Eaton MTL4500/MTL5500 range Analogue Output Modules

can be used to determine the health of the instrument function. The same condition of the actual loop current falling

to a low level will also occur if the wiring between the logic solver and the isolator is open circuit.

Using a field device and logic controller, as defined in section 2. with an MTLx54x then a system-loop can be

implemented that applies functional safety together with intrinsic safety to meet the requirements of protection

against explosion hazards. Note that the transfer of HART communications through the isolator is not considered as

part of the safety function of the isolator.

It should be recognised that the systematic capability of the products limits their application to SIL2 loops.

It is important that the effect of electromagnetic interference on the operation of any safety function is reduced

where possible. For this reason it is recommended that the cable connections from the logic solver to the isolator

modules be a maximum of 30 metres and are not exposed to possible induced surges, keeping them inside a

protected environment.

Similarly, operation of the equipment outside of its environmental ratings induces component stress and

temperature above the normal ambient of 60°C is to be avoided to ensure required performance.

Assessment of Functional Safety

The design features and the techniques/measures used to prevent systematic faults are suitable for the use of

the MTLx546 and MTLx549 modules in safety functions up to SIL2 in a simplex architecture.

The hardware assessment shows that MTLx54x analogue output modules:

•  have a hardware fault tolerance of 0

•  are classified as Type A devices (“Non-complex” component with well-defined failure modes)

•  have no internal diagnostic elements