Mercury Contactors

When ordering specify:

  • Number of poles

  • Load in amps

  • NO or NC (all NC units are filled with Hydrogen and may be used for either an Inductive or Resistive load.  NO units are always filled with Argon and are for Resistive loads ONLY, unless otherwise specified.)

  • Coil voltage (voltage and ac or dc)

  • Inductive or resistive load

  • (p/n example: 360NO-120A means 3 pole, 60 amps max, NO, 120VAC coil)

Mercury Contactors vs Solid State Relays

 Mercury contactors or mercury displacement relays (MDR):

A mercury contactor uses a pool of mercury, inside a glass sealed tube, to electrically connect the two contacts.  This pool may be more than 100 grams of mercury for a single contactor.  Since mercury is a cancer causing agent and is banned in many states, countries, or industries; there are restrictions on shipping, storing, or cleaning up if a contactor should leak or explode. A mercury contactor is not a zero crossing device so there will be noise spikes or surges that may affect nearby electrical products or PLCs.  An ideal on/off cycle time for a mercury contactor is often 20-30 seconds and 3-8 million overall cycles.  Mercury contactors were used frequently for industrial heater control of resistive heaters in the 1960-2000 time period.   When used 24 hours a day in a heater application, there may be 3 million on/off cycles in a single year.  Therefore, the mercury contactors might be replaced many times during the life of the machine. 

Solid state relays (SSR) or Solid state contactors (SSC):

The SSR is more reliable, will last longer (no moving parts), and will make more intelligent decisions. Most controllers can be set to a cycle time of one second or less (for more accurate temperature control) but the mercury contactor won’t be able to keep up with the rapid cycling.  The mercury contactor works best with a 20-30 second cycle time and the SSR works extremely well with a one second or less cycle time.  The shorter cycle time will actually make your heaters last longer, because they won’t experience the large degree of thermal expansion and contraction as they would with a 20-30 second cycle time.

For more information please contact Rick


  at (816) 468-7577