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1 INTRODUCTION SIL
1.1 Application and function
The MTL454x and MTL554x are isolator modules which enable an
analogue 4/20mA control signal to be passed to a device located in
a hazardous area from a safe area. The output current available to
the hazardous area is limited to comply with the requirements of
the process explosion hazard. The modules are also designed and
assessed according to IEC 61508 for use in safety instrumented
systems up to SIL2 when the required function is to de-energise the
output.
For ‘smart’ valve positioners using the HART protocol the units
allow bi-directional communications superimposed on the 4/20mA
signal current.
There are no configuration switches or operator controls to be set
on the modules – they perform a fixed function related to the model
selected. The MTLx546 models are single channel while the MTLx549
models are dual channel, although both channels must not be used in
the same safety instrumented function.
These modules are members of the MTL4500 and MTL5500 Series of
products.
1.2 Variant Description
Functionally the MTL4500 and MTL5500 Series modules are the same
but differ in the following way:
- the MTL4500 modules are designed for backplane mounted
applications
- the MTL5500 modules are designed for DIN rail mounting.
In both models the hazardous area field-wiring connections
(terminals 1-2, and 4-5) are made through the removable blue
connectors on the top of the modules, but the safe area and power
connections for the MTL454x modules are made through the connector
on the base, while the MTL554x uses the removable grey connectors
on the top and side of the module.
Note that the safe-area connection terminal numbers differ between
the backplane and DIN-rail mounting models.
The analogue output models covered by this manual are:
MTL4546 and MTL5546 single channel, open and short cct LFD
MTL4546C, MTL4546Y, and MTL5546Y single channel, open cct LFD
MTL4549, and MTL5549 dual channel, open and short cct LFD
MTL4549C, MTL4549Y, and MTL5549Y dual channel, open cct LFD
Note: To avoid repetition, further use of MTLx54x in this document
can be understood to include both DIN-rail and backplane models.
Individual model numbers will be used only where there is a need to
distinguish between them.
2 System Configuration
An MTLx54x module may be used in single-channel (1oo1) safety
functions up to SIL2.
The figure below shows the system configuration and specifies
detailed interfaces to the safety related and non safety-related
system components. It does not aim to show all details of the
internal module structure, but is intended to support understanding
for the application.
The MTLx54x modules are designed to repeat the current signal from
a safe-area source to a field device such as a current-topneumatic
converter or valve positioner in the hazardous area. The shaded
area indicates the safety relevant system connection, while the
power supply connections are not safety-related. For simplicity the
term ‘PLC’ has been used to denote the safety system performing the
driving function of the process loop.
Note: When using the MTLx549 dual-channel modules, it is not
appropriate for both channels to be used in the same loop, or the
same safety function, as this creates concerns of common-cause
failures. Consideration must also be made of the effect of
common-cause failures when both loops of a dual-channel module are
used for different safety functions.
2.1 Associated System Components
There are many parallels between the loop components that must be
assessed for intrinsic safety as well as functional safety where in
both situations the contribution of each part is considered in
relation to the whole.
The MTLx54x module is a component in the signal path between
safety-related actuators and safety-related control systems.
The current to pressure converter, valve positioner, or other field
device, must be suitable for the process and have been assessed and
verified for use in functional safety applications as well as its
certification for hazardous area mounting.
The safety system PLC shall have a current output with a normal
operating range of 4-20mA but capable of working over the extended
range of 3 to 22mA for under- and over-range. Such controllers will
normally also include a readback facility to enable the detection
of open or short circuits in the wiring.
The transmission of HART data is not considered as part of the
safety function and is excluded from this analysis.
3 Selection of product and implications
The analogue signal levels employed by the MTLx54x are within the
operating range of 4-20mA under normal conditions.
If the wiring between the isolator and field device are open
circuit then the line-fault detect (LFD) operation of the isolator
forces the current taken into the input terminals to fall to a low
value, which is less than the expected under-range value of 3.6mA.
The MTLx546 and MTLx549 models also detect when the resistance in
the field wiring is less than fifty ohms indicating a short circuit
condition, and this also is reflected into a low input current
value.
This diagnostic aspect can be used by logic solvers that include a
readback facility to monitor the output current from their output
cards or modules. Thus the ability to detect that the actual
current being passed is not the desired value 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.
4 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)
• There are no internal diagnostic elements of these products.
5 Installation
There are two particular aspects of safety that must be considered
when installing the MTL4500 or MTL5500 modules and these are:
• Functional safety
• Intrinsic safety
Reference must be made to the relevant sections within the
instruction manual for MTL4500 Series (INM4500) or MTL5500 Series
(INM5500) which contain basic guides for the installation of the
interface equipment to meet the requirements of intrinsic safety.
In many countries there are specific codes of practice, together
with industry guidelines, which must also be adhered to.
Provided that these installation requirements are followed then
there are no additional factors to meet the needs of applying the
products for functional safety use.
To guard against the effects of dust and water the modules should
be mounted in an enclosure providing at least IP54 protection
degree, or the location of mounting should provide equivalent
protection such as inside an equipment cabinet.
In applications using MTL4500 Series, where the environment has a
high humidity, the mounting backplanes should be specified to
include conformal coating.
6 Maintenance
To follow the guidelines pertaining to operation and maintenance of
intrinsically safe equipment in a hazardous area, yearly periodic
audits of the installation are required by the various codes of
practice.
In addition, proof-testing of the loop operation to conform with
functional safety requirements should be carried out at the
intervals determined by safety case assessment.
Proof testing must be carried out according to the application
requirements, but it is recommended that this be carried out at
least once every three years.
Refer to Appendix B for the proof testing procedure of the MTL4500
or MTL5500 modules.
Note that there may also be specific requirements laid down in the
E/E/PE operational maintenance procedure for the complete
installation.
If an MTL4500 or MTL5500 module is found to be faulty during
commissioning or during the normal lifetime of the product then
such failures should be reported to MTL. When appropriate, a
Customer Incident Report (CIR) will be notified to enable the
return of the unit to the factory for analysis. If the unit is
within the warranty period then a replacement unit will be sent.
Consideration should be made of the normal lifetime for a device of
this type which would be in the region of ten years.