B.7 Establishment of LPZs for electrical and electronic systems
Depending on the number, type and sensitivity of the electrical and electronic systems, suitable inner LPZs are defined from small local zones (the enclosure of a single electronic equipment) up to large integral zones (the whole building volume).
Figure B.2 shows typical LPZ layouts for the protection of internal systems providing different solutions suitable for existing structures in particular:
Figure В.2а shows the installation of a single LPZ 1, creating a protected volume inside the whole structure, e.g. for enhanced withstand voltage levels of the internal systems:
This LPZ 1 could be created using an LPS, in accordance with IEC 62305-3, that consists of an external LPS (air-termination, down-conductor and earth-termination system) and an internal LPS (lightning equipotential bonding and compliance with the separation distances).
The external LPS protects LPZ 1 against lightning flashes to the structure, but the magnetic field inside LPZ 1 remains nearly unattenuated. This is because air-terminations and down-conductors have mesh widths and typical distances greater than 5 m, therefore the spatial shielding effect is negligible as explained above.
The internal LPS requires bonding of all services entering the structure at the boundary of LPZ 1, including the installation of SPDs for all electrical and signal lines. This ensures that the conducted surges on the incoming services are limited at the entrance by SPDs.
NOTE Isolating interfaces can be useful inside LPZ 1 in order to avoid low-frequency interference.
Key
E power lines
S signal lines
Figure B.2a - Unshielded LPZ 1 using LPS and SPDs at the entrance of the lines into the structure (e.g. for
enhanced withstand voltage level of the systems or for small loops inside the structure)
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Old installations
New installations
LPZO Q
spd
sS
SPD »D c _
H-T— EE
0/1 1
S SPD
E power lines S signal lines
LPZO Q
і
I-
1
0/1 1
/EC 2802/10
Figure B.2b - Unshielded LPZ 1 with protection for new internal systems
using shielded signal lines and coordinated SPDs in power lines
New installations
Old installations
LPZO 0
0/1 I LPZO Q 0/2 |>2
IEC 2803/10
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E power lines
S signal lines
Figure B.2c - Unshielded LPZ 1 and large shielded LPZ 2 for new internal system
s
I New installations
Old installations
LPZ 1
LPZ 0
S
E
1/2
SPD
LPZ 0
0/1
0/1
...
IEC 2804/10
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E power lines S signal lines
Figure B.2d - Unshielded LPZ 1 and two local LPZs 2 for new internal systems
Figure B.2 - Possibilities to establish LPZ in existing structures
Figure B.2b shows that in an unshielded LPZ 1, new equipment also needs to be protected against conducted surges. As an example, the signal lines can be protected using shielded cables and the power lines using a coordinated SPD system. This may require additional SPDs tested with lN and SPDs tested with a combination wave, installed close to the equipment, and coordinated with the SPDs at the service entrance. It may also require additional Class II “double insulation” of the equipment.
Figure B.2c shows the installation of a large integral LPZ 2 inside of LPZ 1, to accommodate the new internal systems. The grid-like spatial shield of LPZ 2 provides a significant attenuation of the lightning magnetic field. On the left hand side, the SPDs installed at the boundary of LPZ 1 (transition of LPZs 0/1) and subsequently at the boundary of LPZ 2 (transition of LPZs 1/2), should be coordinated in accordance with IEC 61643-12. On the right hand side, the SPDs installed at the boundary of LPZ 1 should be selected for a direct transition of LPZs 0/2 (see C.3.5).
Figure B.2d shows the creation of two smaller LPZs (LPZs 2) inside LPZ 1. Additional SPDs for power as well as for signal lines at the boundary of each LPZ 2 should be installed. These SPDs should be coordinated with the SPDs at the boundary of LPZ 1 in accordance with IEC 61643-12.
Protection using a bonding network
Existing power-frequency earthing systems might not provide a satisfactory equipotential plane for lightning currents with frequencies up to several MHz, because their impedance may be too high at these frequencies.
