Risk zones

Risk is assessed for each risk zone and compared with the tolerable risk in that zone; protection is required unless every zone complies.

Risk component breakdown — governing zone

Recommended protection measures

• Install SPDs designed for LPL III-IV for lightning equipotential bonding at the entrance point of every connected line (IEC 62305-3; Table B.13)
• Install a class III LPS on the structure, including SPDs designed for LPL III-IV for equipotential bonding of the connected lines (Tables B.3 and B.13)

With the recommended measures every risk zone satisfies R <= R_T (projected governing-zone risk 5.83e-06/yr).

Detailed calculation steps

The derivation chain shared by every risk zone, with the substituted values and the clause each step implements — check any step against the standard.

Resolved factors — zone Production hall

Resolved factors — zone Office

Resolved factors — zone Server room

Projected outcome with the recommended measures

The same assessment re-run with the recommended protection measures applied. The projected figures are reproducible from the post-measure factors below, exactly as the unprotected result is from the tables above.

Resolved factors with measures applied — zone Production hall

Resolved factors with measures applied — zone Office

Resolved factors with measures applied — zone Server room

Basis of calculation: IEC 62305-3:2024 (Ed. 3.0)

Air termination

The 15 m cell above is what Table 2 gives for class III. §5.2.2.4 makes it conditional: for the purposes of protecting flat surfaces, a mesh system is considered to protect the whole surface provided all of the following conditions are fulfilled. This design settles b). The other five are properties of the layout — check them against the drawing.

a) Air-termination conductors are positioned on roof edge lines, on roof overhangs, and on roof ridge lines if the slope of the roof exceeds 1/10. (NOTE 1: the mesh method is suitable for horizontal and inclined roofs. NOTE 2: it is suitable for lateral surfaces, to protect against flashes to the side of the structure.)

b) The mesh dimensions meet the values given in Table 2 — computed above.

c) The network of the air-termination system is constructed in such a way that the lightning current will always encounter at least two distinct metal routes to the earth termination.

d) No installation protrudes outside the volume protected by air-termination systems.

e) Metal installations at earth potential on the surface of the roof are required to be protected by an air termination if they do not meet the requirements for a natural air termination.

f) The air-termination conductors follow, as far as possible, the shortest and most direct route.

§5.2.2.4 NOTE 3: damage to a mesh air termination due to snow slabs on roofs with a slope exceeding 1/10 is minimized by using parallel air-termination conductors instead of a mesh, with a spacing between the conductors not greater than the required mesh width. §6.3.1 then provides the general approach to calculating the separation distance, with a more detailed explanation in D.5.2.2.4.

§5.2.2.4 NOTE 5: air terminations installed below the highest probable water level can cause damages.

One dimension of a mesh cell may be increased by up to +25 % provided the cell perimeter is not increased (Table 2 provision).

The elevation below shows the rolling sphere at the class radius 45 m. §5.2.2.1 makes the rolling sphere the general procedure, suitable in all cases, and a structure may be protected in accordance with one or more of the three methods — so it is drawn whichever method sizes the air termination. It bears directly on the mesh method: condition d) above is what the sphere tests.

§5.2.1: for all types of air terminals only the real physical dimensions of the metal air-termination systems shall be used for the determination of the volume protected. Every protected volume in this design is computed from the entered physical height alone — no enhanced or extended radius is claimed for any device. §5.2.1 also states that radioactive air terminals are not allowed.

§5.2.1: the individual air-termination rods should be connected together at roof level to ensure current division.

Down conductors

Fit a test joint on each of the 9 down conductors at its connection to the earth termination, openable for measurement and closed in normal use (§5.3.6).

Separation distance

**s is a distance at a point, and this design has two worth quoting.** §6.3.1 measures l from the point where the separation distance is to be considered, along the air termination *and* the down conductor. The figure above is for the down-conductor run; at the tip of the rod the path is longer and the current has not yet divided among the down conductors, so k_c = 1 over the rod (Annex B, Figure B.4 a) — the values of k_c are considered from the point of strike). Separate any metal installation by the figure for *its own* elevation; the design carries no coordinates, so it cannot say which applies where. The governing figure below is the larger of the two, because a single distance applied everywhere has to be the worst case.

Every internal metal installation closer than s to a down conductor must be bonded or the distance increased.

