For reliable operation of automation components in low-voltage systems, follow the manufacturer’s specified terminal numbering. Auxiliary contacts on most industrial-grade relays are labeled 13/14 for normally open and 21/22 for normally closed. Main power terminals typically use A1/A2 for coil excitation, 1/L1, 3/L2, 5/L3 for input, and 2/T1, 4/T2, 6/T3 for output. Verify the exact configuration in the device’s technical sheet before proceeding–some compact models may reverse the numbering sequence.
Use 0.75 mm² to 2.5 mm² copper conductors for control circuits and 4 mm² to 16 mm² for power depending on motor load. Insert conductors directly into clamp terminals without twisting; ensure no bare wire extends beyond the terminal block. Secure all screws to 1.5 Nm torque–over-tightening damages strands, while under-tightening risks overheating.
Apply a 24 V DC or 230 V AC control voltage to the coil terminals A1/A2, but never mix voltage types on the same device. For motor starting, connect overload relay contacts in series with the coil circuit; set the trip class (Class 10 for general applications) based on motor rating. Test coil continuity with a multimeter before energizing–resistance should match the datasheet value within ±10 %.
Label every conductor at both ends with heat-shrink tubing or adhesive tags. Mark auxiliary contacts with the same identifiers printed on the device cover to simplify troubleshooting. Keep documentation updated with a dated, signed copy of the connection scheme adjacent to the panel.
Electrical Switching Device Connection Schematics
Identify the coil terminals first–mark A1 and A2 to prevent reversed polarity, which causes premature failure in 3RT2 models. Apply 24V DC or 230V AC as specified; exceeding voltage damages insulation and reduces lifespan by 40%.
Use crimp connectors (not solder) for auxiliary contacts–loose strands create resistive hotspots, increasing temperature by 15°C under load. For 3-pole variants, route phase conductors L1, L2, L3 through overload relays before reaching main contacts; skipping this risks undetected overcurrent conditions.
Ground the metallic enclosure via M6 bolt–improper bonding generates stray currents, interfering with sensitive equipment. For IEC 60947-4-1 compliance, maintain 8mm separation between live parts and grounded surfaces to prevent arcing.
| Terminal | Wire Gauge (mm²) | Torque (Nm) |
|---|---|---|
| Coil A1/A2 | 0.75–1.5 | 1.2 |
| Main Contacts 1–6 | 2.5–16 | 2.5 |
| Auxiliary Contacts | 0.5–1.0 | 0.8 |
Label every conductor with heat-shrink tubing–miswiring between NO/NC contacts leads to 60% of startup failures. Test coil resistance with a multimeter before energizing; values below 80Ω indicate shorted windings.
Install arc suppression modules on inductive loads–each switching cycle erodes contacts by 0.3µm; without suppression, pitting occurs after 20,000 cycles. For motor applications, connect thermal overload outputs in series with the control circuit to ensure shutdown before reaching 1.2× FLA.
Understanding Terminal Markings on Electromagnetic Switches
Locate the coil connections first–typically marked A1 and A2–positioned on opposite sides of the device’s base. Verify polarity only if direct current is specified; otherwise, orientation matters little for alternating current operation. Confirm voltage ratings printed adjacent to these points, as mismatches risk coil burnout or unreliable engagement.
Power circuit terminals carry alphanumeric labels 1/L1, 3/L2, 5/L3 for incoming lines, paired with 2/T1, 4/T2, 6/T3 for outgoing loads on three-pole variants. Single-phase units simplify this, using 1/L1 and 2/T1 exclusively. Jumper bars bridge unused terminals–remove these before connecting differing voltage phases to prevent cross-feeding.
Auxiliary Contact Nomenclature
Auxiliary blocks display NO (normally open) and NC (normally closed) pairs, prefixed by 13–14, 21–22, 31–32, etc. Higher numbers indicate additional modules stacked onto the base unit. Factory configurations default NO/NC states; swap internal springs to reverse them if required, ensuring mechanical alignment prevents contact welding.
Test continuity between auxiliary terminal pairs before energizing–accidental mislabeling during manufacture or servicing occurs. Use a multimeter set to ohms mode; expected readings are near zero for NC and infinite resistance for NO contacts under resting conditions. Confirm these values under load after installation, as ambient vibration may alter readings.
Etched symbols accompany each terminal–dot (·) or plus (+) denote coil start (A1), while a hyphen (–) or circle (○) marks its counterpart (A2). Load terminals show directional arrows guiding current flow from L inputs toward T outputs. Misalignment here reverses intended function, damaging inductive loads during startup transients.
Permanent marker labels often fade; replace them with engraved tags or temper-resistant stickers matching the original scheme. Record terminal mappings in device documentation–mismatches during future maintenance introduce system-wide inconsistencies, complicating troubleshooting efforts.
Connecting the 3RT20 Motor Starter with Secondary Circuit Integration
Ensure the main power supply is disconnected before handling any live terminals. Identify the primary coil terminals labeled A1 and A2 on the 3RT20 unit–these accept 24V DC or 230V AC control voltage based on system requirements. Connect the positive lead of your control circuit to A1 and the negative/neutral to A2, securing them with a torque screwdriver to 0.5 Nm.
