Switching Relays: Definition and Comparison with Time and Impulse Relays

Short Answer

An intermediate relay acts as the "muscles" of a control circuit: it receives a low-current signal at the coil and uses its contacts to switch another circuit, providing galvanic isolation. A time relay performs the same function but with a time delay: switching on after N seconds, switching off after N seconds, flashing, or managing a pause during motor starting. An impulse (step) relay changes the contact state with each short pulse to the coil—this is the standard solution for controlling lighting from multiple locations using momentary push-buttons.

The selection process comes down to four questions: which circuit are you switching (current and load type), how many contact groups are required and of what type (changeover CO, normally open NO, or normally closed NC), what is the coil supply (current type and control circuit voltage), and whether time-based or pulse-based logic is required. The most common errors occur when selecting the coil voltage or the contact current rating for inductive or inrush loads—this is discussed below.

Three types of relays: purpose and differences

All three devices are electromechanical or electronic switches, but they serve different purposes and should not be interchanged unnecessarily.

Parameter	Intermediate relay	Time relay	Impulse (step) relay
Purpose	Signal duplication and amplification, galvanic isolation between control and load circuits	Switching with time delays: turn-on/turn-off delays, cycles, star-delta motor starting	State switching per pulse: one press turns it ON, the next press turns it OFF
Contact state	Follows the coil: when power is applied, the contacts switch	Changes based on a timed count after a command	Latches after the pulse is removed (bistable)
Coil energisation	Continuous, as long as switching is required	Continuous or for the duration of a cycle, depending on function	Not required: the coil operates only during the pulse
Typical application	Controller output isolation, intermediate relay circuits, sensor contact duplication	Staircase timers, bathroom fan delays, motor starting, flashing cycles	Corridor and staircase lighting via push-buttons from multiple locations, long control lines
Typical contact currents (catalogue)	6–16 A (modular and miniature series), specific power versions up to 30–50 A	6–16 A	10–16 A

Intermediate relay: contacts, current, and coil

Current and number of contact poles

The most common rated contact currents in the catalogue are 10 A and 16 A, followed by 8, 12, and 6 A; power series can reach up to 30–50 A. It is vital to remember: the rating is specified for resistive loads. For contactor coils, solenoids, motors, and LED drivers with capacitive inrush, the actual permissible current is significantly lower — ensure you include a safety margin or select relays with AgSnO2 / AgCdO contacts designed for inductive and inrush currents.

Regarding the number of poles, the standard range includes 1 to 4 changeover (CO) contacts: 1 and 2 CO configurations are most common, while 3 and 4 CO versions are available in high-volume series. In addition to changeover contacts, purely making (NO) and breaking (NC) groups are used. The selection rule is simple: count how many independent circuits need to be switched by a single signal, and select a relay with that number of poles plus, ideally, one spare.

Coil voltage and current type

The coil is a separate circuit with its own rating, and this is where errors most frequently occur. In the switching relay catalogue, DC versions are often even more prevalent than AC versions. Typical DC coil voltages include: 24 V (the most common), 12, 48, 110, and 220 V; for AC, typical values are 24, 110, 230 V, and various intermediate values. A 230 V AC coil will not function correctly with 24 V DC, and vice versa: always verify both the current type and the voltage rating. For PLC and sensor circuits, 24 V DC is the de facto standard; for intermediate relays in substation auxiliary circuits, 110 V or 220 V DC is standard.

Three typical configurations from the catalogue for comparison — a miniature plug-in relay, a relay with a terminal block, and a 220 V DC intermediate relay:

Time relays: functions and ranges

A time relay is a switching relay combined with a timer. Key characteristics include the set of functions, time delay ranges, the number of output contacts, and the supply voltage.

Which functions are actually required

• On-delay — the most common function, frequently found in time relays. Applications include delayed ventilation startup or stepped switching of load groups to prevent simultaneous inrush currents.
• Off-delay — the contact remains closed for N seconds after the command is removed. A classic example is a bathroom extractor fan that continues to run after the light is switched off.
• Pulse generation and cycling (flashing starting from a pulse or a pause) — periodic switching used for flushing, light signalling, or cyclic purging.
• Star-delta — a specialised function for starting asynchronous motors by switching windings; such relays are specific items in a catalogue rather than a standard feature of any timer.

The delay ranges for multi-range modular timers are very broad: for example, a single-function modular timer from the Finder 83 series offers adjustment from 0.05 s up to 10 days, while compact stairwell timers cover seconds and minutes. In certain catalogue segments, upper limits reach hundreds of thousands of seconds (several tens of days for weekly/daily digital relays). A separate class consists of digital daily and weekly programmable timers: these do not operate based on an event delay, but rather via calendar-based control (e.g., street lighting, shop windows, or scheduled heating).

