Timer switch with contactor: control higher loads
A timer switch and a contactor let you control higher loads on a schedule without passing the load current through the timer contact. The timer operates the contactor coil; the main contacts switch the load. Examples include suitably designed lighting groups, pumps and fans.
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.
Construction and operation
The circuit separates time control from load switching. T1 determines the switching state and K1 makes the power path. In this example, a single-channel timer operates the coil of a four-pole contactor. When the contactor picks up, its four main contacts close and supply the pump assembly. Opening the NO branch of the timer changeover contact releases the contactor and interrupts the illustrated load paths.
The timer remains supplied through L1 and N. The COM terminal of its changeover contact receives a separate L1 feed from the same supply. The timer supply must not be confused with its switched output. For general equipment functions, see the timer switch. The timer-switch circuit without an additional contactor provides a comparison using two directly switched lighting groups.
Circuit diagram: timer and contactor for higher loads
· OFF: COM–NC · ON: COM–NO. NC remains externally unused. Red is not a current measurement.
One changeover contact per channel. The illustrated load uses NO; NC is shown for explanation. Functional arrangement; use the actual manufacturer connection diagram.
Control circuit: from the timer to A1 and A2
NC, COM and NO are at the timer's upper right. COM receives L1; NO supplies switched conductor L′ to A1, while NC has no external wire. Closing the NO branch of the changeover contact connects the control path from L1 through the contact and L′ to A1. A2 is connected to N. A coil suitable for this control supply is energised and operates the contactor. A1 and A2 are coil terminals; they do not carry the pump load current.
Red highlights only the path connected to L1 in the control circuit. It already reaches the open timer contact when the timer is off. This does not mean that operating current flows through the interrupted coil circuit.
The timer's changeover contact: COM, NO and NC
The illustrated channel has three terminals. COM is the common terminal in the middle and receives L1. NO (“Normally Open”) is open with the channel OFF. NC (“Normally Closed”) is connected to COM in that position. ON connects COM to NO and disconnects NC.
NO leaves the timer upwards and runs to coil terminal A1; A2 remains connected to N. The contactor therefore picks up with the channel ON and supplies the assembly through its main contacts. NC has no external connection. With the channel OFF it is nevertheless connected to L1 through COM, so the interactive diagram marks it red. This is not a current measurement or a guarantee that another terminal is safe to touch.
What would change if the coil used NC?
Connecting a suitable coil to NC instead of NO would reverse control: the contactor could be energised with the channel OFF and released with it ON. This can implement timed inhibition rather than timed enabling where the application permits it. The illustrated coil stays on NO, so ON still means “pump running”.
This inversion does not replace a safety function, emergency-stop control or reliable prevention of restart. Actual behaviour on supply loss and restoration depends on the timer and contactor; NC alone does not guarantee it. The time programme, manual or permanent overrides and the required machine behaviour must be considered together.
NO and NC are alternative branches of the same channel, not independent control outputs. Each must be used within its permitted switching duty. Coil selection remains necessary. The physical arrangement is functional; follow the real manufacturer's diagram. Not all timer switches provide changeover contacts.
Main and load circuit: four contacts to the assembly
L1, L2, L3 and N run from the upper rails to the four main inputs. Their outputs lead straight to the pump assembly's terminal block. L1 remains brown, L2 black and L3 grey; closed load paths are shown darker and thicker. N is blue. The incoming conductors remain connected when the contactor is released.
PE bypasses the timer and contactor and runs directly to the assembly enclosure. It is not routed through a switching contact. The timer and coil neutral connections remain present; the diagram does not measure current or voltage.
Why does the pump assembly have a neutral terminal?
This learning model includes an auxiliary supply that needs N. N is explicitly not a fourth winding connection on an ordinary three-phase motor. A basic three-phase motor is commonly supplied through three line conductors and PE. The actual pump manufacturer's diagram determines the required connections. Four switched poles are not a universal requirement for motors.
Try the interactive model
Operate “Channel 1”: its state remains after you release the button. ON energises K1 and rotates the impeller; OFF releases the contactor and stops the wheel. The clock runs independently and is labelled UTC+1, or UTC+2 during summer time. No automatic time programme is simulated. Pausing the motion does not change the electrical switching state. “Static diagram” opens the ordinary enlargeable drawing, which also provides an alternative without JavaScript.