For an LPS installed at high elevations an altitude correction applies (§6.3.2, per IEC 60071-2).

s is the one figure in this design that the finished drawing can correct. Everything else above is fixed by the class and the building; s is computed from the conductor run length and, on the general approach, from the down-conductor and ring spacings — and the values used here for those are stated defaults, not measurements of a layout. Two of them can move s the wrong way: k_c1 grows with the down-conductor spacing c, and rings further apart than assumed make the computed s non-conservative. Once the layout is drawn, re-run this design with the measured figures.

Earth termination

The minimum lengths may be disregarded where an earthing resistance of the Type A arrangement below 10 Ω is achieved — measured at a frequency different from the power frequency and its multiples, to avoid interference (§5.4.2.2).

Ring in contact with the soil for at least 80 % of its length, buried ≥ 0,5 m deep, about 1 m from the external walls (§5.4.3).

Materials & minimum dimensions

All LPS components (conductors, rods, electrodes, clamps) shall meet IEC 62561-2 mechanical/electrical/corrosion requirements (Table 7/8 note a).

Air-termination rods where mechanical stress such as wind loading is not critical may be reduced to 9,5 mm diameter, 1 m long (Table 7 note c).

Copper solid round 50 mm² (8 mm Ø) may be reduced to 28 mm² (6 mm Ø) where mechanical strength is not essential; reduce fastener spacing accordingly (Table 7 note b).

A buried copper earth electrode shall not be used in contact with the structure's steel members, pipes or piles — corrosion (Table 8 note g).

Equipotential bonding (internal LPS)

Install the main bonding bar in the basement or at ground level, accessible for inspection, and connect it to the earth-termination system (§6.2.2 a).

Structure length 42 m exceeds 20 m — use a ring bonding bar or several interconnected bonding bars (§6.2.2 a).

Bond additionally at every point where the separation distance s of the section above cannot be kept (§6.2.2 b). Keep bonding connections as direct and straight as possible.

Live conductors of internal systems are bonded via SPDs — coordinate per IEC 62305-4.

Touch & step voltages

The insulation withstand voltage should be considered in wet conditions (§8.1). Signage shall conform to ISO 3864-1.

Alternative exemption: a measured contact resistance within 3 m of the down conductors of at least 100 kΩ also brings the hazard to a tolerable level. §8.1 c) sets that over the area of a typical footprint (0,02 m²) for touch voltage; §8.2 c) states the 100 kΩ for the soil surface layer without giving an area. A 5 cm layer of insulating material, e.g. asphalt, generally achieves it (§8.1 NOTE 2 / §8.2 NOTE 1).

Inspection & maintenance

Also inspect: during construction (components that become concealed), after LPS installation, after alterations or repairs, and after a known or suspected lightning strike to the structure (§7.4).

Periodic checks: deterioration and corrosion of air-termination elements, conductors and connections; earthing resistance of the earth-termination system; condition of connections, equipotential bonding and fixings (§7.4).

Testing on a 14–15-month cycle is acceptable where earth-resistance readings across different seasons are wanted (D.7.2.2). Consider improving the earthing system when measured resistance rises over successive inspections.

Record all inspection and test results and keep them with the LPS design documentation for the property manager (§7.4).

Calculation trace

Installation notes

• Bond every internal metal installation that cannot keep the separation distance s from LPS conductors.
• Connect the earth termination to the equipotential bonding bar; coordinate SPDs per IEC 62305-4.

Basis of calculation: IEC 62305-4:2024 (Ed. 3.0)

This document specifies the surge protection measures for the structure: the lightning protection zones, the earthing and bonding network, and a coordinated SPD system for every incoming service. The SPD specification is manufacturer-neutral — it states the required test class, discharge current and voltage protection level, and can be issued to any supplier.

Lightning protection zones

An LPZ 1 created by a normal external LPS in accordance with IEC 62305-3 has mesh widths and typical distances greater than 5 m, so its spatial shielding effect is negligible (IEC 62305-4:2024, B.14.5). Effective spatial shielding requires a mesh width typically below 5 m.

Equipotential bonding SPDs are always required at the entrance of an LPZ to bond incoming lines connected to internal systems within that zone. Using an interconnected or extended LPZ can reduce the number of SPDs required (IEC 62305-4:2024, 5.5).

LPZ 0B — IEC 62305-4:2024, 4.3.2 defines this zone as: "Zone protected against direct lightning flashes but where the threat is the full lightning electromagnetic field. The internal systems can be subjected to partial lightning surge currents."

