# Methodics of calculation of SPD load currents

The complexity of solving any task about distribution of currents in SPD circuits is seeming. If the algorithm of the task is well set, any calculation on a modern personal computer of an average efficiency will hardly take more than a minute. We present you a method of calculating SPD load currents.

E. M. Bazelyan, D. Eng. S, Professor;
G. M. Krzyzanowski Power Engineering Institute, Moscow;
acknowledged Russian expert in grounding and lightning protection

## 1. Is there a need in the calculation?

Indeed, is there such a need? There are enough reasons for the doubt. Take a look at fig. 1. It was taken from the Internet. All the currents are strictly determined once and forever. Half of lightning current is taken by the building ground electrode system, which was hit by the lighting, and the rest of the current is equally shared between its metal utilities (power supply line, phone, water pipeline etc).

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## 2. The use of substitution circuit with the distributed parameters at a direct strike into the object

Solution of the task about division of the lightning current between the overhead lines, powering the hit object, its ground electrode system, as it was said above, requires knowledge of linear value of inductivity L and capacity C of the overhead lines, and also its linear resistance R.. The process is described by the equations in partial derivatives.

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## 3. The use of substitution circuit with the distributed parameters at a direct strike into the object or into an overhead line, powering it

One can hardly doubt, that the solution in the frames of the substitution circuit with the distributed parameters in a common case is most authentic. The advantages are very obvious, when it is necessary to have a current and voltage distribution along the studied part of the overhead line. But for a choice of SPD, such information is almost always excessive. It is enough to know currents only at the ends of the overhead lines, where SPDs must be installed.

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## 4. When it is impossible to get along without a circuit with distributed parameters

It is not a surprise, but we will talk not about overhead power transmission lines, but about extended underground utility systems. It can be, for example, metal conduits or simply very long grounding buses. The running inductance of such utility system is approximately the same as of the overhead power transmission line. The capacity per unit length doesn't increase significantly (relative dielectric permeability of soil ε, as a rule is not greater than 10) and that is why, the condensance leakage current in many situations is much less than the current in the grounding devices.

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## 5. Useful recommended practices on the calculation of SPD load currents

The time interval for the calculation is chosen the way to provide the drawing of the lightning current pulse front. For example, at the front time of 10 ms (pulse 10/350 ms, on which SPD of the I protection level are chosen), gives quite a satisfactory result with the interval of 1 ms, at which the front is being drawn by 10 calculation points; at an interval of 0,1 ms (100 calculated points on the front) the result of the calculation appears to be brilliant. The question about the current pulses of the next components appears immediately.

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