Configuring Time Ranges on Siemens 3RP and IDEC GT3 Timers

1. Overview of Industrial Timer Time Range Selection

Industrial timing relays expose a finite set of switchable time ranges rather than a continuous setting. The user selects a coarse decade (for example, 0.5-10 s) and then trims the preset within that decade using a second control. This dual-control approach keeps the trim potentiometer at a usable resolution while spanning several orders of magnitude. The same architecture is used across standalone timer relays such as the Siemens 3RP15 and 3RP20 series, the IDEC GT3 family (GT3A, GT3D, GT3F, GT3S, GT3W), and the Omron H3CR-A. Understanding how the range selector interacts with the time trim is essential for accurate commissioning, particularly for star-delta motor starters and cascade timing chains.

The base time base (T_base) is determined by the position of the rotary range selector, and the actual preset (T_set) equals the trim percentage times T_base. Selecting the wrong decade either wastes trim resolution (preset near 5% of full scale) or saturates the dial (preset pegged at 100% with the actual value clipped). For safety-critical circuits such as motor star-delta transitions, undersizing the range causes nuisance trips; oversizing forces the trim to operate below 10% where linearity and repeatability degrade.

2. Siemens 3RP Series Solid-State Time Relays

The 3RP family replaces older electromechanical 7PU series timing relays with solid-state output stages while preserving the same 45 mm housing, terminal layout, and front-panel time range selector. Two principal product lines are fielded:

• 3RP15 - the established workhorse for ON-delay, OFF-delay, star-delta, and multifunction timing.
• 3RP20 - the successor family with extended voltage ranges, additional multifunction codes, and integrated auxiliary voltage outputs for sensor supply.

Both lines share the same front-panel user interface: a rotary range selector (typically 10 or 11 positions) and a continuous trim dial calibrated 0.05 to 1.0 of the selected range. For detailed ordering data, refer to the Siemens Sirius Relays manual covering the 3RP15 and 3RP20 product groups.

2.1 3RP15 Typical Time Range Matrix

The 3RP15 multifunction relays expose a fixed set of decade ranges selected by the front-panel rotary switch. The table below lists the typical range set; verify the exact ranges printed on the device label against the catalog number (for example, 3RP1505-1BW30 vs. 3RP1525-1BW30 differ in voltage and contact arrangement, not in time ranges).

Note that the first range is sub-second and the last range extends past 24 h. Always size T_base so that T_set falls between 30% and 80% of the range to maximize timing accuracy.

2.2 3RP20 Multifunction Extensions

The 3RP20 series adds higher voltage variants (24-240 V AC/DC universal supply), an additional 5-100 h range position, and codes for specialized functions such as flashing, watchdog, and pass-through pulse. The 3RP2025 (ON/OFF-delay) and 3RP2030 (star-delta) catalog numbers are the most commonly used variants for motor control panels. Time range selection on the 3RP20 follows the same selector-then-trim model.

3. IDEC GT3 Series Timers

The IDEC GT3 series is a modular timer family that pairs a plug-in timer body with a dedicated socket, allowing the same control wiring to accept different timing functions simply by swapping the relay. This is a significant advantage over hard-wired 3RP timers when commissioning a panel with many timing functions.

3.1 GT3A Analog Timers

The GT3A provides four selectable operation modes selected via a side-mounted DIP block: ON-delay, OFF-delay, interval (pulse), and cycle. Time ranges match the standard decade set listed for the 3RP15. The GT3A uses an 8-pin octal base, allowing direct interchange with traditional 8-pin cube relays.

3.2 GT3D Digital Timers

The GT3D is the digital-display model with six discrete time ranges and sixteen selectable timing functions. Maximum setpoint reaches 99.9 hours with three-digit precision. The digital encoder removes the trim-resolution problem of analog dials: the displayed value is the actual T_set within the selected range, with no percentage scaling.

3.3 GT3F True Power OFF Delay Timers

The GT3F is purpose-built for OFF-delay applications where the timing must continue after control power is removed. An internal supercapacitor or battery module (depending on variant) sustains the timing logic through loss of supply. Time range selection is identical in procedure to the GT3A, but the practical application is loss-of-power ride-through rather than delayed drop-out.

3.4 GT3S Star-Delta Timers

The GT3S is the dedicated star-delta timing module. Two distinct time settings are exposed:

1. t1 - the star-to-delta transition delay (typical 30-100 ms range, fully adjustable).
2. t2 - the initial run time in star connection before transition begins (selected from the same decade table as the GT3A).

The transition delay is critical: too short causes a contact overlap that shorts the line and delta windings; too long causes the motor to coast through a dead period and stall on re-engagement. A common preset is 50 ms, but the value must be validated against the motor's load inertia and the break/release time of the contactors.

