Substation lightning protection: what IEC 62305 covers

Short answer

IEC 62305 protects structures: in a substation, the buildings that house equipment, such as the control building, the GIS hall or the switchgear room. For them it calls for a system of air terminals, down-conductors and earthing, protection zones in the rooms, SPDs at the service entrance, shielded and well-routed control cables, and an earth mesh under the structure. The high-voltage yard, with overhead shield wires and masts, is designed with IEEE 998.

Short answer. IEC 62305 is the lightning protection standard for structures. In a substation, that means the buildings that house equipment: the control building with its protection and control panels, the hall of a GIS, or the switchgear room. For them, the standard calls for a lightning protection system (air terminals, down-conductors and earth electrodes), protection zones inside the rooms, SPDs at the service entrance, shielded and well-routed control cables, and an earth mesh of about 5 m under the structure. The high-voltage equipment yard is shielded with overhead shield wires and masts, an IEEE 998 topic covered in substation shielding.

Which part of the substation falls within the standard

Part of the substation	IEC 62305?	Why
Control building, relay room, communications room	Yes	A structure with internal electrical and electronic systems
GIS substation hall, MV switchgear room	Yes	A structure with electrical equipment and its contents
Power and telecommunication lines outside the building	No	Part 1 excludes them from its scope, although their effects on the building are assessed
Outdoor HV equipment yard	Not covered in the text	Parts 3 and 4 do not develop yard shielding; shield wires, masts and IEEE 998 are used

Clause 1 of IEC 62305-1:2010 protects structures “including their installations and contents, and persons”, and excludes from its scope, among others, underground high-pressure pipelines and power lines, pipelines and telecommunication lines outside the structure. Table 1 of that part recalls the effect that matters in the case of a substation: in telecommunication service structures and power plants, loss of service to the public is unacceptable. A structure that is protected can be part of a larger one, so a building inside the substation can be treated separately.

Lightning protection system of the building

For the building, the lightning protection system (LPS) of IEC 62305-3 applies: air terminals on the roof, down-conductors and earth electrodes. The class of the system (I to IV) comes from the risk assessment of IEC 62305-2 and sets the air-termination methods. Table 2 of Part 3 gives, by class, the maximum rolling sphere radius and the mesh size:

Class	Rolling sphere	Mesh
I	20 m	5 × 5 m
II	30 m	10 × 10 m
III	45 m	15 × 15 m
IV	60 m	20 × 20 m

This air termination protects the building against physical damage and persons against electric shock, but it does not protect the electronics inside. Note 3 of clause 4.2 of Part 4 says that an LPS with only equipotential bonding SPDs does not provide effective protection to sensitive systems. That is what the LEMP protection measures (SPM) are for.

Protection zones inside the room

Part 4 divides the interior into LPZ zones. In a control room, one possible division is:

Roof and exterior of the building: LPZ 0A or 0B, depending on whether the air terminal protects against direct strikes.
Interior of the building: LPZ 1, with the current limited by SPDs and bonds at the service entrances. If there is a spatial shield, it also attenuates the field.
Panels, racks or rooms with the most sensitive equipment: LPZ 2, with additional SPDs at their boundary or shielding of the room or enclosure.

All metallic services crossing the boundary of each zone (pipes, power and signal cables) are bonded to bonding bars, directly or through SPDs. The standard gives the example of a large office building, with a reinforcing-steel and metal-facade shield for LPZ 1 and shielded cabinets for the sensitive systems in LPZ 2.

Service entrance and transformer

A substation receives medium- or high-voltage supply and distributes it at low voltage for its own services. IEC 62305-4 (Figure 4) treats the transformer as follows:

Transformer outside the structure (in LPZ 0): only the low-voltage lines entering the building need SPDs.
Transformer inside the building, without SPDs on the high-voltage side (because the building owner often cannot take measures on that side): LPZ 0 is extended inward, and SPDs are installed only at low voltage.

Figure A.6 of the standard shows such a case: LPZ 0B is extended to house a 20 kV supply because it was not possible to install SPDs on the high-voltage side at the entrance. For the low-voltage SPD, see transient overvoltages in plants. For the surge arresters of lines and power transformers, see surge arresters.