Even an LPS designed in accordance with IEC 62305-3, which allows mesh widths typically greater than 5 m, and which includes lightning equipotential bonding as a mandatory part of the internal LPS, might not be sufficient for sensitive internal systems. This is because the impedance of this bonding system may still be too high for this application.
A low impedance bonding network with typical mesh width of 5 m and below is strongly recommended.
In general the bonding network should not be used either as a power, or signal, return path. Therefore the PE conductor should be integrated into the bonding network, but the PEN conductor should not.
Direct bonding of a functional earthing conductor (e.g. a clean earth specific to an electronic system) to the low impedance bonding network is allowed, because in this case the interference coupling into electrical or signal lines will be very low. No direct bonding is allowed to the PEN conductor, or to other metal parts connected to it, so as to avoid power frequency interference in the electronic system.
Protection by surge protective devices
To limit conducted surges due to lightning on electrical lines, SPDs should be installed at the entry to any inner LPZ (see Figure B.2 and Figure B.8, No.3).
In buildings with uncoordinated SPDs, damage to the internal system may result if a downstream SPD, or an SPD within the equipment, prevents the proper operation of the SPD at the service entrance.
In order to maintain the effectiveness of the protection measures adopted, it is necessary to document the location of all installed SPDs.
Protection by isolating interfaces
Power-frequency interference currents through the equipment and its connected signal lines can be caused by large loops or the lack of a sufficiently low impedance bonding network. To prevent such interference (mainly in TN-C installations), a suitable separation between existing and new installations can be achieved using isolating interfaces, such as:
- class II insulated equipment (i.e. double insulation without a PE-conductor),
isolation transformers,
metal-free fibre optic cables,
optical couplers.
NOTE Care should be taken that metal equipment enclosures do not have an unintended galvanic connection to the bonding network or to other metal parts, but that they are isolated. This is the situation in most cases, since electronic equipment installed in domestic rooms or offices is linked to the earth reference through connection cables only.
В.11 Protection measures by line routing and shielding
Suitable line routing and shielding are effective measures to reduce induced overvoltages. These measures are especially important, if the spatial shielding effectiveness of LPZ 1 is negligible. In this case, the following principles provide improved protection:
minimizing the induction loop area;
powering new equipment from the existing mains should be avoided, because it creates a large enclosed induction loop area, which will significantly increase the risk of damage. Furthermore, routing electrical and signal lines adjacent to one another can avoid large loops (see Figure B.8, No. 8);
using shielded cables - the shields of these signal lines should be bonded at least at either end,
using metal cable ducts or bonded metal plates - the separate metal sections should be electrically well interconnected and the overall length bonded at either end. The connections should be performed by bolting the overlapping parts or by using bonding conductors. In order to keep the impedance of the cable duct low, multiple screws or strips should be distributed over the perimeter of the cable duct (see IEC 61000-5-2) [6].
Examples of good line routing and shielding techniques are given in Figures B.3 and B.4.
NOTE Where the distance between signal lines and electronic equipment within general areas (which are not specifically designated for electronic systems) is greater than 10 m, it is recommended to use balanced signal lines with suitable galvanic isolation ports, e.g. optical couplers, signal isolation transformers or isolation amplifiers. In addition, the use of tri-axial cables can be advantageous.
IEC 2805/10
Key
PE, only when class I equipment is used
optional cable shield needs to be bonded at both ends
metal plate as additional shield (see Figure B.4)
small loop area
NOTE Owing to the small loop area, the induced voltage between the cable shield and the metal plate is small.
Figure B.3 - Reduction of loop area using shielded cables close to a metal plate
Key
cable fixing with or without bonding of cable shields to the plate
at the edges, the magnetic field is higher than in the middle of the plate E electrical lines
S signal lines
Figure B.4 - Example of a metal plate for additional shielding
B.12 Protection measures for externally installed equipment
General
Examples of externally installed equipment include: sensors of any kind including aerials; meteorological sensors; surveillance TV cameras; exposed sensors on process plants (pressure, temperature, flow rate, valve position, etc.) and any other electrical, electronic or radio equipment in external positions on structures, masts and process vessels.
