Contactor seal-in circuit
A contactor holding circuit remains on after a brief start pulse because the contactor's own normally open auxiliary contact maintains its coil-supply path. That contact connects in parallel with the start button. Stop and required protective contacts remain effective in the common release path.
The contactor is held electrically, not without a power supply. If its coil supply fails for long enough, an ordinary contactor releases. Self-holding is neither an uninterruptible supply nor mechanical storage of the ON state.
Construction and operation
Power circuit and control circuit
The power contacts switch a suitably rated load, such as a motor. The coil operates the contacts, while the control circuit determines when the coil receives power. Control voltage need not equal load voltage, but the control circuit is not automatically safe extra-low voltage.
The basic example needs a contactor with a suitable NO auxiliary contact, an NO start button and an NC stop button. A separate auxiliary relay is not inherently required for this function.
A normally open contact is open in its unoperated reference state; a normally closed contact is closed. Identify functions from the actual device. Conventional markings such as 13–14 for an NO auxiliary contact and 21–22 for an NC auxiliary contact help explain the drawing but do not replace checking the equipment diagram.
What happens when start is pressed?
Pressing start completes the permitted coil path. The contactor operates, its power contacts switch and its own NO auxiliary contact closes. That contact is connected across the start contact.
After releasing start, the closed auxiliary contact continues to supply the coil. The circuit bypasses the start button, not the power source or the stop path. Ordinary electrical holding still needs a supply.
How stop releases the circuit
The NC stop button is in the common coil path. Pressing it opens that path. The contactor releases and its holding contact opens as well. With start released, letting go of stop leaves the contactor off until another start command is given.
Circuit diagrams and explanations
The existing drawing identifies K1 as the contactor, S1 as stop and S2 as start. K1's NO auxiliary contact, marked 13–14, is connected across S2. A1 and A2 identify the illustrated coil terminals. Actual voltage, current type and any polarity requirements must match the coil.
The left-hand power circuit contains F1, K1 and the motor. On the right, control power passes through F2 and S1 to the parallel start and holding contacts. In this drawing F2 is a control fuse, not a motor-overload relay.
The simplified principle does not show a complete motor-overload device and its trip contact. A real motor application must provide or verify the protective arrangement required by its selected design. Emergency stop and other safety functions are not established by this basic drawing either.
Operating states
| Situation | K1 coil | Holding contact | Expected behaviour |
|---|---|---|---|
| Supply available, no start yet | Off | Open | K1 has not switched the load on |
| Start pressed, stop path closed | On | Closes | Contactor operates |
| Start released | On | Closed | Holding path maintains coil supply |
| Stop pressed | Off | Opens | Holding action released |
| Supply absent long enough for release | Off | Opens | No continued operation through self-holding |
| Supply restored, start released, no other start path | Off | Open | A new start command is required |
The final statement depends on those conditions. A held start button, a sticking contact or another maintained command can change restart behaviour.
Additional drawings: holding path and states
The new control-only drawing uses S0 for Stop and S2 for Start. F0 is control protection and F1 the intended overload NC contact. These identifiers belong to this additional drawing and must not be confused with F2 in the existing combined example.
The highlighted path after Start is released passes through K1's closed auxiliary. S0 and F1 remain effective ahead of it. Colour identifies the analysed state, not actual conductor colour or proof of isolation.
The state sequence shows Ready, Start, Hold and Stop. K1 remains off after Stop only with Start released and no other command path. This is not an unconditional safety-rated restart inhibit.
Seal-in: Start, hold, Stop
Seal-in after releasing Start
Seal-in: state sequence
Existing supplementary drawing
The following original drawings are retained. Their identifiers are explained separately from those of the new examples.
Wiring principles and selection
Wiring and extensions
The key principle is that the holding contact bypasses start only. It must not bypass stop, a required protective contact or another shutdown condition. The common release path must interrupt both the start branch and the maintained branch.
In the basic arrangement, equivalent start buttons connect in parallel. Multiple effective stop buttons use NC contacts in the common series path. This explains the control logic; it is not a complete design for arbitrary numbers of control stations or safety-related functions.
An overload relay can open the coil path through its intended trip contact. An ordinary overload relay does not automatically provide short-circuit protection. A motor-protection circuit-breaker may provide several protective functions, depending on its construction; both devices are not always required together regardless of the design.
Selection depends on load, switching frequency, contact ratings, coil specifications and protection coordination. An auxiliary contact is not automatically suitable for motor current. Likewise, a 24 V coil must not be connected to 230 V merely because its power contacts can switch higher voltages.
Connection work requires appropriate competence and authority, following the verified drawing. Apply the five safety rules even if stop has already switched the motor off. An ordinary stop command does not establish safe electrical isolation.
Functional checks and verification
Following inspection and the required electrical tests, check the sequence with load and machinery hazards controlled.
- Operate start: K1 must pick up under the intended conditions.
- Release start: K1 must remain operated through its own holding contact.
- Press stop: K1 must release and its holding contact open. With start released, releasing stop must not cause a fresh start.
- Check every additional intended stop and protective-release function using suitable procedures.
- Check required supply-loss and restoration behaviour, including any additional command paths.
Humming, chattering or repeated dropout is not successful operation. Possible causes include an unsuitable coil supply, connection problems or an ineffective holding path. The symptom alone does not identify a single failed component. Isolate before examining connections.
Advantages and limitations
Benefits depend on suitable design. Contact duty, protection and behaviour after interruption must match the application.
Advantages
- A brief start command can maintain the intended operating state.
- Suitable standard contacts provide an understandable basic function.
- Loss of coil supply releases ordinary electrical holding; under the stated conditions, a fresh start is then required.
Limitations
- The coil continues to require power while held.
- Contact faults and additional command paths can alter expected stopping and restart behaviour.
- Multiple holding and permissive paths need clear documentation so that no shutdown condition is bypassed.
- This principle does not replace motor protection, short-circuit protection or a verified safety-related prevention of unexpected restart.
Questions and related guides
Common questions
Is a holding circuit the same as an impulse relay?
No. In this circuit the ordinary contactor coil remains energised. An impulse relay changes its intended state in response to pulses and uses a different device concept. Mechanically latched contactors are different again.
Why does the contactor release as soon as start is released?
The holding path may be missing or ineffective, although other faults are possible. Diagnose using the actual diagram and appropriate tests, not by bypassing stop or protective contacts.
Is the stop button also an emergency stop?
That does not follow from this drawing. Any required emergency-stop function is designed and verified as part of the machinery safety concept. An ordinary coil-circuit button alone does not establish it.
Holding is also used as part of reversing contactor circuits and star–delta starting.
Related guides
Contactor control with hold-to-run operation · Contactor control from multiple stations
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.