Separation Distance

01/04/2023

Introduction:

During the passage of lightning impulse current through the components of LPS, sparking may occur between the conductors and any other metal structures. IS/IEC 62305 suggests that the dangerous sparking between an LPS and metal, electrical and telecommunication installations can be avoided by implementing any one of the following options.

1) Isolation or adequate separation in an isolated LPS.

2) Equipotential bonding or by isolation or adequate separation in a non-isolated LPS.

Equipotential bonding avoids the difference in potential and thus reduces the chances of flashing. In this article, we shall explore about calculating the separation distance for LPS as specified by IS/IEC 632305.

Separation Distance:

Separation distance is the minimum distance required between two conductive parts so that no dangerous sparking can occur. The separation distance provides electrical insulation between the conductors and other metal structures. Adequate spacing should be provided between the down-conductors of LPS and any doors, window or metal structures adjacent to it.

As per IS/IEC 62305, the separation distance (s) can be calculated by,

S =

Where,

k_i - depends on the selected class of LPS:

k_m - depends on the electrical insulation material;

k_c - depends on the lightning current flowing on the air-termination and the down conductor and

I - length in metre,

The length is calculated from the air-termination or the down-conductor point to the nearest equipotential bonding point or the earth termination. If internal down-conductors exist, they should be taken into account in evaluating the number n.

The value of k_i for calculating the separation distance for different class of LPS is as follows.

The value of Km for different materials for calculating the separation distance as specified by IS/IEC 62305 is as follows.

The value of partitioning coefficient kc of the lightning current depends on,

Class of LPS,
Type of air termination system,
The overall number the down conductors (n),
Position of down conductors and
The type of earth-termination system.

There are two approaches available for finding the separation distance value. They are,

1) Simplified approach

2) Detailed approach

Simplified approach:

In simplified approach, the current flowing through the lengths of the conductors will be constant. The following conditions have to be considered for calculating the separation distance.

k_c - depends on the (partial) lightning current flowing on the down-conductor arrangement

I - vertical length in metre, along the down-conductor, from the point where the separation distance is to be considered, to the nearest equipotential bonding point.

The value of k_c for different numbers of down-conductors provided by IS/IEC 62305 part 3 is as follows.

Example:

Detailed Approach:

In a meshed air-termination system and interconnected down-conductor system, there will be multiple parallel paths and the lightning impulse current will get divided among the conductors at every junction. Hence the value of current flowing through one down conductor won't be same as the other down conductor. For these cases, IS/IEC 62305 suggests a more accurate evaluation of the separation distance s.

This approach is suitable for evaluation of the separation distance in very large structures or structures with complex shape. For such complex structures, we cannot apply simplified approach to calculate the separation distance.

Let us consider a structure protected by using mesh method and the structure has 24 numbers of down conductors. Based on the current injection point, the current through the down conductor varies. Let us consider three different injection points A, B & C.

Rules for current partitioning:

IS/IEC 62305-3 provides the following instructions for calculating the separation distance.

Current will get divided by the number of possible current paths into the meshed air termination system at the injection point.
Current will be reduced by 50 % at any further joints of the air-termination mesh.
At the down-conductors, current will be further reduced by 50%, but the value of kc must not be less than 1/n. (n- total number of down-conductors).

The path along the roof edge to the down-conductor need not to be considered
If there are fewer meshes between the point of strike and the edge of the roof, only the relevant values of kc have to be used.