MHV connector
The MHV (miniature high voltage) connector is a type of RF connector used for terminating coaxial cable. Description 
The connector looks almost identical to a typical BNC connector, but is designed to not mate with BNC jacks. It features two bayonet lugs on the female connector; mating is fully achieved with a quarter turn of the coupling nut. The dimensions of the connector are specified in the MIL-STD-348B. MHV connectors can be recognized by the slightly protruding insulation on the male plug and the slightly different insulation length in the female jack. Use

The MHV connector is typically rated for 1500 VRMS continuous and 5000 V peak, and 3 amperes. With an operating frequency limited to about 300 MHz their usage is restricted to low frequency applications. It was commonly used in laboratory settings for voltages beyond the rating of BNC connectors. Other applications include:

Geiger counters
Scintillation probes
Transmission lines
High-voltage power supplies
Nuclear control instrumentation
X-ray devices

Safety hazards

MHV connectors are a safety hazard because the central pin is close enough to the end to allow accidental contact with it when the connector is not plugged in, and because the ground connection is broken before the power connection when demating. An additional problem is that MHV connectors are easily mistaken for BNC connectors by users who are unaware of the difference, and MHV and BNC plugs and jacks can be made to mate by brute force. This causes safety hazards, since a user can mate a low voltage cable to a high voltage jack. SHV connectors are designed to prevent these hazards, and cannot be mated to BNC connectors. References

External links 
MIL-STD-348B

RF connectors