We analyze the insulation coordination of arcing horns on the earthing lines of a ±800 kV

5 000 MW UHVDC system. Moreover

we experimentally study the arc extinction characteristic of arcing horns

and introduce U-I characteristic method to investigate the protection region of the arcing horns. The influences of line parameters and arc elongation are analyzed in detail as well. The discharge gap distance of the arcing horns is from 400 mm to 1 500 mm. A pulse-wave generator is chosen as the power supply. The maximum output pulse width is 70 ms

and the peak current is 2 500 A. The results indicate that the arcing horns have limited fault clearing efficiency

especially on long distance and high capacity UHVDC earthing lines. For example

the arcing horns of 1 500 mm discharge gap distance can only protect 31% of the UHVDC earthing line. The fault clearing efficiency is significantly improved with the decrease of the line resistance

but slightly improved with the increase of the tower footing resistance especially at a higher line resistance. The protection region of the UHVDC earthing line increases by 9% even if the tower footing resistance rises from 5 ? to 15 ?. On the other hand

due to the effect of magnetic and thermal buoyancy forces

the actual length of the arc is much longer than the discharge gap length（e.g. the maximum length of the arc for 1 000 mm discharge gap distance is 1 600 mm）. Hence

the effect of arc elongation should be taken into consideration. Since the UHVDC earthing line will provide more power than experimental circuit

the arc will keep burning and elongating. That means the practical performance of the arcing horns is more optimistic.