Wiring Siemens 7MH4710-5BA SIWAREX Ex Barrier: 4-Wire Load Cell

1. Product Overview: SIWAREX IS Ex Barrier 7MH4710-5BA

The 7MH4710-5BA is the standard version of the SIWAREX IS Ex barrier, a passive, intrinsically safe interface module manufactured by Siemens AG. It is installed on the safe-area side of a load-cell weighing circuit and serves as the energy-limiting barrier between a strain-gauge load cell mounted in an ATEX-classified hazardous area and a SIWAREX weighing electronics module located in the safe area.

The barrier performs three essential functions inside the SIWAREX weighing chain:

1. Energy limitation — restricts voltage and current that can flow into the hazardous area so the measurement circuit cannot ignite a surrounding explosive atmosphere under normal or fault conditions.
2. Galvanic isolation — separates the safe-area electronics from the hazardous-area circuit through Zener-diode and resistor-based isolation.
3. Signal transfer — passes the millivolt-level bridge signal back to the SIWAREX module without introducing significant offset, drift, or series resistance that would degrade weighing accuracy.

The device is a passive component (no auxiliary power required), designed for DIN-rail mounting inside the safe-area cabinet or inside a certified enclosure close to the hazardous-area boundary.

2. ATEX Certification and Hazardous-Area Use

The 7MH4710-5BA is associated apparatus, which means it must always be installed in the safe area. Its certification allows the load cell and cabling on the hazardous-area side to be classified down to Zone 0 (for Group IIC gases) when wired as documented.

3. Topology in a SIWAREX Weighing System

The barrier is wired in series between the four load-cell conductors and the four SIWAREX module input conductors. No active electronics, power supply, or bus interface are required at the barrier.

4. Technical Specifications

5. Prerequisites for Wiring

1. Confirm ATEX zone classification for the area where the load cell is installed. The barrier supports Zone 0/1/2 installations on the cell side.
2. Identify the SIWAREX module to be used on the safe-area side (SIWAREX IS, WP231, WT231, etc.) and verify it is approved for use with the 7MH4710-5BA barrier in the SIWAREX IS manual.
3. Check the load-cell data sheet for bridge resistance, sensitivity (mV/V), and cable type. The cell must be a 4-wire or 6-wire strain-gauge type with a minimum 350 Ω bridge.
4. Select cable type with the screen specified in the manual. Siemens typically recommends a shielded, twisted-pair cable (e.g. LiYCY or comparable) for the cell-to-barrier run.
5. Prepare the cabinet layout: barrier on DIN rail in safe area, ≥ 50 mm clearance above and below for ventilation and cable bend radius.
6. Obtain the wiring diagrams from entry 8754899, Fig. 2.3 (p. 28), Fig. 2.4 (p. 30), and Fig. 2.5 (p. 31) before energising.

6. Safety Preconditions

• De-energise the SIWAREX module and any associated 24 V supply before working on the barrier terminals.
• Verify the hazardous area is gas-free and a hot-work permit is in place if the cell is to be replaced.
• Use only cable glands, conduit entries, and enclosure hardware rated for the surrounding zone on the cell side.
• Do not interchange the safe-area and hazardous-area terminal blocks. The two sides are NOT pin-compatible; the Zener diodes face the cell side.
• Connect the DIN-rail earth terminal to a low-impedance protective earth (≤ 1 Ω) — the barrier's Zener clamping depends on this connection.

7. 4-Wire Load Cell — Terminal Assignment

A 4-wire load cell has only the two excitation lines and the two signal lines — no separate sense conductors. The barrier exposes four "Ex-side" terminals (to the cell) and four "safe-side" terminals (to the SIWAREX module). The mapping below is the conventional assignment used across the SIWAREX IS family; cross-check against Fig. 2.3 of entry 8754899 for the exact terminal numbers printed on the 7MH4710-5BA.

For a 4-wire cell, the SIWAREX module's sense terminals (SEN+ / SEN-) are bridged to the excitation terminals (EX+ / EX-) at the module, because no sense pair is returned from the cell. The barrier does not require any modification for 4-wire operation.

