Pepperl+Fuchs KFD2-STC5-EX1 Signal Conditioner – KFD2 Series

KFD2-STC5-EX1: Galvanic Isolation Barrier for 2-Wire HART Transmitters in Hazardous Locations

The Pepperl+Fuchs KFD2-STC5-EX1 is a single-channel, galvanically isolated transmitter power supply engineered for 2-wire 4–20 mA HART field instruments installed in Zone 0, Zone 1, and Zone 2 hazardous areas. Within a process control loop, this module occupies the marshalling layer between the field instrument and the DCS or safety PLC analog input card. Its primary function is threefold: supply loop excitation current to the field transmitter, reproduce a galvanically isolated 4–20 mA signal on the safe-area side, and maintain full HART FSK transparency in both directions without signal attenuation or phase distortion.

Unlike passive Zener barriers, which depend on a low-impedance earth bond for fault energy limitation, the KFD2-STC5-EX1 uses active galvanic isolation. This eliminates the requirement for a dedicated intrinsic safety earth bus, simplifies cabinet grounding architecture, and removes the common-mode voltage constraints that restrict Zener barrier deployment in multi-drop or floating-reference topologies. The result is a module that can be installed in virtually any DCS marshalling cabinet without site-specific earth impedance verification.

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Technical Parameters

Hardware Logical Analysis

The KFD2-STC5-EX1 is built around a DC/DC converter isolation stage combined with a precision current mirror on the output side. On the field-facing input, the module sources a regulated loop voltage to the 2-wire transmitter. The transmitter modulates loop current between 4 mA and 20 mA in proportion to the measured process variable. This current signal, along with the superimposed HART FSK carrier, passes through the isolation transformer. The transformer core is selected for flat frequency response across the HART band (500 Hz–10 kHz), ensuring that the 1200 Hz and 2200 Hz FSK tones are reproduced on the output side with less than 3 dB attenuation — a critical requirement for reliable HART communication over long cable runs.

The EMC design of the KFD2-STC5-EX1 addresses two distinct interference vectors. First, conducted interference from the 24 VDC supply rail is attenuated by an LC filter at the module’s power input, preventing high-frequency switching noise from the power supply from coupling into the signal path. Second, radiated interference from adjacent variable-frequency drives or high-current contactors is mitigated by the module’s shielded housing geometry and the inherent common-mode rejection of the differential current loop topology. The three-way galvanic isolation (field / supply / output) means that a ground fault on the field cable does not create a fault current path through the DCS analog input card — a failure mode that has caused undetected measurement errors in Zener barrier installations.

The fault detection circuit monitors field-side loop current continuously. If the loop current falls below approximately 3.6 mA (open-circuit condition) or exceeds approximately 21 mA (short-circuit or transmitter fault), the red LED activates and the output is driven to a defined fault state. This behavior is configurable in some KFD2 variants but is fixed in the STC5-EX1 to ensure deterministic SIL 2 behavior. The fault response time is within the module’s 30 ms response window, which is compatible with the scan rates of modern safety PLCs operating at 100 ms safety function cycle times.

System Integration Benefits

• No dedicated IS earth bus required: Active galvanic isolation removes the low-impedance earth bond dependency of Zener barriers, eliminating a common source of installation non-compliance in brownfield retrofits.
• Bidirectional HART communication preserved: The FSK signal is reproduced on both sides of the isolation barrier simultaneously, allowing a HART handheld communicator or asset management system (e.g., Emerson AMS, Honeywell FDM) to communicate with the field device from the safe area without loop interruption.
• Deterministic fault propagation to DCS: Open-circuit and short-circuit field faults drive the output to a defined mA value, enabling the DCS to distinguish between a process low reading and a wiring fault — improving diagnostic transparency without additional hardware.
• Power Rail bus compatibility: Snap-on connection to the Pepperl+Fuchs Power Rail eliminates individual 24 VDC wiring per module, reducing marshalling cabinet wiring labor by approximately 40% compared to individually wired barriers.
• High channel density: The 20 mm module width allows up to 36 channels per 720 mm DIN rail, supporting high-density marshalling in space-constrained offshore or modular skid installations.
• SIL 2 pre-assessed for safety loops: The module’s IEC 61508 SIL 2 assessment is documented in the Pepperl+Fuchs Functional Safety Manual, reducing the engineering effort required for SIL verification under IEC 61511 for each loop.
• Wide supply voltage tolerance: Operation from 20–30 VDC accommodates voltage drop across long Power Rail segments and aging power supplies without measurement degradation.
• Backward compatibility with KFD2 infrastructure: The STC5-EX1 is mechanically and electrically compatible with existing KFD2 series DIN rail carriers, Power Rails, and termination boards, enabling channel-by-channel replacement without cabinet redesign.

Quality Assurance & Global Logistics

Every KFD2-STC5-EX1 unit supplied through siemensplc.com is sourced as genuine Pepperl+Fuchs factory production. Each module carries the manufacturer’s serial number and production date code, enabling full traceability back to the factory test record. ATEX, IECEx, FM, and CSA certification marks are verified against the housing label prior to dispatch. Units are supplied in original Pepperl+Fuchs retail packaging with the product datasheet and EU Declaration of Conformity included.

Prior to shipment, each unit undergoes a functional verification check: loop continuity at 4 mA and 20 mA, isolation resistance measurement, and LED indicator function. Any unit that does not pass this check is quarantined and not dispatched.

Logistics operations are based in Xiamen, China. In-stock orders are dispatched within 1–2 business days via DHL Express or FedEx International Priority. Typical transit times are 3–5 business days to Europe and North America, 2–4 business days to Southeast Asia. Full export documentation is provided with every shipment: commercial invoice, packing list, certificate of origin, and manufacturer datasheet. For projects requiring consolidated shipments or specific Incoterms (EXW, FOB, CIF, DAP), contact the sales team directly.

Contact Information

Email: [email protected]
WhatsApp: +86 18359268345
Web: siemensplc.com
Location: Xiamen, China
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