The separation distance s prevents dangerous sparking between the LPS and the metal parts of the structure: s = (ki/km) · kc · l, with ki of 0.08, 0.06 or 0.04 by class, km of 1 in air and 0.5 in concrete, brick or wood, kc of 1, 0.66 or 0.44 by number of down-conductors, and l the length to the nearest equipotential bonding point.
Short answer
The separation distance s is the minimum distance between a part of the external system (air-termination or down-conductor) and a metal part of the structure, a metal installation or an internal system, at which no dangerous spark occurs. IEC 62305-3 calculates it with s = (ki / km) × kc × l, in meters. ki depends on the LPS class, km on the insulating material between the parts, kc on the current sharing among down-conductors and l on the length of the conductor to the nearest equipotential bonding point. It is not required in structures with a metal frame or an electrically continuous reinforced concrete frame.
What it is for
When lightning runs down a down-conductor, a voltage drop appears along it. If another metal part is nearby and at a different potential, a spark can jump between the two, with a risk of fire or explosion. The standard gives two ways to avoid it: equipotential bonding (connecting the parts) or electrical isolation (keeping a distance greater than s). The separation distance is the criterion for the second.
The equation
s = (ki / km) × kc × l (m) (Eq. 4)
| Coefficient | What it represents | Value |
|---|---|---|
| ki | LPS class (Table 10) | I: 0.08; II: 0.06; III and IV: 0.04 |
| km | insulating material between the parts (Table 11) | air: 1; concrete, brick or wood: 0.5 |
| kc | current sharing in air-terminations and down-conductors (Table 12 and Annex C) | see below |
| l | length, in m, along the air-termination and down-conductor, from the point where the distance is considered to the nearest equipotential bonding point or earth-termination | from the project |
A note to Table 11: if there are several insulating materials in series, good practice is to take the smaller value of km. For other insulating materials, the manufacturer must give the construction guidance and the value of km.
The length l along the air-termination can be ignored in structures with a continuous metal roof acting as a natural air-termination.
The coefficient kc: current sharing
The lightning current does not flow down a single down-conductor in full. The coefficient kc depends on the class, on the number n and position of the down-conductors, on the rings that interconnect them and on the type of earth-termination. The more down-conductors, the less current in each and the smaller s.
Simplified approach (Table 12), with l as the vertical length along the down-conductor to the nearest equipotential bonding point:
| Number of down-conductors n | kc |
|---|---|
| 1 (isolated LPS only) | 1 |
| 2 | 0.66 |
| 3 or more | 0.44 |
The standard adds that the simplified approach normally gives results on the safe side. The values hold for all type B arrangements and for type A if the resistances of neighboring electrodes do not differ by more than a factor of 2; if they differ more, kc = 1.
Detailed approach. In an LPS with meshed air-terminations or with rings interconnecting the down-conductors, the current is different along each section. The standard allows a more precise evaluation by summing the sections, each with its own kc and length (Eq. 5), supported by the figures of Annex C. It is suitable for very large structures or those of complex shape. Numerical network programs can be used to calculate kc in each conductor.
What the equation does not solve
- Structures with a metal frame or continuous reinforced concrete: s is not required (the current is shared over so many paths that the voltage drop is low).
- Incoming lines and conductive parts: equipotential bonding at the point of entry is always necessary, with a direct connection or through SPDs.
- Loops in the down-conductor: where a loop cannot be avoided, the distance s measured between two points of the conductor and the conductor length l between them must meet this same condition.
- Doors and windows: keeping the separation distance between the down-conductors and them is recommended.
How it reads in practice
A qualitative example. In a class I building (ki = 0.08) with two down-conductors (kc = 0.66) and a brick wall (km = 0.5), the required separation grows in proportion to the height l to the nearest bonding point. The higher the point considered, the greater s; an intermediate equipotential bond reduces l and, with it, s. A less demanding class or more down-conductors also reduce it. That is why designers combine additional down-conductors, rings and bonding points so that s is achievable. This guide does not solve a specific case: the calculation with your data is part of the study.
- LPS class. Sets ki
- Insulation between parts. Sets km
- Number and position of down-conductors. Set kc
- Length to the equipotential bond. Sets l
- Distance s. s = (ki / km) × kc × l
When bonding fits better than a distance
If the calculation gives a separation that does not fit on site (for example, between a down-conductor and an internal metal installation), the standard leaves two options: increase the number of down-conductors or rings to reduce kc, or bond the parts through equipotential bonding so they are no longer at different potentials. The second moves part of the lightning current to the interior, an effect the standard asks to be taken into account in internal systems. That is why the design of the separation distance and that of equipotential bonding are done together and not separately.
Common mistakes
- Using l to earth when there is an equipotential bond closer. l runs to the nearest bonding point.
- Applying kc = 0.44 with very different electrodes. If the resistances differ by more than a factor of 2, kc = 1.
- Taking km of one material and forgetting layers in series. The smaller one is taken.
- Applying s to an incoming line. The line is bonded at the point of entry.
- Applying the formula to a system with a continuous frame. It is not required.
Guides in the same series
When the distance s cannot be met, lightning equipotential bonding is used. The down-conductors whose separation is calculated are in LPS down-conductors. The system class, which sets the coefficient ki, is explained in LPS, classes I to IV; the earthing arrangement type, in LPS earth-termination, and the bonding network of the building, in bonding network and earthing in 3D.
What we do
We verify the separation distance and propose additional down-conductors and bonds when it is not met. We will quote the study.
Related guides
- IEC 62305: lightning protection, what each part covers
- Lightning equipotential bonding and cable screen cross-section
- LPS down-conductors: number, spacing and step voltage
- LPS materials: air-terminations, down-conductors and electrodes
- Lightning shielding of substations
References
- IEC 62305-3:2010, Part 3: Physical damage to structures and life hazard: Cl. 6.3 (electrical isolation of the external LPS, Eq. 4 and 5, Tables 10 to 12), Annex C (evaluation of the separation distance, Figures C.1 to C.5) and Annex E.6.3.
Frequently asked questions
When is a separation distance not required?
IEC 62305-3 indicates that no separation distance is required in structures with a metal frame or with an electrically continuous reinforced concrete frame.
What if the resistances of neighboring electrodes differ a lot?
The approximate kc values of Table 12 hold for all type B arrangements and for type A provided the resistances of neighboring electrodes do not differ by more than a factor of 2. If they differ more, kc = 1 must be assumed.
Can the separation distance be used instead of equipotential bonding?
They are two alternative ways of preventing dangerous sparking between different parts: equipotential bonding, or electrical isolation by means of the separation distance. Even so, for lines and conductive parts entering the structure, equipotential bonding at the point of entry is always necessary.