Interlocking and motor protection
Interlocking, overload protection and short-circuit protection perform different jobs. An interlock prevents specified combinations. An overload device responds to excessive thermal duty according to its design. Short-circuit protection must handle the associated fault conditions.
Successful interlocking therefore does not prove complete motor protection. An overload relay also does not prevent every forbidden contactor combination.
Construction and operation
Electrical interlocking
In simple mutual interlocking, a suitable K2 NC auxiliary is in the K1 coil path and a K1 NC contact is in the K2 path. Operating K1 opens its auxiliary in the opposite path.
Allocation matters. An NC contact of the same contactor has a different effect from the opposing contactor's contact. Holding branches must not bypass the interlock. The new diagram shows its shared position.
Button interlocking using associated NO and NC button contacts is a different function. It can create a different command sequence but does not generally replace mutual contactor interlocking.
Mechanical interlocking
A suitable mechanical interlock acts between the contactors to prevent simultaneous closing within its designed function. It does not replace correct electrical commands or assessment of every other failure.
Follow the actual starter manufacturer's interlocking requirements. Adjacent contactors do not automatically have an effective mechanical interlock fitted.
Welded main contacts and auxiliary-contact behaviour need separate consideration. Ordinary auxiliary indication is not automatically safety-related position monitoring. Required safety functions need suitable devices, architecture and validation.
Overload and short circuit
| Task | Example | Limitation |
|---|---|---|
| Detect motor overload | Suitable thermal or electronic overload device. | Not automatically complete short-circuit protection. |
| Handle short circuit | Suitable fuse or designated protective device. | Coordinate with the installation and starter. |
| Switch the motor | Appropriately rated contactor. | Switching does not itself provide protection. |
| Prevent forbidden combinations | Electrical and, where required, mechanical interlocks. | Not a complete machine safety assessment. |
A manual motor starter may combine functions. It and a separate overload relay are not always both required regardless of design. Use the documented combination.
For star-delta starters, overload settings also depend on position. A device in the line path measures different currents from one in a winding branch. Do not apply a commonly quoted multiplier without establishing the arrangement.
Resetting and restarting
Protective equipment may reset manually or automatically under defined conditions. Resetting does not necessarily remove the cause. Control-circuit restart behaviour is another question.
Ordinary seal-in drops out after sufficient supply interruption when Start is released. A maintained command or other automatic path can change this. The name seal-in does not establish a required safety-related restart inhibit.
Stop is not automatically emergency stop. Required emergency-stop operation, reset and subsequent starting need a defined machine design. This guide does not provide a universal single-channel safety circuit.
Circuit diagrams and explanations
Mutual electrical interlocking
Switching, overload and short circuit
Wiring principles and selection
Wiring and checks
Define permitted states and transitions first. Assess every holding, start and remote path against common stop and interlocking conditions. An added parallel contact can change the protective logic.
Use appropriate test methods with the hazard area secured. Do not try forbidden states by shorting circuits or bypassing real protective functions. Follow intended product testing procedures.
Document results. A blocked opposing command proves the tested relationship, not every safety property. Electrical verification and required machine-safety validation remain separate.
Advantages and limitations
Benefits depend on suitable design. Contact duty, protection and behaviour after interruption must match the application.
Advantages and limits
Separating tasks improves selection, investigation and later alteration. It allows coordinated design instead of adding plausible-looking symbols.
Unknown motor data, equipment combinations or safety requirements remain genuine gaps. Do not fill them with guessed settings.
Questions and related guides
Common questions
Can overload settings be raised to avoid trips?
Not without investigating the cause and reassessing the design. Tripping may indicate unacceptable duty or a fault.
Is one NC auxiliary always enough?
That depends on the combination and application. Product instructions and the protective design may require more.
Why might protection fail to stop a contactor?
Possible causes include a disconnected trip contact, a bypass path or equipment failure. Check the actual circuit and function.
Related guides
Sources and documentation
Electrical safety: scope, competence and precautions
Put electrical safety first.
- These explanations and simplified diagrams support learning. They are not complete installation designs, site-specific risk assessments or a substitute for competent professional assessment.
- Electrical work can be life-threatening. It requires the competence and authorisation appropriate to the actual task, installation and jurisdiction.
- Errors and omissions can occur. Check the actual equipment documentation, applicable requirements and the complete installation before relying on an example; a working device is not proof of electrical safety.
- Personal protective equipment must suit the assessed hazards and task. It does not replace isolation, appropriate tools, competence or a safe system of work.
- Do not work on exposed live parts on the basis of these guides. Identify every relevant source, isolate it, secure against reconnection and prove dead using suitable equipment and procedures before access.
- Responsibilities for design, installation and verification must be established for the actual work. A website statement does not determine liability or remove duties under applicable law.
- This summary is not exhaustive. Where the circuit, condition or protective arrangement is uncertain, obtain suitable professional assistance before work proceeds.
- Complete the required inspection, testing and documentation before use, and maintain the installation and equipment appropriately.
Electrical terms explained
Electric current
The rate of flow of electric charge, measured in amperes (A). Conventional current direction follows positive charge flow, which is opposite to electron drift in a metallic conductor.
Voltage
The electric potential difference between two points, measured in volts (V). Always identify the reference points, such as line-to-neutral or line-to-earth; a voltage is not a property of one isolated terminal alone.
Resistance
The opposition associated with resistive current flow, measured in ohms (Ω). For a component behaving as an ohmic resistance under the stated conditions, U = IR.
Electrical power
The rate of electrical energy transfer, measured in watts (W). P = UI applies to steady DC; in a sinusoidal single-phase AC circuit, active power is UI cos φ using RMS values. The general average depends on voltage and current waveforms.