LPZ 1 — IEC 62305-4:2024, 4.3.3 defines this zone as: "Zone where the surge current is limited by current sharing and isolating interfaces and/or by SPDs at the boundary. Spatial shielding can attenuate the lightning electromagnetic field."

LPZ 2 — IEC 62305-4:2024, 4.3.3 defines LPZ 2…n as: "Zone where the surge current may be further limited by current sharing, isolating interfaces and/or by additional SPDs at the boundary. Additional spatial shielding may be used to further attenuate the lightning electromagnetic field."

Earthing and bonding network

A Type B (ring or foundation) arrangement is used, as 5.2 recommends.

Meshed, integrated by multiple bonding points; metal components are not isolated from the earthing system. Preferred where internal systems extend over wide zones or many lines enter at several points.

Do not isolate the internal systems' metal parts from the earthing system; tie them into it at several points instead.

One earthing system serves the whole site. Giving the LPS, the SPDs, the power installation and the telecommunications installation an earth each is not an acceptable arrangement for lightning and surge protection; they must form a single integrated system (IEC 62305-4:2024, 5.2).

Bonding bars shall be installed for bonding of: all conductive services entering an LPZ, directly or via suitable SPDs; the protective earth conductor PE; metal components of the internal systems (cabinets, enclosures, racks); the magnetic shields of the LPZ, at the periphery and inside the structure (IEC 62305-4:2024, 5.4).

Where possible, incoming services should enter the LPZ at the same location and connect to the same bonding bar. Where services enter at different locations, each shall connect to a bonding bar and those bonding bars shall be interconnected — bonding to a ring bonding bar is recommended (IEC 62305-4:2024, 5.5).

Treat the bonding network as a protective path only: it should carry no power or signal return current. On that basis the PE conductor is tied into the bonding network and the PEN conductor is kept out of it, along with any metalwork connected to the PEN. A functional earthing conductor may be bonded straight to the low-impedance network. Keeping the PEN separate is what stops power-frequency current finding a way into the electronic systems (IEC 62305-4:2024, B.7 — Annex B is informative, so these are conditions of following its method rather than requirements standing on their own).

For efficient bonding the following installation rules are important: the basis for all bonding measures is a low impedance bonding network; run each bonding bar back to the earthing system by the most direct route the building allows; SPDs should be installed with the shortest possible connections to the bonding bar and to the live conductors, minimising inductive voltage drops; downstream of an SPD, mutual induction should be minimised by reducing the loop area or by using shielded cables or ducts earthed at both ends (IEC 62305-4:2024, 5.4).

Smaller cross-sections are permitted in some countries where the thermal and mechanical requirements of IEC 62305-1:2024, Annex D are satisfied (IEC 62305-4:2024, Table 1, footnote b). That verification is outside this tool's scope, so the tabulated minimum is applied.

For SPDs in power applications the connecting conductors shall also be dimensioned according to IEC 60364-5-53 and IEC 61643-12 (IEC 62305-4:2024, Table 1, footnote c).

Earthing and bonding are required in every design — there is no case in which they can be left out. Every conductive service must be tied to earth where it enters the structure: directly where that is possible, and otherwise through an equipotential bonding SPD (IEC 62305-4:2024, Clause 4.4).

Coordinated SPD schedule

Cascaded SPDs shall be energy coordinated in accordance with IEC 61643-12 and IEC 61643-22; the SPD manufacturer should provide the information needed to achieve energy and voltage protection level coordination between the devices (IEC 62305-4:2024, C.3.4). This tool states the requirement but does not compute the coordination.

This is a telecommunication or signal line, so its discharge currents are not taken from Annex C: Tables C.3, C.4 and C.5 are keyed on the supply system and its modes of protection, and describe power lines only. IEC 62305-4:2024, C.2.2 provides the required current for a signal line in IEC 62305-2:2024, Annex B (Table B.8), and classifies the device under IEC 61643-21 — class D1 where a power SPD would be class I tested, and class C2 where it would be class II. Each source of damage carries its own waveform, so a device at this boundary has both a 10/350 and an 8/20 duty.

These currents are per line conductor, not the total through the cable (IEC 62305-1:2024, Table E.2: "All values refer to each line conductor"). A multi-core cable carries the sum, so a device protecting n cores must carry n times the figure above in total, and manufacturers' data sheets state both a per-line and a total rating. Check which one you are reading before comparing.

