MTL Barries - MTL7756ac - 3- Wire RTDs (grounded bridge)

The MTL7756AC works as an intrinsically safe shunt-diode safety barrier that limits electrical energy to prevent sparks in hazardous areas. It uses a series of Zener diodes, resistors, and fuses to divert excess energy to ground, protecting equipment and people from potential ignition sources. When a fault occurs, the diodes limit voltage and the resistors limit current, while fuses protect the diodes themselves, ensuring safety in both normal and fault conditions.

How it works

• Energy diversion: The barrier is designed to limit the maximum voltage and current that can pass into a hazardous area.
• Fault conditions: In the event of an electrical fault in the "safe" area, the barrier acts to prevent a dangerous amount of energy from reaching the "hazardous" area.
• Voltage and current limitation:

• Zener diodes: When the voltage exceeds the Zener breakdown voltage, the diodes conduct and shunt the excess voltage to ground.
• Resistors: In conjunction with the diodes, the resistors limit the current to a safe level.

• Failsafe design: The MTL7756AC includes two stages of voltage limitation, meaning that if one stage fails, the other can still provide protection.
• Protection: A fuse protects the entire assembly from overcurrent, and a series of resistors and diodes ensures that the voltage and current passed to the field device remain within safe, non-sparking limits

The MTL7756AC is a passive shunt-diode intrinsic safety (IS) barrier designed to prevent ignition in hazardous areas by limiting the electrical energy that can enter those environments. It operates using a simple, robust circuit comprising Zener diodes, a resistor, and a fuse.

RTD Connection

Working Principle

The core function of the MTL7756AC is to safely divert excess electrical energy to the earth ground connection, ensuring that no sparking or overheating can occur in the hazardous location.

1. Normal Operation: Under normal, fault-free conditions, the current flows from the safe area (e.g., control room) to the field device in the hazardous area. The barrier acts as a normal connection, potentially providing a small voltage and current limitation due to the series resistor.
2. Fault Condition Protection: In the event of an electrical fault in the safe area (e.g., overvoltage or power surge), the barrier's components work to protect the hazardous area:

• Zener Diodes: These diodes are connected in shunt (parallel) to the circuit. When the voltage exceeds a predetermined safe limit (the Zener voltage), the diodes conduct, effectively "clamping" the voltage to that safe level and diverting the excess energy to the dedicated, high-integrity earth ground.
• Resistor: A series resistor is included to limit the current flow, working in conjunction with the Zener diodes to ensure the current remains below a level that could cause ignition. The resistor is typically "infallible," meaning it will not fail in a way that compromises safety.
• Fuse: A fuse is incorporated to protect the Zener diodes from damage under extreme or prolonged fault conditions, ensuring the safety mechanism remains intact.

Key Features and Application

• Intrinsic Safety: The device is certified 'ia' for use in all zones and 'IIC' for all explosive atmospheres, meaning it is designed to be safe even with two component failures.
• Application: The MTL7756AC is specifically designed for variable resistance measurements, such as those involving 3-wire Resistance Temperature Detectors (RTDs).
• Installation: It is a passive device designed for easy DIN-rail mounting, which also provides a reliable earth connection.

You can find more technical details in the Eaton MTL7700 Range Datasheet.

 Full Datasheet