Siemens / SIRIUS contactors, motor starters and overloads

Every 3RT2 rating with its matching overload, the coil voltage codes, and why your contactor keeps failing.

Contactors and overloads are consumables. Contacts erode, coils burn out, and they wear on a schedule rather than failing at random. Most plants hold spares, run out of the one they need, and then discover the replacement will not plug onto the existing overload.

We supply SIRIUS current production as a distributor and hold surplus and used stock of the older 3RT1 generation.

Match the frame size or nothing plugs together

A contactor and its overload relay must be the same SIRIUS frame size. Mismatched frames will not plug together and cannot be assembled as a direct-mount motor starter.

The previous generation follows the same logic: 3RT1 contactors with 3RU1116, 3RU1126, 3RU1136 and 3RU1146 overloads.

Every S00 and S0 rating

From the Siemens IC 10 catalogue. AC-3 and AC-3e ratings at 40 degrees C.

Derate hard for AC-4 duty

AC-3 is normal motor starting. AC-4 is plugging, reversing and inching, and it costs you a lot of capacity.

Read that last column. A 3RT2028 rated 18.5 kW on normal starting is a 6 kW contactor if you want 200,000 operations of reversing duty. If you are replacing the same contactor over and over on a reversing or inching application, it is almost certainly undersized rather than faulty.

Coil voltage is in the suffix

Two contactors with the same base number and different suffixes are different parts. This is the second most common ordering error after frame size.

A is AC and B is DC. So 3RT2025-1AL20 is a 230 V AC coil and 3RT2025-1BB40 is the 24 V DC version of the same contactor.

In New Zealand panels 230 V AC and 24 V DC are the common two. If the panel is PLC driven off 24 V DC outputs, you want a B suffix.

The rest of the number

The format is 3RT2 then the frame and rating digits, then a generation digit, then the coil code. So 3RT2025-1AL20 reads as SIRIUS contactor, size S0 at 17 A, 230 V AC coil.

Some contactors have an extended operating range of 0.7 to 1.25 times the control supply voltage, useful on a site with a soft or noisy control supply.

Power ranges by size and pole count

The 4-pole 3RT23 with four NO contacts has the same performance range as the 3-pole 3RT20: in S00 to 22 A at up to 690 V, in S0 to 50 A.

The 3RT25 is derated against the 3RT20. Same frame, fewer kilowatts, because the fourth pole changes the thermal picture. Three-pole to four-pole is not like-for-like on rating.

The families

Common accessories

Thermal or electronic overload?

Thermal, the 3RU2 range. Cheaper, simple, no auxiliary supply. Fine for a straightforward motor.

Electronic, the 3RB3 range. Wider setting range so one unit covers several motor sizes, better repeatability, typically lower power loss.

That wider setting range is the real argument. If you are stocking spares for a plant with many different motors, one electronic relay can replace several thermal ones on the shelf. Fewer part numbers to hold.

Auxiliary contacts

Size S00 contactors have one auxiliary contact, NO or NC, integrated. Beyond that, 3RH29 auxiliary switch blocks snap onto the front and release with a lever.

One thing to check on safety circuits. The solid-state compatible 3RH291 series blocks for S00 contactors have hard gold-plated contacts, intended for small voltages and currents and dusty atmospheres. Siemens states that the NC auxiliary contacts on those are not mirror contacts.

If your safety circuit relies on mirror contacts to detect a welded main contact, that variant will not do the job.

Why your contactor keeps failing

If you are replacing the same contactor repeatedly, the contactor is usually not the problem.

Check the duty class first. See the AC-4 table above. Reversing and inching duty can cut the rating to a third.

Fit surge suppression on the coil. 3RT2 contactors take RC elements, varistors, suppressor diodes or diode assemblies for damping opening surges. There is space for them next to a snap-on auxiliary switch block.

This is the same issue we cover on the SM 322 output module page from the other end. An unsuppressed contactor coil destroys the PLC output driving it and shortens the life of the contactor. Fit a diode or Zener across a DC coil, a varistor or RC element across an AC coil.

If you are replacing both a contactor and an output module on the same circuit, suppression is almost certainly what is missing.

Check the coil voltage actually present. A coil running below its pull-in voltage chatters, and chatter destroys contacts fast.

Repair kits and replacement coils

Both exist and both are cheaper than a new contactor. Contact kits for worn main contacts, replacement coils for burnt-out ones. On the larger frames especially this is worth doing.

Tell us the contactor number and what has failed, and we will quote the kit as well as the whole unit so you can compare.

IO-Link and AS-Interface

S00 and S0 contactors with a communication interface are required for mounting the SIRIUS function modules that connect over IO-Link or AS-Interface. A standard contactor will not take them.

Confirmed part numbers

The SIRIUS range runs to thousands of variants across frame sizes, pole counts, coil voltages and terminal types. Send us the rating and coil voltage you need and we will build the correct number.

Send us your requirement

For a replacement, send the full number off the contactor including the suffix. If the label is unreadable, tell us the motor kW, the control voltage, the duty class and whether the terminals are screw or spring.

For a new panel, send the motor list and we will specify contactors, overloads and assemblies as matched sets. Quotes in NZD including GST.

Request a quote

Industech is an independent supplier of new, surplus and used industrial automation equipment. We are not an authorised distributor of, approved by, sanctioned by, or affiliated with Siemens AG. All trademarks, brand names and article numbers are the property of their respective owners and are used here for identification purposes only.