Reduction of field intensity on lightning protected installations with Ex zones to avoid upward leaders

Installations with explosion hazard zones (Ex zones) are to be protected so that an ignition or explosion of flammable atmospheres by lightning impact is avoided with high probability. A direct lightning strike into installations with Ex zones, mostly exhausts, must be prevented by air termination system and some measures against hazardous sparking caused by lightning currents, potential rises or induced voltages are to be implemented due to equipotential bonding. In addition, an ignition of Ex atmospheres can happen due to the energy of unconnected upward leaders, which emanate from exhausts. Therefore certain field intensity must occur on exhausts when a downward leader is approaching. The investigation is concerned with the extent to which the field intensity is reduced on exhausts by air terminations or shielding electrodes, so that upward leaders are unlikely there. Analytical and numerical 3D field calculations show that the shielding effect by installed lightning rods and horizontal conductors is too low. Shielding electrodes of appropriate dimensions can cause an effective field reduction.