Eaton MTL MTL7728- Single Channel 4-20mA Current Loop Zener Intrinsic Safety

The MTL7728- is a high-performance 1-channel shunt-diode safety barrier (Zener barrier) from Eaton's Crouse-Hinds series MTL7700 range, specifically engineered for analogue and digital I/O applications requiring intrinsic safety protection in hazardous areas with negative polarity configuration.

This passive device limits the voltage and current that can reach a hazardous area under fault conditions, preventing ignition of flammable atmospheres. With a rated supply voltage of 28 V DC, safety description of 28 V, 300 Ω, 93 mA, and negative polarity configuration, the MTL7728- is the ideal solution for protecting controller outputs, solenoids, proximity detectors, and switch inputs in Zone 0, 1, and 2 hazardous locations (gas groups IIC, IIB, IIA).

The "-" suffix indicates negative polarity, distinguishing it from the positive polarity MTL7728+ and the AC version MTL7728ac. This model is the direct replacement for the discontinued MTL728- from the MTL700 range.

• Negative Polarity Configuration: Specifically designed for systems requiring negative polarity — ideal for certain control system architectures and floating power applications
• IIC Gas Group Certification: Certified for the most hazardous IIC gas groups (hydrogen, acetylene) — suitable for use in Zone 0, 1, and 2 explosive atmospheres
• 1-Channel General Purpose I/O: Single channel rated for 93 mA switching current — ideal for protecting controller outputs, solenoids, proximity detectors, and switch inputs
• 28 V Working Voltage: Provides safe voltage limiting for analogue/digital circuits, preventing ignition under fault conditions
• 300 Ω End-to-End Resistance: Matches typical loop resistance requirements for 4-20 mA signal circuits while ensuring intrinsic safety compliance
• 93 mA Maximum Current: Adequate current handling for most field instrumentation including transmitters, sensors, and switch inputs
• Matched Power Output: 0.651 W worst-case power transfer — 28V x 93mA / 4
• Removable Terminals: Unique removable terminal blocks for easy cabling, maintenance, and loop disconnect by simply unplugging terminals from the side of the module
• Bussed Power Capability: Integrated bussed power distribution reduces cabling requirements when used with the MTL7798 power feed module
• Angled Wire Entry: Wire entry is angled to assist wiring within limited space enclosures
• Ultra-Slim DIN Rail Mounting: Compact 12.6 mm width for standard 35 mm T-section DIN rail
• Dual-Stage Protection: Contains two stages of pulse-tested Zener diodes and an 'infallible' terminating resistor for maximum reliability
• Direct Replacement for MTL700 Series: Compatible terminal numbering and safety descriptions allow direct replacement of older MTL700 series barriers (MTL728-)
• Global Approvals: ATEX, IECEx, CSA, FM, and LR Marine certifications for international installation flexibility

The MTL7728- is a passive shunt-diode safety barrier that operates on the Zener barrier principle. It limits both voltage and current that can be delivered to a hazardous area circuit under fault conditions.

• Voltage limiting: Zener or forward-connected diodes clamp the output voltage to a safe level (28V) if the safe-area voltage rises above normal operating levels
• Current limiting: A series resistor (300Ω) limits fault current to a safe value (93mA)
• Fusing: A fuse protects the diodes, and the two stages of voltage limitation ensure continued safety if either stage should fail
• Negative polarity: The barrier is designed for negative ground applications — common in certain industrial control system architectures
• No active output current limiting circuits: Pure passive design for maximum reliability
• Grounding: The barrier requires a connection to a high-integrity earth ground — made simultaneously when clipping onto standard T-section DIN rail

The MTL7728- is specifically designed for general-purpose I/O protection in hazardous area installations requiring negative polarity:

• Solenoid valves (low-power)
• Proximity detectors (NAMUR)
• Switch inputs (limit switches, pressure switches)
• 2-wire transmitters (with bussed power)
• Analogue I/O circuits
• Conventional fire detectors

• Negative Polarity Configuration — Specifically designed for negative ground systems
• Industry Standard for IIC Applications — Direct replacement for the MTL728-
• Unique Removable Terminals — Simplifies cabling, maintenance, and barrier replacement without rewiring
• Bussed Power Capability — Integrated bussed power distribution reduces cabling requirements
• Angled Wire Entry — Assists wiring within limited space enclosures
• Dual-Stage Protection — Two stages of voltage limitation ensure continued safety
• Direct Replacement for MTL700 Series — Compatible with existing installations
• Global Approvals — Certified for use in major industrial markets worldwide
• Proven MTL Heritage — Established as the worldwide standard since 1984
• Simple Installation — DIN rail mounting with simultaneous IS earth connection