In an existing building it is not always possible to apply every LEMP protection measure (SPM) of IEC 62305-4. Annex B sets out the work in order: collect data with checklists, assess the risk, define LPZ zones, design a bonding network (or at least a ring at each floor), add coordinated SPDs, isolating interfaces, better cable routing and protection of external equipment. Old TN-C networks and large loops are the typical problems.
Short answer. In an existing building, measures against the lightning electromagnetic impulse (LEMP protection measures, SPM) cannot be applied as they would be in a new one: the structure, installations and equipment are already given. Annex B of IEC 62305-4 proposes an order of work: collect data with checklists, assess the risk, define protection zones (LPZ), improve the equipotential bonding and earthing network, add coordinated SPDs, use isolating interfaces, correct the cable routing and protect the external equipment.
Checklists
Clause B.2 calls for collecting data with the lists in Tables B.1 to B.4 and, with them, carrying out the IEC 62305-2 risk assessment to decide whether protection is needed and which is the most economical. The data are also used in the design. The questions cover four groups:
| Table | What is surveyed |
|---|---|
| B.1 Structure and environment | Material (masonry, brick, timber, reinforced concrete, steel, metal facade); single structure or blocks joined by expansion joints; dimensions; electrical continuity of the reinforcing steel; type of metal roof; bonded facades and window frames; window size; whether there is an external LPS and its quality; soil type; height, distance and earthing of neighbouring structures |
| B.2 Installations | Incoming services (underground or overhead); antennas and external devices; type of supply (high or low voltage, overhead or underground); line routing (risers, ducts); use of metal ducts; whether the equipment is self-contained within the structure; metal conductors to other structures |
| B.3 Equipment | Type of interconnection of internal systems (shielded or unshielded cables, coaxial, analogue or digital, balanced or unbalanced, optical fibre); whether the withstand level of the electronic system was specified |
| B.4 Other | Earthing arrangement of the supply (TN-S, TN-C, TN-C-S, TT or IT); location of the equipment; interconnection of the functional earth conductors with the bonding network |
Design sequence
Figure B.1 organises the design into steps. If the risk assessment indicates that SPM are needed:
- Collect data and assess risk. Checklists B.1 to B.4 and analysis per IEC 62305-2
- Define the LPZ zones. From an equipment enclosure up to the whole building
- Design the basic bonding network. 5 m mesh or, at least, a ring at each floor
- Design the basic measures per zone. LPZ 1, LPZ 2 and LPZ 3
- Design the coordinated SPD system. At the boundary of each zone
- Additional measures. Isolating interfaces, cable routing and shielding
- External equipment and links between structures. Antennas, sensors, cables between buildings
If new systems must be installed in a structure that already has measures, the process is repeated for the location of those systems.
How to create the LPZ zones
Depending on the number, type and sensitivity of the systems, zones can range from a small enclosure (an equipment cabinet) to the whole volume of the building. Figure B.2 shows four frequent solutions in existing structures:
- Unshielded LPZ 1, with the LPS and SPDs at the line entrance. Protects against conducted surges, but the interior magnetic field is hardly attenuated, because the air terminals and down-conductors have meshes greater than 5 m.
- Unshielded LPZ 1, with protection only for the new systems. Shielded signal lines and coordinated SPDs on the power lines, and possibly double-insulated class II equipment.
- A large LPZ 2 with a grid shield inside LPZ 1, to house the new systems.
- Two small local LPZ 2 inside LPZ 1, with additional SPDs in each.
The basis of protection is an internal shielding and bonding network with meshes no larger than 5 m in any direction. If the structure does not allow it, at least one ring conductor is installed inside the outer wall at each floor, connected to each down-conductor of the external LPS. For LPZ 1, the ring is connected to the one in the outer wall at no fewer than two points as far apart as possible. An LPZ 2 or LPZ 3 larger than 5 m × 5 m is subdivided into meshes no larger than 5 m × 5 m.
To improve the shielding of an existing LPS (B.6), metal facades and roofs are integrated into the external system, continuous reinforcement from the roof to the earthing system is used, the spacing of down-conductors and the air-termination mesh are reduced to typically less than 5 m, and flexible bonding conductors are installed at expansion joints between reinforcement blocks.
