Auxiliary contacts and interlocking
Industrial Control · Lesson 7 · Advanced
Purpose
Use auxiliary contacts and interlocks to express machine states while accounting for contact failure and timing.
Learning objectives
- Trace main and control paths
- Select from declared utilization data
- Coordinate protection and control
- Identify dangerous failure states
Auxiliary contacts provide holding, indication, permissive and interlocking logic. Their drawn normal state is the device’s de-energized state, not necessarily the machine’s safe or process-normal state.
Core theory
A normally-open seal-in contact parallels a momentary start command; normally-closed stop, overload and permissive contacts interrupt the coil path according to the design.
Electrical interlocking commonly places the opposing contactor’s NC auxiliary contact in series with each coil. Mechanical interlocking can additionally prevent simultaneous main-contact closure.
Contact welding, delayed release, broken wires, common-cause supply faults and restart after voltage recovery must be considered. Ordinary auxiliaries are not automatically safety-rated or force-guided.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Normally open | Open when the referenced device is in its defined normal/de-energized state | Not applicable | Not applicable |
| Seal-in | Holding logic maintaining an energized command | Not applicable | Not applicable |
| Mechanical interlock | Physical mechanism preventing incompatible contactor positions | Not applicable | Not applicable |
Assumptions: All ratings are supplied fictional design data; The machine risk assessment and manufacturer instructions remain governing inputs.
- State change: The reverse NC auxiliary should open.
- Effect: The forward coil path is interrupted.
- Boundary: Verify mechanical interlocking and welded-contact failure behavior where required.
The electrical interlock blocks a normal forward-coil command, but does not alone prove every failure safe.
Reasonableness check: The result is checked against electrical duty, starting behavior and protective boundaries rather than accepted from one nameplate number.
- Using motor kW alone to select switching equipment
- Treating an overload relay as short-circuit protection
- Assuming a stopped motor or dark display is isolated
Where this appears in practice
Industrial motor control must coordinate the motor, switching device, short-circuit protection, overload protection, control voltage, driven load and machine safety functions.
Knowledge check
Does ‘normally closed’ mean closed while the machine runs normally?
No. Normal refers to the referenced device’s defined de-energized/unactuated state.
Answer: No. Normal refers to the referenced device’s defined de-energized/unactuated state.
Practical exercise
Translate a forward/reverse diagram into a state table including power loss, simultaneous commands and welded auxiliary/main contacts.
Summary
- Main and control circuits have different jobs
- Every rating has a declared duty and condition
- Protection, stopping and isolation are distinct functions
Sources and review
- IEC 60947-4-1:2023: Electromechanical contactors and motor-starters: IEC; 2023, corrected version 2026-03; International
- IEC 60204-1:2016+AMD1:2021: Electrical equipment of machines: IEC; Consolidated edition 6.1; International
- IEC 61800-5-1:2022: Adjustable-speed drive safety: IEC; 2022 with current corrigenda; International
Editorial review date: 2026-08-22. Professional electrical review is pending.