Contactor Faults

Contactors are the only moving part in a battery pack and although simple there are a number of contactor faults that will stop the operation of the battery pack.

contactor schematic
Introduction to Contactors

There are two main types of contactors: Normally Open (NO) and Normally Closed (NC). A NO contactor does not allow current to flow when the actuation circuit is not powered. However, an NC contactor connects the external circuit when the contactor actuation coil is not energized. Therefore, we use EV contactors as NO to switch off the circuit in case of loss of actuation circuit supply.

The faults are typically:

  • permanently closed
  • soft welding
  • permanently open
  • overheating
  • overheating of actuator coil

Although contactors are quite a simple part, the pathway to these faults can be complex.

There are a number of reasons for contactor failure:

  • Poor contactor design
  • Manufacturing quality issues
  • Incorrect sizing and selection
    • electrical
      • LV and HV side need to be considered
      • short circuit current- only want fuse to blow and contactor or other components to not need to be replaced
    • thermal sizing
    • mechanical vibration and shock loads
  • Incorrect orientation
  • Incorrect design of associated busbars and fixings
  • Incorrect control
  • Incorrect LV supply
    • overheating of actuator coil due to long term low voltage supply

This post has been built based on the support and sponsorship of: AVANT Future MobilityQuarto Technical ServicesTAE Power Solutionsh.e.l group and The Limiting Factor. 

Actuator Failure

This can be sub-divided into the elements:

  • Coil failure
    • breakdown of enamel coating
    • spike in low voltage system
  • Iron core failure
    • poor quality laminations
    • foreign matter between layers
  • Armature failure
    • armature stuck
      • poor design and tolerance stack
      • foreign body included in manufacture

Contactor Failure

The contactors carry the current and normal operation is for them to operate under no load. However, in extreme circumstances they are required to open under electrical load, if the contact surfaces become damaged (eg due to arcing) they can then increase in resistance and hence heat generation. Also, the contacts can become welded together due to arcing.

  • Pitting
    • electrical loads too high
    • closing onto high loads
    • opening under high loads
    • high vibration input causing bouncing
  • Arcing
    • loss of quenching gas
    • prior damage of surface
    • contamination of gas
  • Deposits
    • carbon deposits can build and lead to arcing, melting and then welding
  • Incorrect operation
    • opening too soon or closing onto a high load due to poor control
    • opening due to interrupted low voltage power supply

Case Failure

Some contactors contain a gas that quenches any arc formation. Hence it is important that the gas does not leak from the enclosure. The structure of the contactor also needs to remain intact to maintain the operation of the contactor and safe isolation of parts. Some possible failures:

  • Thermal
  • Vibration
  • Mechanical stress
    • shock input from crash
    • over-tightening of mechanical fixings
    • flexure of base system
    • over heating of busbars and hence expansion inputting high force into terminals


  1. What are the common faults of relays?, QuiSure
  2. Relays in safety-related control systems, Electronic Specifiier
  3. US Military Relay Failures
  4. Finding Relay Failures, Pickering

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