Page 229 - Electrical Safety of Low Voltage Systems

212 Chapter Twelve

electrodes depends on both the voltage applied across them and their
reciprocal distance [Eq. (12.4)]:

V i = k m Es (12.4)
6
where V i is the overvoltage induced by the lightning strike ; E is the
dielectric strength of the air in dry conditions; k m is a multiplier that
takes into account separation materials more “dense” than air, for
which it assumes values greater than one. s can be calculated from
Eq. (12.3) as the minimum space between the two conductive parts
betweenwhichnodangerouselectricalsparkingcanoccurinlightning
conditions.
If metal parts cannot be separated by at least the safety distance
(i.e., a < s) due, for example, to structural constraints, protective mea-
sures must be assumed to prevent the adverse effects of the lightning
current.
A possible alternative solution is to provide equipotential bond-
ing connections to bridge the gap where a < s. In this case, though,
part of the lightning current flows to ground through the metal part
within the building, which results in parallel to the down-conductor.
This amount of current is reduced if multiple down-conductors are
installed.
Alternatively, the adoption of separation materials with higher
impulse dielectric capability, by increasing the multiplier k m , reduces
the value of the safety distance, thereby allowing a closer distance
between down-conductors and metal parts.

Endnotes

1. “Protection for Safety—Protection Against Voltage Disturbances and Electromagnetic
Disturbances,” IEC 60364–4-44, 2001.
2. IEEE Standard 100 “The Authoritative Dictionary of IEEE Standard Terms,” 7th ed.
3. The dielectric strength of the air in standard dry conditions is 3000 kV/m, but
this value can be greatly reduced by the presence of humidity and/or atmo-
spheric motes.
4. At 50/60 Hz, the inductance of ground electrodes can be estimated as 1 H/m.
5. 0 = 1.257 × 10 −6 H/m.
6. For more details see: M. Mitolo, “Shall Masts and Metal Structures Supporting
Antennae Be Grounded?,” Proceedings of the IEEE-IAS Industrial & Commercial
Power Systems Technical Conference, Edmonton, Canada, May 6–10, 2007.