Electromechanical relay — Failure Modes & Failure Rate

Mechanical contact switched by a solenoid coil. Used wherever a low-power signal must control a higher-power circuit. Failure rate driven by contact wear from arcing — a function of switching current, voltage, and cycle count, not just calendar time.

λ typical

5.0×10-7 / h

Range

1.0×10-7 – 2.0×10-6

Source

NPRD-2016

Failure modes

Contact wear / pitting

Root causes: Arc erosion at make/break, especially with inductive loads; contact-material transfer (anode→cathode) shifts the surface roughness over millions of cycles.
Detection: Increasing contact resistance measurable at the terminal pair; intermittent operation under light load; sometimes audible arcing.
Mitigation: Spec relays with arc-suppression contact materials (AgSnO₂, AgNi); RC snubbers across inductive loads; size the relay to ~50% of its rated AC1 / DC1 cycle life expectation.

Contact welding (stuck closed)

Root causes: High inrush current from capacitive or motor-start loads; current exceeding rated make capacity for one cycle; debris bridging the contacts.
Detection: Output stays energised after the coil drops out; downstream equipment will not stop. Often only detected during proof test or fault.
Mitigation: Force-guided (positively-driven) contacts in safety-critical applications — IEC 60947-5-1 — so a welded primary contact mechanically prevents the auxiliary monitor contact from closing. Dual relays with diverse coils for SIL 3+.

Coil open-circuit

Root causes: Magnet-wire fatigue at the terminal; insulation breakdown and short to the core; corrosion of wire-to-pin junction in humid atmospheres.
Detection: Coil current drops to zero; relay does not pick on demand; ohm-out check at proof test.
Mitigation: Conformal coating on PCB-mount relays; encapsulated / sealed coils for harsh environments; coil-current monitoring on safety-critical channels.

Mechanical jam / sluggish armature

Root causes: Spring fatigue; corrosion of the pivot; foreign matter in the air gap.
Detection: Pick-up time exceeds spec; chatter; intermittent operation. Hard to detect without an active proof test.
Mitigation: Sealed (gas-tight) relay construction for high-reliability and SIL applications; periodic actuation to prevent stiction in standby duty.

Typical applications

Power switching for motors, heaters, solenoids, lamps; safety-relay outputs in SIS / E-stop circuits; load-shedding in power-distribution panels; logic-level isolation between PLC and field power.

How to model in a fault tree

For functional-safety work the contact-welding mode (fail-dangerous, stuck closed) usually dominates — model it as a separate basic event with its own λ_DU rather than lumping into a generic relay λ. Force-guided safety relays (per IEC 60947-5-1) explicitly defeat single-contact welding by exposing the failure on the monitor contact, so the FTA sub-tree gets a clear AND structure: two welded contacts must coincide for the dangerous top event. See Beta-factor CCF for the dependency model on dual-relay channels driven from the same coil supply.