Breakdown Voltage or Dielectric Strength, measured in Volts, is the voltage which may be applied across the leads of a reed contact in open condition, without causing contact damage, arcing, or causing excessive leakage.
When checking the breakdown voltage for a reed switch, the second breakdown is always lower and more stable than the first. Measuring the breakdown voltage of a reed contact is a destructive test and could erode the contact.
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to experience electrical breakdown and become electrically conductive.
For diodes, the breakdown voltage is the minimum reverse voltage that makes the diode conduct appreciably in reverse. Some devices (such as TRIACs) also have a forward breakdown voltage.