When is a timer switch alone insufficient?
A timer output has specified switching limits. A prominent current rating is not approval for every load. Operating current, load type, inrush and switching frequency must match the manufacturer's conditions. Different load types can have different limits, as illustrated by the separate lamp and load ratings in Theben's single-channel timer product data, in German.
A load that exceeds the timer contact's permitted switching conditions must not be connected directly through that contact. One solution is a suitably rated contactor between time control and load. Another approved load-switching device may suit a particular application. There is no universal wattage above which every timer always needs a contactor.
| Application | Decision criterion |
|---|---|
| Suitable small load | Direct switching can be sufficient if all specified switching conditions are met. |
| High inrush or a higher load | Compare timer and load data; use a suitable contactor or another load-switching device where required. |
| Several conductors switched together | Determine the necessary main contacts and permitted switching arrangement for the installation. |
A seemingly small LED group can be more demanding at switch-on than its operating power suggests. Conversely, a high power rating alone does not establish suitability for motor switching. The decision follows from the actual equipment and load data.
Where are timers with contactors used?
- Lighting: jointly controlled groups whose load or inrush requires a suitably selected switching device.
- Pumps and fans: scheduled operation where the equipment and process permit this switching method.
- Heating loads: timed enabling of suitable equipment; temperature control and independent protective functions remain necessary.
For equipment with its own electronics, first establish whether its supply may be switched externally at regular intervals or whether a designated control input should be used. A timed enable does not replace operating protection, such as required dry-run or overtemperature protection.
Which specifications need to match?
Main contacts and the actual load
The contactor must suit the load and switching duty. Predominantly resistive loads and motor operation use different utilisation categories. AC-1 and AC-3 are not interchangeable ratings: AC-3 includes starting squirrel-cage motors and switching them off during normal running. See Schneider Electric's explanation of AC-1 and AC-3. Other operating modes require their own assessment.
Contactor coil and timer contact
The illustrated control circuit is supplied between L1 and N. The coil must be rated for that supply; its voltage cannot be inferred from the three-phase load side. Coil consumption, energising duty and any manufacturer-specified suppression must also be compatible with the timer contact. The contactor removes the main load current from the timer output, but it does not make that output unlimited.
Protection and neutral switching
A contactor does not replace overload or short-circuit protection and is not automatically a suitable means of isolation. Motor protection, wiring, protective devices and the switching device must be coordinated. Where N is switched, the permitted arrangement and any required contact sequence must be considered. The model moves all four contacts together and does not simulate a product-specific sequence.
The drawing shows a functional principle. Selecting, connecting and verifying a real installation require appropriate electrical planning and the requirements applicable at the installation site. No generic conductor sizes, fuse ratings or interchangeable Austrian, German and Swiss rules are specified here.
Functional test and typical fault symptoms
In the model, check both states, the related colours and the rotating wheel. For the real installation, also check clock time, weekday, programme and operating mode. A successful manual test does not verify the time programme or electrical safety.
| Channel | COM–NO | COM–NC | Contactor / main contacts | Pump |
|---|---|---|---|---|
| OFF | open | closed | released / open | stopped |
| ON | closed | open | energised / closed | running |
- The timer indicates ON but the contactor does not pick up: possible causes include a missing contact feed or a problem in the relevant coil circuit. The display alone does not prove the voltage at A1/A2.
- The contactor picks up but the pump does not run: the load supply, protective devices and assembly require appropriate testing. Mechanical operation does not establish that every power path is sound.
- The load switches at the wrong time: check settings and active manual or permanent overrides. Repeated switching can also originate from another control or protective function.
Benefits and limitations
The roles are clearly divided: the timer makes the time decision and a suitable contactor switches the load. Several required main contacts can operate together. This adds space requirements, coil consumption, possible switching noise and extra components. It does not increase the permitted connected load of the existing installation.
Automatic operation must also suit the process when power returns or an unintended restart occurs. A contactor alone is not a complete safety control system. For direct control without a time programme, explore the other contactor circuits.
Sources and further reading
Further references: Schneider Electric on contactor selection, Fuji Electric on load types and switching duty, and Grundfos as an example of three-phase pump supply. These sources support the technical explanation; the illustrated assembly and its neutral-fed auxiliary supply do not reproduce any of those product diagrams.