Protection of external equipment
Wherever possible, the equipment should be brought under the protective zone LPZ 0B using for example a local air terminal to protect it against direct lightning flashes (see Figure B.5).
On tall structures the rolling sphere method (see IEC 62305-3) should be applied, to determine if the equipments installed on the top or sides of the building are possibly subject to a direct flash. If this is the case, additional air-terminations should be used. In many cases handrails, ladders, pipes etc. can adequately perform the function of an air-termination. All equipment, except some types of aerials, can be protected in this manner. Aerials sometimes have to be placed in exposed positions to avoid their performance being adversely affected by nearby lightning conductors. Some aerial designs are inherently self-protecting because only well-earthed conductive elements are exposed to a lightning flash. Others might require SPDs to be installed on their feeder cables to prevent excessive transients from flowing down the cable to the receiver or the transmitter. When an external LPS is available the aerial supports should be bonded to it
.
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1 lightning rod
2 steel mast with antennas
3 hand rails
4 interconnected reinforcement
5 line coming from LPZ 0B needs an SPD at entry
6 lines coming from LPZ 1 (inside the mast) may not need SPDs at entry r radius of the rolling sphere
Figure B.5 - Protection of aerials and other external equipment
B.12.3 Reduction of overvoltages in cables
High induced voltages and currents can be prevented by running cables in bonded ducting, trunking or metal tubes. All cables leading to the specific equipment should leave the cable duct at a single point. Where possible, the inherent shielding properties of the structure itself should be used to maximum advantage by running all cables together within the tubular components of the structure. Where this is not possible, as in the case of process vessels, cables should run on the outside but close to the structure and make as much use as possible
of the natural shielding provided by metal pipes, steel rung ladders and any other well bonded conducting materials (see Figure B.6). On masts which use L-shaped corner members, cables should be placed in the inside corner of the L for maximum protection (see Figure B.7).
Key
process vessel
rung ladder
pipes
NOTE А, В, C are good alternatives for cable tray positioning.
- Inherent shielding provided by bonded ladders and pipes
Key
ideal positions for cables in corners of L-girders
alternative position for bonded cable tray within the mast
- Ideal positions for lines on a mast (cross-section of steel lattice mast)
Improving interconnections between structures
General
Lines interconnecting separate structures are either
isolating (metal-free fibre optic cables), or
metallic (e.g. wire pairs, multicores, waveguides, coaxial cables or fibre optic cables with continuous metal components).
Protection requirements depend on the type of the line, the number of lines and whether the earth-termination systems of the structures are interconnected.
Isolating lines
If metal-free fibre optic cables (i.e. without metal armouring, moisture barrier foil or steel internal draw wire) are used to interconnect separate structures, no protection measures for these cables are needed.
Metallic lines
Without proper interconnection between the earth-termination systems of separate structures, the interconnecting lines form a low impedance route for the lightning current. This may result in a substantial portion of the lightning current flowing along these interconnecting lines. In this case:
the required bonding, directly or via an SPD, at the entries to both LPZs 1 will protect only the equipment inside, whereas the lines outside remain unprotected;
the lines might be protected by installing an additional bonding conductor in parallel. The lightning current will then be shared between the lines and this bonding conductor;
it is recommended that the lines be run in closed and interconnected metal cable ducts. In this case the lines as well as the equipment are protected.
Where proper interconnection between the earth-termination systems of separate structures is implemented, the protection of lines by interconnected metal ducts is still recommended. Where many cables are run between interconnected structures, the shields or the armouring of these cables, bonded at either end, can be used instead of cable ducts.
Integration of new internal systems into existing structures
When adding new internal systems to an existing structure, the existing installation might restrict the protection measures that can be adopted.