8. 4-Wire Wiring Procedure (Step-by-Step)

1. Mount the barrier on the 35 mm DIN rail inside the safe-area cabinet. Snap onto the rail and verify the earth clamp contacts the rail.
2. Terminate the cell cable at the barrier's Ex-side terminal block (terminals 1–4 per the table above). Tighten to the torque specified on the device label (typically 0.6–0.8 N·m for the SIWAREX IS barrier family).
3. Route the cell cable through the cable gland on the hazardous-area enclosure. Maintain separation from power cables (≥ 200 mm) and avoid parallel runs > 1 m to minimise common-mode noise pickup.
4. Connect the screen to the barrier's screen terminal on the Ex side, then continue the screen to the load-cell enclosure and bond to the cell body. Do not double-ground the screen at the cell end if the manual specifies a single-end screen termination.
5. Wire the safe-side terminals (5–8) to the matching SIWAREX module input. Use a shielded twisted pair, with screen bonded to the SIWAREX module's screen clamp and to the cabinet earth.
6. Bridge the sense terminals on the SIWAREX module (SEN+ to EX+, SEN- to EX-) per the module's manual, because this is a 4-wire cell.
7. Tighten the cabinet earth bond on the DIN rail. Measure the resistance from the barrier earth clamp to the cabinet protective earth bus — should be ≤ 1 Ω.
8. Apply identification to both ends of every conductor and to the barrier itself (loop number, cell serial, ATEX certificate reference).

9. Wiring Diagram (4-Wire Cell ↔ 7MH4710-5BA ↔ SIWAREX)

10. Cable Specification Notes

• Cell-to-barrier cable: shielded, twisted; cross-section 0.5–1.5 mm². Use a cable with individually shielded pairs if multiple cells are connected via a junction box.
• Barrier-to-SIWAREX cable: shielded twisted pair; cross-section 0.5–1.5 mm². Maximum length per the SIWAREX IS manual — the barrier contributes negligible series resistance, so the limit is set by the module's input specification.
• Screen termination: bond at the barrier earth clamp and at the SIWAREX module screen clamp; if the cell is in a separate enclosure, also bond at the cell gland plate to prevent shield-loops acting as single-turn secondary windings.
• Segregation: maintain ≥ 200 mm from VFD output cables, motor feeders, and 400 V+ power conductors. Cross power cables at 90° if a crossing is unavoidable.

11. Connection to Common SIWAREX Modules

12. Commissioning and Verification

1. Visual inspection: confirm terminal torques, screen bonding, DIN-rail earth clamp, and absence of stray strands.
2. Insulation test (optional, with barrier isolated): measure > 100 MΩ at 500 V between the bundled Ex-side conductors and the cabinet earth.
3. Energise the SIWAREX module without load on the cell. Verify the module's raw load-cell count is in the expected idle range (typically within ±10 % of the cell's zero-balance spec).
4. Apply a known test weight and confirm the displayed mass matches the cell's mV/V calibration within tolerance. A 4-wire cell installed behind the barrier typically shows a small additional zero drift; perform the module's auto-calibration / zero adjust with the cell mounted and unloaded.
5. Verify excitation voltage at the safe-side barrier terminals (5 and 6) — should equal the SIWAREX module's setpoint (typically 5 V DC or 10 V DC depending on module configuration).
6. Document: update the loop sheet, ATEX dossier, and the SIWAREX module's "DR_Ex" or equivalent configuration tag indicating that the cell is wired through a 7MH4710-5BA barrier.

13. Troubleshooting Matrix

14. Spare-Parts and Ordering Notes

Verify the exact MLFB on the existing device label before ordering a replacement; the suffix (5BA vs 5CA) is significant and entity parameters may differ.

15. Related Documentation

• SIWAREX IS manual — Siemens Support entry 8754899 (primary wiring source; Fig. 2.3 p. 28, Fig. 2.4 p. 30, Fig. 2.5 p. 31)
• SIWAREX IS Ex barrier standard version product page