As ususal, looks like the best description made it into Wikipedia http://en.wikipedia.org/wiki/High-voltage_direct_current http://en.wikipedia.org/wiki/High-vol

As ususal, looks like the best description made it into Wikipedia http://en.wikipedia.org/wiki/High-voltage_direct_current http://en.wikipedia.org/wiki/High-voltage_direct_current The ABB breaker contains four switching elements, two mechanical (one high-speed and one low-speed) and two semiconductor (one high-voltage and one low-voltage). Normally, power flows through the low-speed mechanical switch, the high-speed mechanical switch, and the low-voltage semiconductor switch. The last two switches are paralleled by the high-voltage semiconductor switch. Initially, all switches are closed (on). Because the high-voltage semiconductor switch has much greater resistance than the mechanical switch plus the low-voltage semiconductor switch, current flow through it is low. To disconnect, first the low-voltage semiconductor switch opens. This diverts the current through the high-voltage semiconductor switch. Because of its relatively high resistance, it begins heating very rapidly. Then the high-speed mechanical switch is opened. Unlike the low-voltage semiconductor switch, which is only capable of standing off the voltage drop of the closed high-voltage semiconductor switch, this is capable of standing off the full voltage. Because no current is flowing through this switch when it opens, it is not damaged by arcing. Then, the high-voltage semiconductor switch is opened. This actually cuts the power. However, it only cuts power to a very low level; it is not quite 100% off. A final low-speed mechanical switch disconnects the residual current.