Figure B.8 shows an example where an existing installation, shown on the left, is interconnected to a new installation, shown on the right. The existing installation has restrictions on the protection measures that can be employed. However design and planning of the new installation can allow for all necessary protection measures to be adopted.
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New installations
Existing installations
/EC 2810/10
Key
existing mains (TN-C,TT,IT)
new mains (TN-S,TN-CS,TT,IT)
surge protective device (SPD)
class I standard insulation
class II double insulation without PE
isolation transformer
opto-coupler or fibre optic cable
adjacent routing of electrical and signal lines
shielded cable ducts
E electrical lines
S signal lines (shielded or unshielded)
Ey earth-termination system
BN bonding network
PE protective earthing conductor
FE functional earthing conductor (if any)
ITT З-wire electrical line: L, N, PE
/Т 2-wire electrical line: L, N
• bonding points (PE, FE, BN)
Figure B.8 - Upgrading of the SPM in existing structures
Overview of possible protection measures
Power supply
Existing mains supply (see Figure B.8, No.1) in the structure is very often of the type TN-C, which can cause power frequency interference. Such interference can be avoided by isolating interfaces (see below).
If a new mains supply (see Figure B.8, No. 2) is installed, type TN-S is strongly recommended.
Surge protective devices
To control conducted surges on lines, SPDs should be installed at the entry into any LPZ and possibly at the equipment to be protected (see Figure B.8, No.3 and Figure B.2).
Isolating interfaces
To avoid interference, isolating interfaces between existing and new equipment can be used: Class II insulated equipment (see Figure B.8, No. 5), isolation transformers (see Figure B.8, No. 6), fibre optic cables or optical couplers (see Figure B.8, No. 7).
Line routing and shielding
Large loops in line routing might lead to very high induced voltages or currents. These can be avoided by routing electrical and signal lines adjacent to each other (see Figure B.8, No. 8), thereby minimizing the loop area. It is recommended to use shielded signal lines. For extended structures, additional shielding, for example by bonded metal cable ducts (see Figure B.8. No. 9), is also recommended. All these shields should be bonded at both ends.
Line routing and shielding measures become more important the smaller the shielding effectiveness of the spatial shield of LPZ 1, and the larger the loop area.
Spatial shielding
Spatial shielding of LPZ against lightning magnetic fields requires mesh widths typically less than 5 m.
An LPZ 1 created by a normal external LPS in accordance with IEC 62305-3 (air-termination, down-conductor and earth-termination system) has mesh widths and typical distances greater than 5 m, resulting in negligible shielding effects. If higher shielding effectiveness is required, the external LPS should be upgraded (see Clause B.4).
LPZ 1 and higher may require spatial shielding to protect internal systems not complying with radiated radio frequency emission and immunity requirements.
Bonding
Equipotential bonding for lightning currents with frequencies up to several MHz requires a meshed low impedance bonding network having a typical mesh width of 5 m. All services entering an LPZ should be bonded directly, or via a suitable SPD, as closely as possible to the boundary of the LPZ.
If, in existing structures, these conditions cannot be fulfilled, other suitable protective measures should be provided.
Upgrading a power supply and cable installation inside the structure
The power distribution system in older structures (see Figure B.8, No. 1) is very often TN-C. Interference at 50/60 Hz arising from the connection of earthed signal lines with the PEN conductor can be avoided by
isolating interfaces using class II electrical equipment or double insulated transformers. This can be a solution if there is only a small amount of electronic equipment (see Clause B.5),
changing the power distribution system to a TN-S (see Figure B.8 No. 2). This is the recommended solution, especially for extensive systems of electronic equipment.
The requirements of earthing, bonding and line routing should be fulfilled
.Annex С
(informative)
Selection and installation of a coordinated SPD system
C.1 Introduction
Lightning flashes to a structure (source of damage S1), near the structure (S2), to a service connected to the structure (S3) and near a service connected to the structure (S4) can cause failures or malfunction of internal systems (see 5.1 of IEC 62305-1:2010).
