Back flashovers

When lighming strikes a tower (or a GW) as illustrated in Figure 2.36, the lighming current flows into the tower and causes a sudden increase in tower voltage. When the voltage difference between the tower and a phase wire (PW) reaches its electrical withstand voltage, a flashover from the tower to the PW occurs. This is called back flashover (BFO), because... 

Lightning overvoltages in overhead power transmission systems are mainly caused by back flashovers (BFOs) of tower insulations. The electromagnetic field around a transmission tower when it is hit by lightning changes dynamically, while electromagnetic waves make several round trips between the shield wire and the ground. 

Structure of a 500-kV transmission-line tower analyzed using the MoM in the time domain. (Mozumi, T. et al., Numerical electromagnetic field analysis of archom voltages during a back flashover on a 500 kV twin-circuit line, IEEE Trans. Power Deliv. 18(1) 207-213. 2003 IEEE.)... 

A back flashover can occur when a lightning strikes the ground wire. In addition, a lightning strike can directly hit the phase conductor due to the shielding failure. In these cases, the lightning surge in the phase conductor can directly propagate into the cable core, which leads to the sheath... 

In another case, the MCB controlling the downstairs socket outlet ring main tripped after the bath, on the first floor, had been used. There was a defective joint in the waste pipe under the bath and polluted water from it ran down a hollow partition wall nearby and entered a socket outlet box recessed in the partition, causing tracking across the insulation between the terminals at the back of the socket outlet and again triggering a flashover when wet. 

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