Description

Key Technical Specifications (For Spare Parts Verification)

Product Model: 8937-HN
Manufacturer: MTL Instruments (Eaton)
Series: MTL8000
Input Signal: 4–20 mA (passive, loop-powered)
Output Signal: 4–20 mA (isolated replica)
Power Supply: Loop-powered (no external power required)
Isolation Voltage: 2.5 kV AC (input to output)
HART Compatibility: Transparent pass-through supported
Safety Certification: ATEX, IECEx, FM, CSA for use in Zone 0/1 (gas) and Class I Div 1
Mounting: Standard DIN rail (35 mm)
Operating Temperature: -20°C to +60°C

System Role and Impact of Failure

The MTL 8937-HN is typically installed at the interface between field instrumentation in hazardous areas (e.g., pressure transmitters in oil & gas plants) and safe-area control systems such as DCS or PLC I/O cards. It performs two critical functions: it breaks ground loops that could distort analog signals, and it limits energy entering the hazardous zone to prevent ignition—fulfilling intrinsic safety requirements. If this module fails open-circuit, the 4–20 mA signal is lost, causing the control system to register a fault or default value (e.g., 0 mA = low alarm), potentially triggering unnecessary shutdowns. If it fails shorted or degraded, it may compromise the intrinsic safety barrier, creating a compliance and safety hazard. In safety instrumented functions (SIFs), its failure could invalidate SIL-rated loops.

Reliability Analysis and Common Failure Modes

Despite being a passive device with no active electronics requiring power, the 8937-HN is not immune to aging. Its reliability stems from robust design, but decades of service expose inherent vulnerabilities.

Common failure modes include:

Degradation of internal isolation transformer insulation due to thermal cycling or moisture ingress, reducing isolation integrity.
Corrosion of terminal blocks or PCB traces from environmental contaminants (e.g., H₂S in oil fields), leading to intermittent connections.
Drift in precision components (e.g., shunt resistors), causing signal offset errors outside acceptable tolerance (±0.1%).
Mechanical fatigue in DIN rail clips, causing modules to loosen or fall out during vibration events.

Design limitations include lack of diagnostic LEDs or health monitoring—failures are often detected only after process upset. Additionally, being loop-powered, it places a small but non-zero burden voltage on the transmitter loop; in marginal designs, this can contribute to signal dropout over time.

Preventive maintenance recommendations:

Perform annual loop calibration checks including zero/span verification through the isolator.
Inspect terminal tightness and signs of oxidation, especially in high-humidity or corrosive environments.
Use thermal imaging during routine rounds to detect abnormal heating (indicative of internal short or overload).
Maintain a stock of verified functional spares; test unused units every 24 months by installing in a dummy loop.

MTL 8937-HN

Lifecycle Status and Migration Strategy

MTL (Eaton) has formally discontinued the 8937-HN and the broader 8000 series. No new units are produced, and official technical support is limited to legacy documentation. Continuing to operate with this module carries risks: inability to source matched replacements, potential non-compliance with updated safety standards, and lack of traceable certification for new installations.

As an interim solution, facilities can procure tested surplus or refurbished units from specialized industrial suppliers, ensuring each unit undergoes full functional and safety validation before deployment.

The recommended migration path is to replace the 8937-HN with a modern equivalent such as the MTL4041 or MTL5041 series. These offer:

Active or passive variants with improved accuracy (±0.05%)
Enhanced HART and WirelessHART support
Full compliance with IEC 61508 and IEC 60079-26
Optional diagnostics via IO-Link or remote configuration

Migration typically requires minimal rewiring, as the form factor and terminal layout are similar. However, active versions need an external power supply, so panel power infrastructure may need updating. For passive-only applications, the MTL4041P provides a near drop-in replacement with extended lifecycle support and global certifications.