Analogue or Digital

Analogue models (using a potentiometer or scale) are cheaper and simpler: they typically offer one function and one range, which is sufficient for fan delays or stairwell timers. Digital models provide precise settings, multi-functionality, and weekly programming. Both classes are available in catalogues: analogue and digital time relays, as well as modular versions for DIN-rail mounting in distribution boards.

Impulse (step) relay: lighting controlled from multiple locations

An impulse relay is bistable: a short pulse from a push-button toggles the contacts, which then remain in that position without requiring the coil to be held. This offers two practical advantages over two-way switch wiring or standard relays:

1. Unlimited number of control points. All push-buttons are simply connected in parallel to the same pair of wires. Adding a control point means adding a button, rather than rerouting cables between two-way and intermediate switches.
2. The coil does not heat up or consume power in standby mode—power is only required during the pulse. For large buildings with hundreds of lighting groups, this results in significant energy savings and increased component longevity.

Typical staircase lighting circuit: push buttons on each floor are wired in parallel → the impulse relay coil in the distribution board → the relay contact switches the lighting circuit. Stepping relays may have 2 or 4 fixed states (for example, switching lighting groups sequentially: one group, both, or none). A typical contact for modular series is 2NO 16 A; versions for high inrush current loads are also available (for instance, some models are rated for 160 A / 20 ms, which is critical for LED drivers). Coils are available for 12–240 V AC/DC depending on the series, including universal versions.

Typical Application Schemes

Controller and Load Isolation — Intermediate Relay

The output of a PLC or a low-voltage sensor (24 V DC, tens of milliamperes) must not directly switch a contactor or a 230 V line. An intermediate relay is used: the 24 V DC coil is connected to the controller output, while the 10–16 A contacts are connected to the power circuit or the contactor coil. This provides galvanic isolation and protects the expensive controller output: in the event of a fault, you only replace a low-cost relay rather than the entire controller module.

Bathroom Fan — Time Relay with Off-Delay

The time relay coil is wired in parallel with the light, and the contact is connected to the fan, using the off-delay function. Once the light is switched off, the fan continues to run for a pre-set duration. For this single function and specific range, a standard analogue relay is sufficient.

Staircases and Long Corridors — Impulse Relay

Momentary push buttons from all floors are wired in parallel to the coil, and the relay contact switches the lighting line. If guaranteed automatic shut-off is required, use a staircase timer (time relay) instead of an impulse relay, or a stepping relay with a central shut-off command.

Motor Starting — Star-Delta Time Relay

A specialised time relay controls two contactors: it manages the duration in the "star" configuration, the transition pause, and the switch to "delta". Settings are determined by the drive requirements; a general-purpose timer cannot replace a specialised device here due to the requirement for a fixed transition pause.

Selection Errors

• Incorrect coil type or voltage rating. AC and DC versions are distinct items with different part numbers. A 230 V AC relay will not operate in a 24 V DC circuit, and a 24 V DC coil will burn out if connected to 230 V. Always verify the "Control Supply Voltage" attribute and the rated voltage $U_s$.
• Contact ratings based on resistive loads for inductive loads. Contactors, motors, transformers, or groups of LED luminaires with drivers all require contact headroom or specialised versions (specific contact materials or inrush current resistance).
• Using a standard relay instead of an impulse relay. While a "self-holding" circuit using an intermediate relay can control lighting from multiple locations, the coil remains constantly energised, and the logic requires extra contacts. An impulse relay solves this with a single device.
• Time relays lacking the required function. "On-delay" and "off-delay" are different functions; they are not interchangeable in single-function relays. Check the specific function attributes rather than relying on the word "timer" in the product name.
• Ignoring mounting requirements. DIN rail modular relays (17.5 mm width, IP20) are designed for distribution boards; miniature relays with sockets are for automation cabinets; PCB-mount relays cannot be installed in a board without a socket. In most catalogues, most time relays allow DIN rail mounting, while some are designed for front-panel mounting.
• Insufficient contact changeover for signalling. If the circuit status needs to be output to a lamp or a remote monitoring system, select a relay with an additional contact group immediately—a 2CO (double pole) relay costs only slightly more than a 1CO (single pole) version.

The enclosure protection rating for modular relays is typically IP20: they are designed for installation inside a distribution board. Requirements for enclosures and accessibility to live parts are determined by the relevant electrical installation standards (such as HD 60364) for the specific environment.

Related Categories

Control relays are a separate class: they do not switch a load upon command, but rather monitor network parameters and trip or signal an alarm when a setpoint is exceeded.

Summary: use an intermediate relay for signal isolation and expansion, a time delay relay where logic is based on specific durations or schedules, and an impulse relay for controlling loads via short pulses from multiple control points. Selection should be based on four key criteria: contact current and material, the number and type of contact groups, the type of current and coil rating, and the specific function (time-delay or bistable).