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IEC 61812-1: Time Relays for Industrial and Residential Use – Design and Application Guide
Key Insight
IEC 61812-1 defines the requirements and test methods for time relays used in industrial and residential control applications, standardizing timing functions, accuracy classes, and environmental ratings to ensure interoperability and reliable timing performance.
IEC 61812-1 defines the requirements and test methods for time relays used in industrial and residential control applications, standardizing timing functions, accuracy classes, and environmental ratings to ensure interoperability and reliable timing performance.
1. Scope and Timing Function Classification
IEC 61812-1, published in 2011, applies to time relays with specified timing functions used in industrial control, building automation, and household equipment. The standard defines eight standard timing functions designated by the IEC 61812 function code system (F1 through F8), replacing the historically confusing variety of manufacturer-specific designations. These functions cover on-delay, off-delay, interval timing, symmetrical flashing, and multifunction combinations — enabling a designer to select a single product type that can be configured for multiple timing modes via DIP switches or software.
The standard classifies time relays by their timing accuracy: Class 1 (accuracy ±1% of full scale), Class 5 (±5%), and Class 10 (±10%). These accuracy classes apply at reference conditions (23 °C ± 2 °C, nominal supply voltage, fresh from storage). Under operating conditions across the full temperature and voltage range, the accuracy may degrade by an additional factor specified by the manufacturer. For precision timing applications such as process control sequencing, Class 1 relays with quartz-controlled oscillators are typically specified.
Engineering Insight: The F1 (on-delay) function — where the timing period begins when the supply voltage is applied and the output contacts transfer after the set time — accounts for approximately 60% of all time relay applications in industrial control. The F5 (interval) function, where the output is energized for a fixed duration triggered by a momentary signal, represents another 20%. Understanding the application mix is critical when designing control panels with standardized timing modules.
2. Timing Accuracy and Setting Resolution
2.1 Accuracy Under Reference and Operating Conditions
IEC 61812-1 defines both intrinsic accuracy (at reference conditions) and operating accuracy (over the full rated temperature, voltage, and aging range). The standard specifies that the operating accuracy be calculated as the root-sum-square of the individual influence factors: temperature coefficient (typically 0.05%/°C for RC timers, 0.005%/°C for crystal-based), voltage coefficient (0.1%/V for RC, negligible for crystal), and aging drift (0.1%/1000 h for RC, 0.005%/1000 h for crystal).
| Accuracy Class | Intrinsic Accuracy | Temperature Coefficient | Typical Technology | Typical Application |
|---|---|---|---|---|
| Class 1 | ±1% | ≤ 0.01%/°C | Quartz crystal / microprocessor | Precision sequencing, batch control |
| Class 5 | ±5% | ≤ 0.05%/°C | RC oscillator with trim | General industrial, HVAC control |
| Class 10 | ±10% | ≤ 0.10%/°C | Simple RC (potentiometer set) | Lighting, fan control, non-critical timing |
2.2 Timing Range and Setting Resolution
The standard requires that the setting resolution (the minimum distinguishable change in time setting) be better than 5% of the maximum value of the selected time range. For multifunction time relays with ranges from 0.1 s to 100 h, this translates to at least 20 distinguishable positions per range, typically implemented with a 10-turn potentiometer or digital thumbwheel switches. The repeatability (precision when set to the same value repeatedly under identical conditions) must be better than 1% of the set value for Class 1, 2% for Class 5, and 5% for Class 10 relays.
Design Warning: Time relay accuracy specifications can be misleading. A Class 1 relay specified as “±1% of full scale” means that on a 0–60 second range, the accuracy is ±0.6 seconds regardless of the set point. At a setting of 2 seconds, this represents 30% error! For applications requiring precision at short intervals, ensure the time range is selected so that the normal operating point falls in the upper 20–80% of the range, or specify accuracy as “±1% of reading” (which is available on higher-cost microprocessor-based relays).
3. Electrical and Environmental Requirements
IEC 61812-1 specifies comprehensive electrical requirements for time relays:
- Supply voltage range: Must operate correctly from 85% to 110% of nominal voltage for AC types, and 90% to 120% for DC types. During the timing period, the relay must tolerate supply interruptions of up to 10 ms without resetting the timing cycle (memory retention).
- Output contact rating: Minimum 5 A / 250 V AC for general-purpose types (resistive load), with inductive load ratings typically derated to 30–50% of the resistive rating. The standard requires verification of both making and breaking capacity under specified load types.
- Recovery time: After completing a timing cycle, the relay must be ready to start a new cycle within 500 ms (Class 1), 200 ms (Class 5), or 100 ms (Class 10) — counter-intuitively, simpler relays recover faster because they have less internal state to reset.
- Dielectric strength: 2 kV AC, 50/60 Hz between supply and contact circuits for 1 minute; 4 kV impulse voltage withstand per IEC 60947-1.
Environmental requirements include operation from -20 °C to +55 °C (industrial) or -10 °C to +50 °C (residential), with storage from -40 °C to +70 °C. Damp heat testing at 40 °C / 93% RH for 4 days verifies resistance to humidity-induced timing drift and insulation degradation.
4. Frequently Asked Questions
Q1: What is the difference between on-delay (F1) and off-delay (F2) timing functions?
On-delay (F1): The timing period starts when the supply voltage is applied. The output contacts transfer only after the set time has elapsed. If the supply is removed before the time elapses, the timing resets. Off-delay (F2): The output contacts transfer immediately when supply is applied. The timing period starts when the supply is removed, and the contacts remain in the timed position until the set time elapses. Off-delay is typically used for cooling fan run-on, stairway lighting hold, or machine lubrication post-circulation.
Q2: Can I use a single multifunction time relay for all timing functions in a control panel?
Yes — and this is a common practice in modern panel design. Multifunction time relays complying with IEC 61812-1 typically offer 4–8 selectable timing functions (F1–F8) in a single device. However, be aware that the accuracy specification applies to all functions; some manufacturers achieve multifunction flexibility by using an internal microprocessor, which introduces timing jitter from firmware execution. Verify that the jitter (typically ±10–50 ms for microprocessor-based relays) is acceptable for your application’s timing precision requirements.
Q3: How does temperature affect time relay accuracy, and what can be done to mitigate it?
Temperature affects the timing capacitor’s value (typically +250 ppm/°C for polyester capacitors) and the charging resistor’s value. RC-based time relays can experience timing drift of 0.05–0.1%/°C, meaning a relay calibrated at 23 °C may be 3–6% slow at 55 °C. Mitigation strategies include: (a) selecting Class 1 crystal-based relays for temperature-sensitive applications, (b) mounting the relay away from heat-generating components (contactors, power supplies), and (c) using the manufacturer’s temperature correction curves (required by IEC 61812-1 to be provided in the technical documentation).
Q4: What is the significance of the “repeatability” specification versus “accuracy”?
Accuracy describes how close the actual time is to the set time (absolute error). Repeatability describes how consistent the timing is from one cycle to the next under identical conditions (random error). For many industrial applications, repeatability is more important than absolute accuracy — for example, in a conveyor system where the timing determines the interval between successive products, consistency matters more than whether the interval is exactly 5.00 s or 5.15 s. IEC 61812-1 specifies both parameters separately.
