Choosing a lighting circuit
Choose a lighting circuit by describing the required operation first. How many positions should control the same lighting? Are there two independent groups? Should the light stay on or switch off after a period? Only then assess which arrangement fits the installation.
More components do not automatically make a better solution. This comparison explains conventional functions and the questions worth answering before a change. It supports learning and planning rather than authorising connections in an existing installation.
Compare the main functions
| Required function | Typical arrangement | Distinguishing feature |
|---|---|---|
| One lighting group, one control position | One-way switching | One contact controls the group |
| Two groups at one position | Two-gang switching | Two independent functions, not lamps connected electrically in series |
| One group from two positions | Two-way switching | Two changeover switches and two travellers |
| One group from three or more positions | Intermediate switching | Changeover switches at the ends, intermediate switches between them |
| Multiple momentary controls, maintained state | Impulse-relay circuit | Push-buttons operate a state-holding device |
| Push-button operation with timed switch-off | Staircase timer | Timer function and product-specific modes |
| Visible indication of the switched state | Suitable indicator arrangement | Indication must match the circuit topology and purpose |
The alternative two-way circuit is a particular two-position arrangement. Whether it saves conductors depends on the real routing and required functions. Its name is not enough to establish a saving or approve its use.
One lighting group or two?
A lighting group can contain one or several suitable luminaires that operate together. The common control function matters more than the number of individual lamps.
For two independent groups at one position, two-gang switching is a straightforward option. If both groups also need operation from multiple positions, one two-gang switch alone cannot provide the complete arrangement. Each group's control must be planned accordingly.
The German term Serienschaltung can be misleading. In this installation context it means two switching functions in one control position. It does not describe lamps connected electrically in series. English terms also vary by country; this category explains the depicted function explicitly.
How many control positions?
A conventional two-way circuit is an understandable choice for two positions. An intermediate circuit extends the arrangement to three or more. Each intermediate switch connects the two traveller paths straight through or crossed over.
A push-button circuit with an impulse relay puts the maintained switching function in a control device. Push-buttons provide suitable commands. This can be convenient with several positions, but introduces another component and its installation requirements.
There is no universal number of positions at which one solution always becomes mandatory or cheaper. Cable routes, existing wiring, space, loads and future changes all influence the decision.
Maintained light or timed light?
A basic impulse function changes between on and off for each valid pulse. A staircase timer instead switches on for a defined period. Retriggering, early switch-off, permanent-on modes and advance warning are additional product-dependent functions.
Describe the actual use rather than relying on a room name. A corridor may need maintained lighting. A short time period is unsuitable if users regularly stay longer without appropriate extension or other suitable control.
An ordinary staircase timer is not automatically an emergency-lighting or other safety system. Such functions need their own appropriate design.
Consider loads, existing wiring and usability
Lamp wattage alone does not establish switch suitability. LED drivers can create a different switching demand from an equally rated resistive load. Match the switch or relay to the actual lighting; a headline current marking does not replace the relevant manufacturer's load information.
For an existing installation, establish the available conductors and their functions. A protective conductor must not become an extra switched core. Borrowing a neutral from another circuit is not a shortcut to a new function.
Consider access and clarity as well. Can users reach the controls? Is the expected response understandable? Does suitable manual operation remain available? An indicator may help explain state, but it does not verify electrical safety.
Questions to settle before choosing
- Which luminaires should operate together and which independently?
- What response is expected at every control position?
- Is automatic switch-off required or unwanted?
- Which existing equipment and routes can be retained appropriately?
- What load, installation and verification conditions apply to the selected products?
A state table can establish the intended function before installation. Mains work requires the necessary competence and appropriate verification. The switching-device guides explain individual components; these circuit guides explain how they work together.
Where do fluorescent circuits fit?
The inductive, capacitive and DUO guides explain conventional fluorescent equipment. Their question is different: how do the lamp, ballast and associated components operate together?
They therefore form a separate section. Those explanations do not establish universal LED-retrofit compatibility. Any conversion depends on the actual luminaire and an appropriate approved replacement arrangement. These English articles retain the technical and national context of the original site rather than silently becoming UK or US installation instructions.
Sources and further 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.