Because they are far more resistant to damage due to overcurrent or back-voltage, ignitrons are still manufactured and used in preference to semiconductors in some installations. For example, specially constructed "pulse rated" ignitrons are still used in certain pulsed power applications. These devices can switch hundreds of kiloamperes and hold off as much as 50kV. The anodes in these devices are often fabricated from a refractory metal, usually molybdenum, to handle reverse current during ringing (or oscillatory) discharges without damage. Pulse rated ignitrons usually operate at very low duty cycles. They are often used to switch high energy capacitor banks during electromagnetic forming, electrohydraulic forming, or for emergency short-circuiting of high voltage power sources ("crowbar" switching). Although the basic principles of how the arc is formed, along with many aspects of construction, are very similar to other types of mercury-arc valves, ignitrons differ from other mercury-arc valves in that the arc is ignited each time a conduction cycle is started, and then extinguished when the current falls below a critical threshold. In other types of mercury-arc valve, the arc is ignited just once when the valve is first energised, and thereafter remains permanently established, alternating between the main anode(s) and a low-power "auxiliary anode" or "keep-alive circuit". Moreover, control grids are required in order to adjust the timing of the start of conduction. The action of igniting the arc at a controlled time, each cycle, allows the ignitron to dispense with the auxiliary anode and control grids required by other mercury-arc valves. However, a disadvantage is that the ignition electrode must be positioned very accurately, just barely touching the surface of the mercury pool, which means that ignitrons must be installed very accurately within a few degrees of an upright position.