flash back in mercury arc rectifiers

Mercury Arc rectifier in now an obsolete technology.It is obsolete not because it is in efficient technology or because it may back fire if care is not taken but semiconductors are much cheaper and non fragile and easy to handle and are having longer life . They were earlier used for battery charging, electroplating,Arc welding and spot welding machines and HF Induction heating equipments. Grid controlled Mercury Arc rectifiers were used as high power relay and rectifier in HF generators.

The mercury arc rectifiers can conduct reverse when the anode attains higher temperature than the mercury cathode. For Thyratron ( grid controlled mercury arc rectifiers while starting if the cathode is not sufficiently pre heated they may back fire. This is the reason the rectifier is allowed to conduct only after the pre heating time. They should not be stored in any position other than vertical position as any mercury deposit on top side will cause back fire. This is the reason when they are used for first time they are pre heated for almost 8 to 12 hours and ensure that what ever mercury spilled on the glass is totally vapourised before initial firing to avoid any back fire.

If more information is required read the following link

http://www.electricstuff.co.uk/mercarticle.html