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Three is better than one in case of emergency

Safety relays with long proof intervals
Three is better than one in case of emergency

Safety relays are required in a wide range of applications in the process industry: in case of emergency, there must be a reliable means of activating sirens, warning lamps or active cooling systems and of switching off motor controls or emergency shutdown valves. The Pepperl+Fuchs solution – safety relays with a 1oo3 architecture equipped with test inputs for checking relay outputs – provides a higher level of availability compared with solutions offering only single or double force-guided contacts.

Safety relays with long proof intervals

Three is better than one in case of emergency
Safety relays are required in a wide range of applications in the process industry: in case of emergency, there must be a reliable means of activating sirens, warning lamps or active cooling systems and of switching off motor controls or emergency shutdown valves. The Pepperl+Fuchs solution – safety relays with a 1oo3 architecture equipped with test inputs for checking relay outputs – provides a higher level of availability compared with solutions offering only single or double force-guided contacts.
Wherever possible, safety applications are designed in the form of DTS (de-energised-to-safe) loops, to ensure that components such as motor controls or valves are switched off in case of an emergency. The benefits of this mode of operation become clear when you consider the high proportion of DTS (around 90 %) as opposed to ETS (energised-to-safe) applications (only around 10 %) which are responsible for switching on sirens, warning lamps or active cooling systems. If the power supply fails, the safety relay enters a safe state. A lack of a control signal at the actuator caused by a line fault on the field or control side of the relay module will also cause the safety loop to enter this safe state. Given the large number of safe errors, the proof intervals for the loop as a whole are quite long.
ETS solutions, on the other hand, are more complex to implement. Any faults that mean the actuator can no longer be switched on are considered dangerous errors. As a result, the proof test intervals for loops of this kind are usually shorter than those for DTS applications, as more dangerous errors can occur. For SIL 3 safety functions, a redundant power supply is generally also required.
Requirements for safety relays
Safety applications in process automation are characterised by the relatively low demand rates of the safety function. In view of these low demand rates, it no longer makes sense to permanently monitor the position of the switch contact. However, concepts based on interval checks of the switch contacts require short proof test intervals. In order for the signal circuit to function correctly, the safety relay must be compatible with the relevant control panel. Diagnostic functions, such as those provided by process control systems, must in no way cause devices to malfunction.
To enable these requirements to be met, safety relays from Pepperl+Fuchs are based on a 1oo3 (1-out-of-3) architecture. This means that the switch contacts offer three-way redundancy – three contacts arranged in series for DTS applications or in parallel for ETS applications. The benefit of this set-up is that the safety function of the device is retained even if up to two of the contacts fail.
In an ETS application, the 1oo3 architecture also offers enhanced reliability: with force-guided contacts, the closure of a contact does not necessarily mean that a current is flowing. In extreme cases, dirty or corroded contacts may cause contact resistance to such an extent that the safety function can no longer be reliably executed. In this case, the feedback provided about the status of the contact may be dangerously misleading, suggesting that a conductive connection is established via the switch contact. Modules with force-guided contacts offer the benefit that the condition of the switch contact is monitored automatically on an ongoing basis. As a result, any information about malfunctions following a switch request is provided immediately. The complex wiring and additional evaluation channels required in the control panel to return information on the switch contact position are only economically viable in applications where there is a high demand rate for the safety function. This is because a switching event is necessary for the diagnostic measure to be useful.
Working on the basis of 10 % PFD (probability of failure), the proof interval for safety relays is therefore in excess of 30 years for DTS applications or 10 years for ETS applications. The modules are equipped with proof test inputs for checking the relevant relay outputs. The device series also ensures compatibility between the field device and the control panel – a point that is becoming increasingly important in light of the growing need for diagnostic functions.
Integrated diagnostics
Digital output (DO) cards on control panels generally offer built-in diagnostic functions. In addition to delivering dynamic diagnostics through “test pulses”, the field circuit is also statically checked at regular intervals. To implement this function, the loop currents are measured and evaluated by the DO card (both when switched on and when switched off). The input on the safety relay filters the test pulses coming from the DO card. This prevents the field device from being inadvertently switched during diagnostics or a line fault inadvertently displayed on the control panel. What‘s more, this input enables currents to flow from the DO card – a minimum load is supplied in the “on” state while a trickle current can flow in the “off” state. This current flow has no impact on the switching function.
A further component of the input is the input current limitation function, which prevents peak currents from overloading the DO card by minimising the input currents required for switching. When combined with safety relays, DO cards can switch significantly higher currents and AC voltages. This greatly increases the load range of the DO card. This function is extremely useful when expanding or modernising plants if a new control level is to be linked to an existing actuator infrastructure. The inputs are compatible with the DO cards for a multitude of control panels.
Modules for typical implementations
The safety relays are available in DTS and ETS versions, with up to 5 A (20 mm wide KF version) or 3 A switching current (12.5 mm wide KC version) in each case. The DTS versions feature an integrated fuse to prevent contacts from melting in the event of an overload. This also saves extra space on the DIN mounting rail. The 20 mm DTS versions feature a replaceable fuse. The single-channel safety relays are loop powered. All modules are approved in line with Atex Zone 2 and offer SIL 3 as per IEC 61508 ed2 and EN 62061. Corresponding safety manuals are available.
The modules are part of the K-System, which has been successfully used in the process industry for more than 25 years. With 200 isolated barriers (hazardous area) and 150 signal conditioners (safe area), the K-System offers a wide range of products for various signal types and applications in the process industry. The K-System sets itself apart through its cost-efficient mounting on the Power Rail, high level of availability and ease of maintenance. In the event of a fault, removable terminals allow modules to be replaced quickly without having to undo the wiring.
www.cpp-net.com search: cpp0316pepperlfuchs

Andreas Grimsehl
Andreas Grimsehl
Product Marketing Manager Interface Technology, Pepperl+Fuchs
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