Dead end / horizontal insulator strings

[sparky1976]

All,

I've read the REA Bulletin 1724E-200 about design manual for high voltage trasmission lines. Page 8-2 on 8.2.3 "Deadends : In situations where the insulator string is in line with the conductor , the number of bells should be two more than what was used for tangent structure".

I try to find the reason of that additinal 2 bells for horizontal strings.

In my experince the vertical insulator is more prone to flash over than horizontal because the dirt on horizontal is easier to wash by rain than the vertical string.

Some one told me that for the practical reason, because the work on horizontal string is hard/complicated (its more complicated if you do it hot) than to work on vertical strings, so if one bell damage on horizontal we still can say it still safe.

please advise.

regards
Pitat

 

[cuky2000]

In a transmission line often conductor is routed down or around the insulator string in a dead-end or in a heavy angle structures.

These arrangements modify the field around to the insulator compromising the dielectric strength of the insulator string. Reinforcing this weak point with additional insulators is intended to maintain the minimum designed dielectric characteristic (BIL/BSL) with hope to minimize flashover in the event of surge overvoltage caused by traveling wave.

 

[jghrist]

Lightning surges can double at deadends because of surge wave reflections. The extra insulators increase the BIL (Basic Impulse Insulation Level).