Bently Nevada 21747-085-00 Proximitor Extension Cable – 3300 Series

Bently Nevada 21747-085-00 Proximitor Extension Cable — Signal Transmission Architecture in 3300 Series Machinery Protection Loops

The Bently Nevada 21747-085-00 is an 8.5-metre (28 ft) coaxial extension cable engineered exclusively for the 3300 Series Proximitor System. Its function within a machinery protection loop is precise: it bridges the gap between the eddy-current probe tip and the Proximitor signal conditioner, maintaining the calibrated electrical characteristics of the entire transducer chain. Any deviation in cable impedance, capacitance, or shielding continuity at this junction directly corrupts the DC gap voltage and AC vibration signal that the monitor card interprets. The 21747-085-00 is therefore not a passive conductor — it is a calibrated transmission element whose electrical parameters are matched to the 3300 Series probe-Proximitor system at the factory.

In rotating machinery protection applications — steam turbines, gas compressors, centrifugal pumps, and gearboxes — the extension cable routes from the probe connector at the bearing housing, through conduit or cable tray, to the Proximitor mounted in a junction box or instrument enclosure. The 8.5 m length covers the majority of standard turbine pedestal-to-local-panel runs without requiring field splicing, which would introduce impedance discontinuities and potential ingress points for moisture and EMI.

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Technical Parameters

Hardware Logical Analysis

The 21747-085-00 operates within a three-component eddy-current transducer chain: probe → extension cable → Proximitor. The Proximitor drives the probe tip with a high-frequency oscillator signal (typically 1.0 MHz for 3300 Series 8 mm probes). The probe’s eddy-current field interacts with the target shaft surface, and the resulting impedance change modulates the oscillator. The extension cable carries this modulated RF signal back to the Proximitor demodulator stage.

Impedance Matching: The 95 Ω characteristic impedance of the 21747-085-00 is not arbitrary — it is the value at which the Proximitor’s output driver and the probe’s input impedance are conjugate-matched at the operating frequency. Substituting a generic 50 Ω or 75 Ω coaxial cable introduces a standing-wave ratio (SWR) that shifts the Proximitor’s calibration curve, producing a DC gap voltage offset that cannot be corrected by field zero adjustment alone.

EMC Shielding Architecture: The braided shield with ≥ 95% optical coverage provides a continuous Faraday enclosure around the signal conductor. In turbine environments where variable-frequency drives, high-current bus bars, and ignition systems generate broadband electromagnetic interference from 50 Hz to several hundred kilohertz, an incomplete shield allows capacitive coupling of noise onto the signal conductor. The 21747-085-00’s shield is terminated at both connector shells, creating a low-impedance return path that shunts induced currents to chassis ground rather than allowing them to appear as differential noise on the signal line.

Fluoropolymer Jacket Integrity: The PTFE-grade outer jacket resists hydrocarbon lubricants, steam condensate, and cleaning solvents encountered in turbine bearing areas. Unlike PVC jackets, fluoropolymer does not plasticize or crack under sustained thermal cycling between −35 °C and +85 °C, which preserves the dielectric constant of the cable and prevents moisture ingress that would alter the cable’s capacitance per unit length — a parameter the Proximitor’s calibration depends on.

Connector Termination Geometry: The OEM connectors are crimped and soldered to the cable center conductor and shield braid using factory tooling that controls contact resistance to below 5 mΩ. Field-fabricated connectors using generic coaxial tooling rarely achieve this tolerance, and elevated contact resistance at the probe-end connector introduces a resistive voltage divider effect that reduces the Proximitor’s effective sensitivity (mV/mil or mV/mm) below the factory-calibrated value.

System Integration Benefits

• Preserved calibration traceability: Because the 21747-085-00 is manufactured to the same electrical specification as the original system cable, the probe-Proximitor-cable assembly retains its factory calibration certificate without requiring field recalibration after cable replacement.
• Deterministic gap voltage baseline: The matched impedance and capacitance ensure the Proximitor’s DC output voltage accurately represents the physical air gap between probe tip and shaft, giving the monitor card a reliable reference for alert and danger setpoint logic.
• Reduced false-trip probability: Shield continuity and impedance matching suppress common-mode noise pickup, lowering the noise floor on the vibration channel and reducing nuisance trips caused by electrical interference rather than actual shaft motion.
• Direct plug-in replacement: OEM connector geometry eliminates the need for adapter fittings or field-modified terminations, reducing installation time and eliminating mechanical stress points at the connector-cable interface.
• Thermal stability across operating range: The fluoropolymer dielectric maintains a stable permittivity (εr ≈ 2.1) from −35 °C to +85 °C, ensuring the cable’s electrical length does not shift with ambient temperature changes in outdoor or steam-exposed installations.
• Compatibility with 3500 Series rack diagnostics: When used with 3500 Series monitor cards, the cable’s matched parameters allow the rack’s built-in transducer health diagnostics to correctly interpret probe OK/not OK status based on gap voltage thresholds, without false not-OK indications caused by impedance mismatch.
• Compliance with API 670 transducer chain requirements: API 670 (Machinery Protection Systems) specifies that extension cables must be part of the qualified transducer system. Using the OEM 21747-085-00 maintains compliance with this standard, which is a contractual requirement in many EPC and O&G plant specifications.
• Simplified spare parts management: A single part number covers the 8.5 m extension requirement across multiple 3300 Series probe diameters (5 mm, 8 mm, 11 mm), reducing the number of distinct cable SKUs that a maintenance department must stock for a mixed-probe installation.

Quality Assurance & Global Logistics

Every 21747-085-00 unit supplied by siemensplc.com is sourced through verified industrial channels with full part-number traceability. Physical inspection covers connector shell integrity, cable jacket condition, OEM labeling accuracy, and part number conformity against Bently Nevada documentation. Continuity and shield integrity checks are performed using calibrated instruments prior to dispatch.

Shipments originate from our warehouse in Xiamen, China, a major export hub with direct access to international express carriers. Standard dispatch for in-stock units is 1–3 business days. Available shipping services include DHL Express, FedEx International Priority, and UPS Worldwide Expedited. For bulk procurement (5+ units), sea freight consolidation is available with full export documentation: commercial invoice, packing list, certificate of origin, and MSDS where applicable. All shipments comply with applicable export control regulations. A 12-month warranty covers manufacturing defects from the date of shipment.

Contact Information

📧 Email: [email protected]
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🌐 Web: siemensplc.com
📍 Location: Xiamen, China
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