. In the following inner zones, shielding is achieved with closed metal racks or cabinets, or with the metal enclosure of the equipment.
Shielding factor SF
Table A.3 gives the attenuation of a grid against a plane wave, in decibels:
| Material | 25 kHz (first positive stroke) | 1 MHz (subsequent strokes) or 250 kHz (first negative stroke) |
|---|---|---|
| Copper or aluminium | SF = 20 × log(8.5 / wm) | SF = 20 × log(8.5 / wm) |
| Steel (relative permeability ≈ 200) | SF = 20 × log[(8.5 / wm) / √(1 + 18 × 10⁻⁶ / rc²)] | SF = 20 × log(8.5 / wm) |
Here wm is the mesh width in metres and rc the radius of the grid rods, in metres. If the formula gives a negative value, SF is zero. If there is also a meshed bonding network under clause 5.2, SF increases by 6 dB. The field inside the zone is H1 = H0 / 10^(SF/20) (Equation A.11).
An example from the standard itself: a copper grid with a mesh width of 2 m gives SF = 12.6 dB. The field is divided by about 4.3. With the meshed bonding network, SF rises to about 18.6 dB, and the reduction is about 8.5 times.
Safe volume
Close to the shield the field is high because partial lightning currents flow through it. For that reason the internal systems are installed inside a safe volume that keeps a distance ds from the shield wall (Figure A.4). For an LPZ 1 shield in the case of a direct strike, ds/1 = wm × SF / 10 if SF ≥ 10, and ds/1 = wm if SF < 10. For the case of a nearby strike, the standard gives the same relationships with ds/2. In the example of wm = 2 m and SF = 12.6 dB, the safety distance comes to about 2.5 m. Sensitive equipment should not be located within that band.
Shielding and routing of internal lines
Where there is no effective spatial shield, or in addition to one, the standard calls for:
- Running cables close to the metal parts of the bonding network, which are earthed. It is useful to route them inside metal ducts that are part of the network, such as U-section conduits or metal trays.
- Routing power and signal cables together, so that the loop between them is small. Even so, some separation between unshielded power and signal lines may be needed to avoid interference.
- Using shielded cables or closed metal ducts, whose screens are bonded to earth at both ends. Duct sections are well interconnected with one another, with screws or with straps distributed around the perimeter, to keep their impedance low.
- Taking care near the zone shield, especially in LPZ 1, where the field is higher.
Between separate structures, cables should run in metal ducts bonded at both ends to the bonding bars of each structure. If the cable screens, bonded at both ends, can carry the expected partial lightning current, no additional ducts are needed. For external lines entering the structure, shielding helps, although it is often not decided by whoever plans the SPM, because the lines belong to the network operator.
In an existing installation, Annex B.11 recommends avoiding feeding new equipment from the existing network when that creates a large loop, and keeping power and signal lines adjacent. Where there are more than 10 m between a signal line and an electronic device in general areas, the standard recommends balanced lines with galvanic isolation ports, such as optocouplers or signal isolating transformers.
- Measure the loop. Length and width of the loop between power cables, signal cables and equipment
- Bring cables close to the bonding network. Earthed metal tray or duct
- Route power and signal together. The loop area is reduced
- Shield if the field is still high. Shielded cable or closed duct, with the screen bonded at both ends
- Install equipment inside the safe volume. Away from the shield wall
Shield materials
At the boundary between LPZ 0A and LPZ 1, the shields (grids, cable screens, enclosures) meet the requirements of Part 3 of the standard: the minimum thickness of sheets, ducts and screens in Table 3 and the arrangement and cross-section of grids in Table 6. That sizing is not required if the shield is not intended to carry lightning current: at LPZ 1/2 boundaries or higher, if the separation distance of Part 3 is met, or at any boundary, if the number of dangerous events due to lightning strikes on the structure is negligible (ND less than 0.01 per year).
Where it applies in a substation or plant
The control, protection, metering and communication cables of a control building or switchgear room run through loops between panels, trays and equipment. The routing and shielding of those cables, together with the bonding network and the SPDs, are part of the secondary design. Equipment immunity is covered in electromagnetic compatibility of switchgear and EMC and shielding of drives.
Guides in the same series
The SPM are grouped in SPM against LEMP. The magnetic field and induced voltage that justify shielding are calculated in magnetic field and induced voltage in a loop; the bonding network is in equipotential bonding network and 3D earthing, and the case of buildings already constructed is in SPM in existing structures.
When to call us
The shielding factor and the induced voltage depend on the building, the mesh, the routing of each cable and the protection level. If you are designing a control room or reviewing one with communication failures or erratic measurements during storms, our engineering design and studies team can support you on the secondary design and the earthing. Write to us through the contact page.
Related guides
- IEC 62305: lightning protection, what each part covers
- Equipotential bonding network and 3D earthing system
- Magnetic field and lightning-induced voltage in a loop
- Coordinated SPDs: classes I, II and III and Iimp current
- Electromagnetic compatibility of switchgear
References
- IEC 62305-4:2010, Protection against lightning, Part 4: Electrical and electronic systems within structures: clause 6 (magnetic shielding and line routing), Annex A.3 (Figures A.3 to A.5, Table A.3), Annex A.4 (Equations A.5, A.11 and A.15) and Annex A.5 (Equation A.34), Annex B.11.
- IEC 62305-3:2010, Tables 3 and 6.
Frequently asked questions
What is the shielding factor SF of a metal mesh?
It is the attenuation of the magnetic field produced by a grid-type shield, in decibels. In IEC 62305-4, Table A.3, for copper or aluminium it equals 20 × log(8.5 / wm), where wm is the mesh width in metres. The field inside the zone is the outside field divided by 10 raised to SF/20. If the result is negative, SF is taken as zero.
When is a grid shield effective against the lightning field?
When the mesh width is typically less than 5 m. The standard states that the shielding effect can be neglected if the zone is formed by a normal LPS of Part 3, with meshes and distances typically greater than 5 m. A large building with a steel frame and many columns does provide significant shielding.
How is the induced voltage in a control cable reduced?
By reducing the area of the loop it forms, or by attenuating the field that passes through it. Cables are run close to earthed metal parts, power and signal cables are routed together, shielded cables with the screen earthed at both ends or closed metal ducts are used, and sensitive equipment is installed inside a safe volume away from the shield wall.