For auxiliary NO (normally open) contacts, locate terminals 13/14–these act as secondary signaling points. Attach a 1.5 mm² wire from terminal 13 to your PLC input or pilot lamp, then from 14 to the common return. For NC (normally closed) contacts, use terminals 21/22, wiring them in series with safety circuits or alarm relays to interrupt current during faults.
Verify the coil voltage rating matches your control supply. If using a 230V AC coil, connect a freewheeling diode across A1/A2 for DC coils (cathode to A1) to suppress voltage spikes. Omit this step for AC coils. For remote operation, wire pushbuttons or switches in series between the control source and A1, ensuring no parallel paths bypass the intended control path.
Phase sequencing must align with the mechanical layout of the 3RT20. Connect L1, L2, and L3 from the upstream breaker to the input terminals (1/L1, 3/L2, 5/L3) using conductors sized per IEC 60947-4-1 (minimum 2.5 mm² for 16A models). Output terminals (2/T1, 4/T2, 6/T3) link directly to the load–tighten connections to 1.2 Nm to prevent overheating.
Engage the overload relay protection by connecting its terminals in series with the load side (e.g., 95/96 for NC alarm contacts). Set the trip class (CLASS 10A is standard) via the dial on the relay housing. For three-wire control, connect the start button in series with the holding contact (NO terminal 13/14) and the stop button in series with the NC contact (21/22) to the coil (A1/A2).
Test the assembly with a multimeter in continuity mode. Energize the coil–NO contacts should show near-zero resistance, NC contacts infinite. Simultaneously check phase rotation at the load terminals; reversed phases risk damaging motors. If using a soft starter or VFD downstream, ensure the 3RT20’s control voltage doesn’t share the same reference as the drive’s logic to avoid interference.
For dual-voltage coils (e.g., 110/230V), configure the bridging link between A2 and the adjacent terminal as specified in the technical manual–incorrect settings will damage the coil. Ground the unit’s chassis via the PE terminal with a 4 mm² conductor, even in IT systems. For outdoor use, apply corrosion-resistant grease to terminals and encase in an IP65-rated enclosure.
Label all conductors at both ends–use heat-shrink sleeves for durability. Document the configuration with a one-line schematic, noting coil voltage, contact arrangement, and load details. Recheck all connections after 24 hours of operation; thermal cycling can loosen terminals under vibration. For redundant systems, wire a second 3RT20 in parallel, using auxiliary contacts to interlock the units.
Integrating an Industrial Switch with a Triple-Phase Motor Alongside a Current Protection Device
Select an electromechanical switching unit rated for 120% of the motor’s full-load current to accommodate startup surges. The model’s coil voltage must match the control circuit supply–typically 24V DC, 110V AC, or 230V AC–to ensure reliable actuation. Verify the auxiliary contacts include at least one normally open and one normally closed for signal feedback and interlocking.
Mount the switching device within 30 cm of the current protection relay to minimize voltage drop in the control leads. Secure all connections with crimp terminals or ring lugs, torqued to the manufacturer’s specification–usually between 1.2–2.0 Nm for M4 screws. Use stranded copper wire rated for 75°C or higher, sized according to the table below:
- Motor rating < 5 kW: 1.5 mm² (16 AWG)
- 5–15 kW: 2.5 mm² (14 AWG)
- 15–30 kW: 4 mm² (12 AWG)
- > 30 kW: 6 mm² (10 AWG)
Connect the main terminals in sequence: L1 to T1, L2 to T2, L3 to T3, following the phase rotation indicated on the motor nameplate. Reverse any two phases at the input if the motor spins backward. Route the neutral through the current protection device only if the control circuit requires it; otherwise, keep it isolated from the power path.
The current protection relay should be set to trip at 105–110% of the motor’s rated current, with a class 10 time-delay to allow transient overloads during startup. Link the relay’s normally closed contact in series with the switching unit’s coil circuit so a trip immediately drops the mechanical latch. Test the arrangement by manually tripping the relay–verify the unit de-energizes within 100 ms.
For auxiliary circuits, wire a green-yellow ground conductor from the motor frame to the metal enclosure, ensuring continuity resistance below 0.1 Ω. Add a pushbutton for manual override, but install it downstream of the relay contacts to prevent accidental bypass. If remote monitoring is needed, connect a 4–20 mA transducer across the main terminals, scaled to 1.5× the motor’s full-load current.
Avoid daisy-chaining multiple motors off a single switching device; each unit should have its own dedicated conductor and current protection relay to prevent nuisance tripping. If space is constrained, stack modular units vertically, keeping a 25 mm air gap between adjacent components to comply with IEC 60947-4-1 thermal derating curves.
After assembly, perform a 50 Hz insulation test between each phase and ground at 1000 V DC for one minute–leakage current must remain below 1 mA. Energize the circuit with no load first, then gradually increase to rated current while monitoring for abnormal humming or vibration. Record coil voltage, pickup time, and dropout time; deviations greater than ±5% indicate contact misalignment or faulty magnetic circuit.