Control and protection cables

Between the yard and the control building run control, measurement and protection cables that form loops in which lightning induces voltage. Part 4 (clause 6) calls for shielding and good routing of those cables:

Metal trays and ducts bonded to the earthing network, with screens connected at both ends.
Power and signal cables together, to reduce the loop area.
Sensitive equipment kept away from the shield wall, where the field is higher.

Shielding of the external lines entering the structure helps, but it is often not defined by whoever plans the SPM, because the lines belong to the network operator.

Earth mesh

Clause 5.2 of Part 4 calls for the earth ring around the structure, or the ring in the concrete of the foundation perimeter, to be integrated with a mesh under and around the building, with a typical width of 5 m. In structures with electronic systems, the type B electrode (ring) is recommended. With several buildings, such as a substation with a control building, GIS hall and services building, Part 3 (Annex E.5.4.3.6) indicates interconnecting the earthing of the buildings and of the equipment to obtain a meshed earthing system. This lowers the potential difference between them and reduces interference on the cables linking them. That lightning earth is joined to the substation’s fault earth in the design of the whole: see earth grid and step and touch voltage.

Which standard governs each part of the substation

Is the part inside a building (control building, GIS hall, switchgear room)?
- Yes: Does it protect electrical or electronic equipment?
  - Yes: IEC 62305: building LPS (Part 3) and SPM (Part 4)
  - No: IEC 62305-3 for the LPS and the protection of persons
- No: Is it the outdoor high-voltage equipment yard?
  - Yes: Shield wires, masts and IEEE 998 methods
  - No: Lines outside the structure: outside the scope of IEC 62305

You can estimate the resistance of an electrode or a grid with our ground resistance tool.

Inspection and maintenance

The LPS and the SPM of the building are inspected under clause 7 of Part 3 and clause 9.3 of Part 4: during construction, on completion, periodically and after alterations, repairs or a lightning strike. Note b of Table E.2 includes among the “critical situations” structures with sensitive internal systems, for which it sets a complete inspection every year. A control building, with relays and communication systems, fits that description.

Guides in the same series

The building LPS is described in LPS classes I to IV and the earthing in LPS earthing. The interior measures are in SPM against LEMP, equipotential bonding network and 3D earthing system, magnetic shielding and cable routing and coordinated SPDs.

When to call us

Lightning protection of a substation combines the building LPS, the earthing of the whole site, protection and control, and the cables. If you are designing a substation or extending a control building, our engineering design and studies team can support you on the electrical side, and our testing and commissioning services cover earthing measurements, in Colombia or elsewhere on request. Write to us through the contact page.

References

IEC 62305-1:2010, Protection against lightning, Part 1: General principles: clause 1 (scope), definition of structure to be protected, and Table 1.
IEC 62305-3:2010, Part 3: Physical damage to structures and life hazard: clauses 5 and 7, Table 2 and Annex E (Table E.2, E.5.4.3.6).
IEC 62305-4:2010, Part 4: Electrical and electronic systems within structures: clauses 4.3, 5.2 and 6, Figure 4 and Annex A.3.
IEEE 998: guide for direct lightning stroke shielding of substations.

Frequently asked questions

Is IEC 62305 used to shield a substation yard?

That is not its purpose. IEC 62305-1 protects structures with their installations and contents, and leaves out power and telecommunication lines located outside the structure. For the high-voltage equipment yard, with shield wires and masts, other methods are used, such as IEEE 998. IEC 62305 applies to the substation buildings.

What rolling sphere radius is used for a substation building?

It depends on the class of the lightning protection system: 20 m for class I, 30 m for II, 45 m for III and 60 m for IV, per Table 2 of IEC 62305-3. The class comes from the risk assessment of IEC 62305-2.

Where do the SPDs go when the transformer is inside the building?

If the transformer is outside the structure, only the incoming low-voltage lines need SPDs. If it is inside and there are no SPDs on the high-voltage side, because the building owner cannot act there, IEC 62305-4 (Figure 4) extends zone LPZ 0 inward and the SPDs are installed only on the low-voltage side.