3.5 GT3W Dual Time Range Timers

The GT3W provides eight selectable operation modes and dual independent ON/OFF timing ranges, with maximum setpoint up to 300 hours. Typical applications include sequential start/stop chains where one timer cascades into another without an external interposing relay.

4. Timer Function Descriptions

Industrial timer relays expose a small set of canonical functions. Selecting the right function is the first step; selecting the right time range is the second. The diagrams below use SVG timing charts where the upper trace is the trigger input and the lower trace is the relay output.

4.1 ON Delay (On-Delay Timer, TON)

The output turns ON after T_set has elapsed from the rising edge of the trigger. If the trigger falls before T_set completes, the timing resets without an output transition.

4.2 OFF Delay (Off-Delay Timer, TOF)

The output turns OFF after T_set has elapsed from the falling edge of the trigger. The output follows the trigger ON edge immediately. If the trigger rises again before T_set expires, the timing resets and the output stays ON continuously.

4.3 Pulse Timer (One-Shot / Interval)

The output turns ON on the rising edge of the trigger and turns OFF after T_set. The output duration is independent of the trigger pulse width - a brief trigger still produces a full T_set output. This is the function used for fixed-length cleaning pulses, lubrication shots, and brake release timing.

4.4 Star-Delta Transition Timing

The star-delta function combines three sequential elements:

1. Line contactor (K1) closes on run command.
2. Star contactor (K2) closes after a short pre-arc delay (typically 50 ms).
3. After T_set elapses, K2 opens, a transition dead time (t1) passes, and delta contactor (K3) closes.

5. Time Range Setting Procedure

The procedure below applies to both 3RP and GT3 relays; refer to the device label for the exact range table applicable to the catalog number in hand.

1. Identify the required T_set from the process specification. Record the value in seconds even if the relay will be set in minutes or hours.
2. Calculate the optimal range. Choose the smallest range whose maximum is greater than or equal to T_set. The trim should land between 30% and 80% of full scale.
3. Select the range position on the rotary switch. Power down the relay before turning the selector to avoid internal arcing on the wiper contact.
4. Apply power and verify trigger response using a test input or the manual test button on the front face.
5. Trim the preset while observing the LED status indicator. For 3RP15/3RP20 the trim is a percentage; for GT3D the trim is the actual time in HH:MM:SS or SS.s format.
6. Measure the actual time with a stopwatch or panel meter against the trigger and output transitions. Tolerances of ±5% are typical for analog electromechanical designs; solid-state versions achieve ±1%.
7. Record the setting on the panel legend and in the commissioning log. Include the catalog number, range position, trim value, and measured actual time.

5.1 Worked Example: Star-Delta Timer Set to 8 s

Required T_set = 8 s. Available ranges: 0.5-10 s (option 3) and 5-100 s (option 4). Range 3 yields a trim of 80%, range 4 yields a trim of 8%. Range 3 is the correct choice because the trim sits in the linear 30%-80% band. The selected time base is 10 s, and the trim reads 0.8.

6. Cascading Timers for Extended Time Ranges

When the required T_set exceeds the maximum available range position, cascade two timers. The output of the first timer (T1) drives the trigger of the second timer (T2). The total elapsed time from trigger to T2 output is:

T_total = T_set1 + T_set2

Cascading is preferable to selecting an oversized range position because it preserves trim resolution on both segments. For example, a 45-minute delay is poorly served by a single 0.5-10 h range (45 min = 7.5% of full scale). Cascading a 30-minute and a 15-minute timer on separate range positions yields trim settings of 50% and 50% respectively, both well inside the linear band.

7. Verification and Commissioning

After setting the time range and trim, perform the following verifications before releasing the panel to operations:

1. Trigger integrity: confirm the trigger input receives a clean step. Chattering inputs defeat timing accuracy. Apply a debounce filter (RC network of 100 ms time constant) if the source is a mechanical limit switch.
2. Power supply tolerance: solid-state timers are sensitive to undervoltage. Measure supply at the relay terminals, not at the panel bus. Tolerances typically ±10% of nominal.
3. Temperature stability: at panel locations exceeding 50 °C, timing drift of analog units can exceed ±5%. Specify solid-state or digital units for high-temperature cabinets.
4. Functional test: cycle the trigger 5 times and record the measured output transition time on each cycle. The maximum-to-minimum ratio should be within the published repeatability spec.
5. Fail-safe check: for motor control, verify that loss of control power drops the timer output within the dropout time of the contactor. If the dropout time is not zero, the star-delta sequence may not start, leaving the motor in an undefined state. Add an undervoltage relay on the line contactor if needed.

8. Troubleshooting Matrix

9. Comparison: 3RP15 vs. 3RP20 vs. GT3 Series

Selection guidance: use 3RP15/3RP20 for fixed-function panels where wiring density is high and the timing function rarely changes. Use GT3 series where modular replacement is desired or where a single panel mixes many timing functions across different motor starters.

10. Frequently Asked Questions