Ohm's law
For an ohmic resistance, current is proportional to the applied voltage: I = U/R. Resistance can vary with temperature, and non-linear devices do not necessarily obey one constant-resistance relationship.
Kirchhoff's laws
The current law expresses charge conservation at a circuit node. The voltage law states that the algebraic sum of voltages around a closed loop is zero within the applicable circuit model; induced effects must be represented appropriately.
Direct current (DC)
Current that maintains one direction. Its magnitude may be constant or vary; pulsating DC is still distinguished from current that reverses direction.
Alternating current (AC)
Current whose direction reverses with time, commonly periodically. Public supplies normally use approximately sinusoidal AC, while electronic equipment can produce other waveforms.
Frequency
The number of cycles per second of a periodic quantity, measured in hertz (Hz). Many European public supplies operate at a nominal 50 Hz.
Phase difference
The relative position of periodic quantities of the same frequency, often expressed in degrees or radians. For sinusoidal quantities, a time displacement can be related to that phase angle.
Circuit diagram
A drawing representing circuit components and their electrical connections using symbols. It is distinct from a physical layout or a complete installation design.
Capacitor
A component that stores energy in an electric field. Its capacitance relates stored charge to voltage under the stated conditions; a practical capacitor also has voltage, current and other operating limits.
Strappers in lighting circuits
The pair of conductors linking the changeover contacts in conventional two-way lighting switching, including intermediate switching where used. They carry the alternative switching paths and are not transformer windings; identify their actual functions from the circuit.
Coil or winding
A conductor arranged in turns, commonly to produce magnetic effects. Its behaviour includes inductance as well as the resistance and other properties of the actual winding.
Inductance
The relationship between flux linkage and current in the applicable model, measured in henries (H). Changing current produces an induced voltage; for a constant inductance its magnitude relates to L di/dt with the appropriate sign convention.
Capacitance
The relationship between stored charge and voltage, measured in farads (F). For a constant capacitance, changing voltage produces current according to i = C dv/dt.
Transistor
A semiconductor device used for switching, amplification or other control functions. Its operating principle and control quantities depend on whether it is, for example, a bipolar transistor or a field-effect device.
Semiconductor
A material whose electrical behaviour can be controlled through composition, electric fields, light or other conditions. Semiconductor devices use this behaviour for functions such as rectification, switching and amplification.
Diode
A device with a strongly directional current-voltage characteristic. Real diodes have forward losses, reverse leakage and limits; they do not act as perfect one-way valves at every voltage.
Relay
A device that changes an output in response to a control input. An electromechanical relay uses a coil and contacts, while a solid-state relay uses electronic switching elements.
Switch
A device that makes, breaks or changes electrical connections. Its rating, contact arrangement and permitted duty determine the applications for which it is suitable.
Electrical network
An interconnection of electrical components and branches. Series, parallel and more complex arrangements can be analysed with an appropriate circuit model.
Transformer
A device transferring energy between magnetically coupled windings through changing flux. Turns ratio influences voltage and current relationships; losses, limits and any conductive connection depend on its construction.
Electromagnet
A magnetic arrangement energised by electric current, commonly using a coil and magnetic core. Contactors, relays and actuators use electromagnetic force for mechanical operation.
Impedance
The relationship between voltage and current phasors in a sinusoidal steady-state model, measured in ohms. It combines resistance and reactance and can vary with frequency.
Short circuit
An unintended low-impedance connection between points at different potentials. Current is limited by the complete source and fault-path impedance, not literally unrestricted.
Electric circuit
An arrangement of sources, conductors and components. Current flows where a complete conductive path exists, although a circuit can also be described in an open or disconnected state.
Series connection
Components in one unbranched path carry the same current. Their voltages add algebraically; the individual voltage shares depend on their electrical behaviour.
Parallel connection
Branches connected between the same two nodes share the same voltage. The branch currents depend on their loads and add algebraically at the connection points.
Fuse
An overcurrent protective device with an element designed to melt and interrupt the circuit under specified conditions. Its rating, characteristic and breaking capacity must suit the application.
Overcurrent protection
Protection against overload and short-circuit effects, commonly using fuses or circuit-breakers. A sensing relay may require a separate interrupting device; an ordinary RCCB does not provide this protection by itself.
Earthing
A deliberate connection of a system point or conductive part to earth. Protective earthing forms part of an electric-shock protection arrangement; it must not be confused with neutral, functional earth or a guarantee of zero touch voltage.
Frequency converter or variable-speed drive
Electronic equipment that produces a controlled output frequency, commonly with controlled voltage, to operate a suitable motor. Its pulsed output, protection and compatibility requirements differ from a direct mains supply.
Electromagnetic compatibility (EMC)
The ability of equipment to operate satisfactorily in its electromagnetic environment without causing unacceptable disturbance to other equipment. Both emissions and immunity matter.
Power loss
Energy dissipated per unit time, commonly as heat in conductors, cores, insulation or semiconductor devices. Ideal inductors and capacitors exchange stored energy without average loss; practical components have additional loss mechanisms.
Short-circuit current
The current resulting from a short circuit. Prospective fault current is assessed to select protective devices with adequate interrupting capability and to check thermal and mechanical effects.
Power factor
The ratio of active power to apparent power. For sinusoidal voltage and current it equals cos φ; with distortion it also reflects waveform effects. It is not the same as energy-conversion efficiency.
Insulation resistance
Resistance measured between specified conductive parts separated by insulation under stated test conditions. The test voltage, connected equipment and interpretation must suit the circuit; it is not a complete assessment of insulation performance on its own.