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1 INTRODUCTION SIL
1.1 Application and function
The binary input modules that are the subject of this manual are intrinsic safety isolators that enable a safe-area load to be controlled by a proximity detector or switch located in a hazardous area of a process plant. They are also designed and assessed according to IEC 61508 for use in safety instrumented systems up to SIL2.
In addition to the transfer of the switch status, the detection and signalling of line faults is indicated by an LED on the top of the module and also provided through a separate relay output in some models.
All of the 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 1-3, and 4-6) are made through the removable blue connectors, but the safe area and power connections for the MTL45xx modules are made through the connector on the base, while the MTL55xx 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 the DIN-rail mounting models.
The binary input models covered by this manual are:
MTL4504 single channel, switch/proximity detector repeater, independent LFD
MTL4511 and MTL5511 single channel, switch/proximity detector repeater
MTL4514/B and MTL5514 single channel, switch/proximity detector repeater, independent LFD
MTL4516/C and MTL5516C dual channel, switch/proximity detector repeater,
MTL4517 and MTL5517 dual channel, switch/proximity detector repeater, independent LFD
Note: To avoid repetition, further use of MTLx51x in this document can be understood to include both DIN-rail and backplane models specified above. Individual model numbers will be used only where there is a need to distinguish between them.
2 System Configuration
The modules 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 modules are designed to power a proximity detector or a switch in the hazardous area and to reflect the open or closed condition of the field switch through relay contact output to the safe-area load. The shaded area shows the safety related system connection, while the line fault connections are not safety-related but may be used to assist diagnosis of field circuit conditions.
For simplicity the term ‘PLC’ has been used to denote the safety system performing the monitoring function of the process loop variable. Note: When using the dual channel module variants both channels must not be used in the same safety function to avoid concerns of common-cause failures.
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. In both situations the contribution of each part is considered in relation to the whole.
The module is a component in the signal path between safety-related sensors and safety-related instruments or control systems.
3 Selection of product and implications
For systematic capability of SIL2 the normally open contact of the relay should be used, with the input such that the relay is in the normally energised state. This gives the operating condition where the safe state is relay de-energised, output contact open.
Consideration should be given to the affect of the wetting current and voltage that are applied to the contacts of the relay. If the modules are used at high voltage or current within the rating of the relay specification it is advisable not to subsequently apply the module for service at low currents or voltages. This is to avoid possible problems due to degradation of the contact surfaces. The rating of the relay contact shall be limited to a maximum of 500mA/35V dc and a minimum of 50mW e.g. 10mA at >=5V dc in the ON state.
Using an input sensor and logic controller as defined in section 2 and these modules, a system-loop can be implemented that applies functional safety together with intrinsic safety to meet the requirements of protection against explosion hazards.
4 Assessment of functional safety SIL
The design features and the techniques/measures used to avoid systematic faults make the modules suitable for use in an instrument loop implementing safety functions up to SIL2.
The hardware assessment shows that these Switch/Proximity Detector Interface 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 SIL
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.
For functional safety applications the equipment should not be subjected to shock impacts of greater than 5g to ensure that the mechanical relay contacts are not affected.
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.
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. Note also that the maximum number of switching cycles under the load conditions given in section 3 is 22 x 104 at 500mA dc, rising to 9 x 106 at 10mA dc.
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