MTL Barries - MTL7787 Analog Input/Output (high level) - Digital On/Off Input and Output

The MTL7787 is a Zener barrier that works on the principle of limiting electrical energy to a hazardous area by using a series of diodes and resistors. It creates an intrinsically safe barrier between a safe area and a hazardous area, ensuring that any faults in the safe area are prevented from creating sparks or excessive heat that could cause an ignition in the hazardous area. This is achieved through two stages of voltage limitation via Zener and forward-connected diodes, and a terminating resistor that limits current, with a fuse to protect the diodes.

How it works

• Diode-based voltage limitation: The barrier uses two stages of Zener diodes to clamp the voltage to a safe level.
• Current limitation: The barrier incorporates a terminating resistor to limit the current that can flow into the hazardous area.
• Fault protection: In the event of a fault on the safe-area side, such as a short circuit, the diodes and resistor limit the energy to prevent a dangerous spark.
• Dual-stage safety: The two stages of voltage limitation ensure that the barrier remains safe even if one of the diode stages fails.
• Fuse protection: A fuse protects the diodes from damage in case of a severe fault condition.
• Intrinsic safety: The overall effect is to create an intrinsically safe circuit that is incapable of releasing enough electrical energy to cause ignition of a flammable atmosphere.
• Power feed: The MTL7700 range, which includes the MTL7787, can also provide a bussed power feed to other modules, which helps to reduce wiring complexity.

RANGE BARRIER APPLICATIONS
ANALOGUE INPUTS (HIGH LEVEL)	DIGITAL (ON/OFF) INPUTS
DIGITAL (ON/OFF) OUTPUTS	ANALOGUE OUTPUTS

 The MTL7787 is a passive shunt-diode (Zener) barrier that operates on the principle of Intrinsic Safety (IS). Its working principle is to reliably divert excess electrical energy to the ground, thus preventing the ignition of flammable atmospheres in hazardous locations.

Working Principle

The MTL7787 uses a simple, robust circuit design to limit the voltage and current that can enter a hazardous area from a safe area (e.g., a control room) in the event of an electrical fault.
The core components within each channel of the barrier are:

• Two stages of Zener (or forward-connected) diodes: These diodes are connected in a shunt configuration (parallel to the signal line and to the safety earth). When the voltage on the line exceeds a specific safe threshold (the Zener voltage), the diodes conduct, diverting the excess voltage and associated energy to the earth connection.
• An "infallible" terminating resistor: This resistor is in series with the input line. Its purpose is to limit the current flowing into the hazardous area under fault conditions, even if the diodes fail or the maximum input voltage is applied.
• A fuse: The fuse is strategically placed to blow if a persistent overvoltage or overcurrent condition occurs. This disconnects the safe-area circuit from the hazardous-area circuit, protecting the Zener diodes from sustained power dissipation that could cause them to fail.

Summary of Operation

1. Normal Operation: During normal operation within specified working voltages, the Zener diodes do not conduct significant current, and the signal passes through the barrier with only a small voltage drop across the series resistor.
2. Fault Condition (Overvoltage/Overcurrent): If an electrical fault in the safe area causes an excessive voltage or current (e.g., from a power surge or wiring error), the Zener diodes instantly begin conducting, clamping the voltage at a safe, predetermined level. Simultaneously, the series resistor limits the current.
3. Safety Redundancy: The two-stage diode design ensures continued safety. If one diode stage fails, the second stage maintains the voltage limitation, and the fuse will eventually blow to permanently isolate the fault.

Key Features of the MTL7787

• Intrinsic Safety (IS): The device is certified 'ia' for use in all hazardous zones and 'IIC' for all explosive atmospheres, meaning it prevents energy levels from ever reaching the point where they could cause a spark capable of ignition.
• Passive Design: It does not use active current-limiting circuits, relying instead on passive components for robust, predictable failure modes.
• Earthing: The reliable operation of the Zener barrier is dependent on a high-integrity connection to a safety earth via the DIN rail.

For detailed product specifications and installation guidelines, you can refer to the official Eaton MTL 7700 Range Datasheet.

 Full Datasheet