The figures above are those IEC 62305-2:2024, Table B.8 gives for overhead unshielded lines, which is its worst case. Its NOTE 3 permits the currents to be halved for a buried line, and halved for a shielded line. This design applies neither reduction, so the requirement stated here is conservative for a line that is buried, shielded, or both.

No voltage protection level is stated for this signal line. Every U_p ceiling in this schedule is a fraction of the equipment's rated impulse withstand U_W, and U_W is resolved from Table C.1, whose rows are line-to-neutral voltages of the *power* supply. The withstand of a telecommunication or signal port is a property of that interface and is declared by its manufacturer; select U_p against it, and against IEC 61643-21, rather than against the value used for the power lines here.

The voltage drop in the SPD connecting leads is computed from the approximation of IEC 62305-4:2024, eq. C.4 — 0,1 x I_SPD kV per metre. The standard notes that the true value depends on the current waveform, the conductor arrangement and the loop geometry; published manufacturer figures for the same case run up to about 20 % higher. The margin this consumes is small only while the leads are kept within 0,5 m, which is why that limit is a requirement and not a recommendation.

SPD selection and installation requirements

U_c, the maximum continuous operating voltage of each SPD, shall be equal to or higher than the value IEC 60364-5-53:2019+AMD1:2020, Table 2 gives for its mode of protection and the earthing arrangement, and the device shall withstand the temporary overvoltages of IEC 61643-11 for the same combination. Both are checked against a candidate's data sheet in the verification section, which needs the line-to-line voltage U of the supply — the Table C.1 band above does not fix it, since its 300 V row covers 230/400 V and 277/480 V alike.

Where SPDs are cascaded, a decoupling element or a minimum line length between them is required for correct energy coordination (IEC 62305-4:2024, Figure D.1). The required value is given in IEC 61643-12 and by the SPD manufacturer; a combined device may be used instead.

The back-up overcurrent protective device shall be the maximum permissible type specified by the SPD manufacturer, and shall be coordinated with the upstream system OCPD; where those two requirements conflict, the lower rating is generally the safer choice (IEC 62305-4:2024, C.2.8). That coordination is not computed here. The SPD's own short-circuit current rating I_SCCR shall not be lower than the maximum prospective short-circuit current at its connection points (IEC 60364-5-53:2019+AMD1:2020, 534.4.4.6), and the same holds for the follow current interrupting rating I_fi where the manufacturer declares one (534.4.4.7). Neither requirement applies to an SPD connected between the neutral conductor and PE in a TN or TT system — both clauses exempt that case, as the product standard IEC 61643-11 already covers it. Both are checked in the verification section against a prospective current you state, since it is a property of the installation rather than of the lightning protection design.

The behaviour of the back-up overcurrent protective device under partial lightning current or high impulse current should be considered as one of three cases (IEC 62305-4:2024, C.2.8.1). Case 1 — No melting and no tripping. The OCPD withstands the expected surge current through the SPD without tripping or melting. Case 2 — Melting and tripping permissible. The lightning impulse energy is enough to operate the OCPD. Case 3 — Mechanical destruction permissible. The impulse energy exceeds the OCPD's withstand level.

In cases 2 and 3 the installation is no longer protected against subsequent strokes within the same flash; additional SPDs on specific downstream circuits can maintain protection. Take care before replacing a blown OCPD or re-closing a tripped MCB, since the SPD may have failed into a short circuit. Because lightning behaves as an ideal current source, operation or destruction of the back-up OCPD does not reduce the impulse current the SPD must carry (IEC 62305-4:2024, C.2.8).

Where the circuit between the SPD and the equipment is long, propagation can lead to oscillation raising the overvoltage at an open circuit to as much as 2 x U_p/f, even where U_p/f <= U_W is satisfied (IEC 62305-4:2024, C.2.3.4). The effect is negligible where the SPD is installed at the equipment input.

Services connected to different ports of the same equipment shall not be connected to different points on the earthing system, except where those points are equipotential bonded (a densely meshed foundation electrode of the order of 1 m x 1 m, or a ring electrode welded or bolted to the steel reinforcement), or the internal services are shielded or in a metallic duct bonded at both ends, or a multiservice SPD able to withstand S1/S3 stress is provided between the ports (IEC 62305-4:2024, C.2.6).

Calculation trace

Every requirement above, with the clause that governs it — check any step against the standard.