Equipotential bonding
Existing 50/60 Hz earthing systems may not provide a satisfactory equipotential plane for lightning currents of up to several MHz, because of their impedance (B.8). Even an LPS designed to Part 3 may be insufficient for sensitive systems. A low-impedance network with a typical mesh of 5 m or less is strongly recommended. The network is not used as a power or signal return: the PE conductor is integrated into the network, but the PEN is not. A functional earth conductor (a “clean earth”) may be connected directly to the network, but never to the PEN or to metal parts connected to it.
SPDs and isolating interfaces
SPDs are installed at the entrance of each inner zone and, if necessary, at the equipment. With uncoordinated SPDs, a downstream SPD or one inside the equipment can prevent the one at the service entrance from operating properly, and damage results. To maintain effectiveness, the location of all installed SPDs is documented (B.9). SPD selection by zone is also covered in transient overvoltages in plants.
To avoid interference currents at 50/60 Hz through large loops or through the lack of a low-impedance bonding network, particularly in TN-C installations, Annex B.10 proposes separating the old from the new with isolating interfaces: double-insulated class II equipment without a PE conductor, isolation transformers, metal-free optical fibre and optocouplers.
Cable routing
Routing and shielding are especially important when the shielding of LPZ 1 is negligible. The principles of Annex B.11 are to minimise the induction loop area, to avoid feeding new equipment from the existing network because it forms a large loop, to run power and signal lines adjacent to each other, to use shielded cables with the screen bonded at both ends, and to use metal ducts or bonded plates, well interconnected with each other and with the full length bonded at both ends. To keep the duct impedance low, several screws or straps are distributed around its perimeter. With more than 10 m between a signal line and the electronic equipment, the standard recommends balanced lines with galvanic isolation, such as optocouplers.
External equipment and cables between structures
Antennas, cameras, weather sensors and exposed sensors on processes or masts (B.12) are brought, where possible, into zone LPZ 0B with a local air terminal. On tall structures the rolling sphere method is used to see whether the equipment can take a direct strike, and if so air terminals are added; handrails, ladders and pipes can serve that function. Some antennas may require SPDs on the feeder. Cables leave the duct at a single point and use the natural shielding of pipes, ladders and well-bonded profiles.
Between separate structures (B.13), fibres with no metal parts need no protection. Metal cables, if the structures’ earthing is not interconnected, form a low-impedance path for lightning: bonding at the entrance protects only the internal equipment, not the line. It is recommended to run them in closed, interconnected metal ducts, or to add a bonding conductor in parallel. With many cables, screens or armours bonded at both ends can replace the duct.
Power supply of the installation
Old networks are usually TN-C, which cause interference at power frequency. For a new network TN-S is strongly recommended. To improve an old one, isolating interfaces are used (feasible with little electronic equipment) or it is changed to TN-S, the recommended solution for extensive systems.
Guides in the same series
The upgrades are explained in equipotential bonding network and 3D earthing system, magnetic shielding and cable routing and coordinated SPDs; the general framework of the measures is in SPM against LEMP.
When to call us
Retrofitting a control room or an old plant is decided from the data survey, the risk assessment and the real condition of the earthing and the cables. If you are going to extend or modernise an installation, our engineering design and studies team can support you on the electrical side, and our testing and commissioning services cover the field measurements, in Colombia or elsewhere on request. 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
- Equipotential bonding network and 3D earthing system
- Magnetic shielding and cable routing against lightning
- Extending an industrial plant substation: what to review first
References
- IEC 62305-4:2010, Protection against lightning, Part 4: Electrical and electronic systems within structures: Annex B (implementation of SPM in an existing structure), Tables B.1 to B.4 and Figures B.1 to B.8.
- IEC 62305-2:2010, Part 2: Risk management.
Frequently asked questions
What can be done if a meshed bonding network cannot be built in an existing building?
IEC 62305-4, Annex B.3, calls for at least a ring conductor inside the outer wall at each floor, connected to each down-conductor of the external LPS. The standard warns that improving the shielding of an existing building is often impractical and uneconomic, and that in such cases SPDs are an effective alternative.
What problem does an old TN-C supply network bring?
The PEN conductor carries load current, and if earthed signal lines are connected to it, interference currents at 50/60 Hz appear. Annex B.16 proposes separating the installations with isolating interfaces, if there is little electronic equipment, or changing to a TN-S system, which is the recommended solution for extensive systems.
Do cables between two buildings need protection?
It depends on the cable. An optical fibre with no metal parts needs no measures. Metal cables between structures without interconnected earthing can carry a significant part of the lightning current, so it is recommended to run them in closed, interconnected metal ducts, or to add a bonding conductor in parallel.