SIL3 coupling relay connects safety controllers to actuators

In addition to ensuring safety when circuits are opened, accounting for safety when circuits are closed is also becoming increasingly important. Modules for electrically isolating actuators and for adjusting power are used especially in the process industry. For this purpose, the Phoenix Contact PSR-ETP coupling relay has been developed specifically for this industry and is certified in accordance with SIL 3.

In a typical application, a secure digital output controls an alarm that provides an acoustic warning to employees in the event of a hazardous situation. Since the current drawn by the alarm exceeds the maximum current-carrying capacity of the output, an intermediate coupling relay is activated. In the resulting safety circuit, the user must monitor and evaluate all three components respectively. At the same time, the particular technical features related to safe controller outputs must be taken into consideration, such as current monitoring and cyclical test pulses.

Filter element ensures long service life

Safety controllers are typically able to perform dynamic self-tests of their outputs or the existing wiring. To do this while an output is switched on, the signal is switched from high to low for a short time, for example, 2 ms, and then back to high. If the output is switched off, the test is run in reverse order, meaning a short high pulse is sent. For a connected load such as a safety coupling relay, this results in a brief voltage interruption or a short voltage pulse, which must not initiate a switching operation in the device.

Short pulses of this kind do not necessarily cause the relay contact to open or close. Instead, they produce a pre-excitation or a de-excitation of the coil. This can lead to armature movement, resulting in premature wear and thus relay failure. In order to counteract this effect, a filter element must be used to filter out the test pulses from the control signal. This effectively prevents a reduction of the safety coupling relay’s service life.

Control circuit adjusts current consumption

The capability of monitoring short circuits and line breaks is another special feature of safety controller outputs. This monitoring feature requires maintaining the current consumption of the switched load within a relatively narrow range. On the one hand, it must not fall below a minimum value, so that an open circuit is not detected. On the other, it should not exceed a certain maximum value, which would trigger the short-circuit monitor. Current peaks should therefore be avoided, especially at the instant when circuits are closed. This scenario calls for using an internal circuit to hold the current at a constant level precisely within the required window between the minimum and maximum values. If such measures are not taken, these test pulses frequently cause premature relay wear. Integrating a test pulse filter into the safety coupling relay and using an adjusted current control circuit helps to ensure that the device has a long service life and that it maintains a high level of compatibility with the various safe controllers.

The PSR-ETP safety coupling relay has been specially adapted to failsafe controllers used in process engineering, while also meeting all requirements for use in applications up to SIL 3 in accordance with IEC 61508 and IEC 61511. The device has six internal contacts connected both in parallel and in series. The resulting redundancy is important for achieving the required level of safety. The specific requirements of safety controllers are taken into account through an integrated test pulse filter to extend the service life and a control circuit to adjust current consumption.

Carefully selecting components is a vital requirement

In developing the PSR-ETP safety coupling relay, Phoenix Contact has once again paid special attention to carefully selecting the integrated relay. Unlike in safety disconnection applications, contacts often remain open for years in fire and gas applications before they are activated for the first time, causing them to close. Therefore, when selecting suitable relays and predicting potential malfunctions, the considerations differ from those of safety-related disconnection applications.

One criterion in this context is the contact friction path. This path is beneficial for removing any tarnish layers from the contact surfaces, thus ensuring a conductive connection in the event of a hazard. In order to minimise the formation of the tarnish layer in the first place, the manufacturer must use low-outgassing materials. It becomes apparent here why the relay’s NO contact is used to perform the safety function, as the relay coil remains unpowered until the application is required. This causes significantly less intrinsic heat build-up in the relay and saves energy. Fully automating the relay production process is another important factor. By eliminating the need to adjust the contacts manually, the interior of the relay is less vulnerable to contamination from foreign particles. Since each relay is characterised by different strengths, Phoenix Contact has paid attention to both redundancy and diversity when designing the PSR-ETP safety coupling relay. Having a relay design that is specifically matched to the given application is the best way to ensure a high level of safety and availability.

Line/load monitoring decreases installation cost

The PSR-ETP’s optional line/load monitoring feature supports load-dependent configuration and allows performing integrated diagnostics from the controller through to the actuator. A wire breakage or a missing or incorrect load is thus indicated via the wiring between the digital output and the coupling module. This means that additional installation costs and additional digital inputs to read back the actuator state can be eliminated.

Wiring errors frequently necessitate costly and time-consuming troubleshooting when starting up systems or system components. This is why system cabling is playing an increasing role as a component of integrated solutions. Phoenix Contact’s optionally prewired termination carrier thus combines the benefits of modular rail-mounted devices with the advantages of plug-and-play quick cabling to form an integrated systems engineering design. A diode-decoupled power supply is also provided on the termination carrier’s mechanically decoupled circuit board. In addition, there is a relay with a floating contact for indicating any malfunctions in the power supply.

The PSR-ETP safety coupling relay also provides all the necessary parameters and certificates for facilitating loop calculation, project planning and documentation preparation.