Safety relay mounts into PLC rack

3 min read

Safety is an issue of paramount importance for today's manufacturer which cannot be ignored. Yet tight margins, lost time and rising costs remain high on the agenda. A common cause of costly operational downtime can be traced to the operation and resetting of traditional stand alone safety circuits.

Mitsubishi's innovative approach to safety relays closely integrates the stand alone protection and performance of safety circuits and the diagnostic capability of the PLC either by incorporation of the new QS safety relay onto the PLC rack, or by direct high speed CC-Link for the stand alone networked version. This configuration means that for the first time, detailed intelligence is instantly available on the operational condition of the safety relay as well as the status of each of the connected safety circuits, thereby allowing rapid identification of safety conditions on the plant.

“This is a major step forward for plant owners and machine operators,” says Jeremy Shinton, Q Series Product Manager for Mitsubishi Electric.

“Typically a manufacturing plant will have a number of safety circuits on it, each with its own stand alone safety relay protecting one particular aspect of the process. If one of these circuits trip, the whole plant may be effectively shut down and production lost while engineers inspect the machine or process line looking for the appropriate relay and circuit to identify its cause and correcting the reason before restarting operations."

“With today’s tight margins any plant down time in manufacturing is bad news."

“The New Mitsubishi intelligent QS Safety Relay addresses this on two levels. Firstly the tripped circuit is instantly identified at the control system. This information can then be visualised by HMI, SCADA or simple panel indicators which dramatically reduce the circuit search and locate time. Secondly a history of trips and their causes can be logged and analysed, leading to identification of recurring issues which can thus be addressed.”

In developing the QS Safety Relay, Mitsubishi also realised the importance of the fact that something like half of all safety shutdowns are merely nuisance trips – typically someone brushing against an E-stop or a light curtain being broken by a badly wielded broom. Each inadvertent stoppage adversely affects productivity and impacts the bottom line of the host business.

The QS Safety relay sits on the rack of Mitsubishi’s Q series PLC and integrates fully with the plant or machine’s functional control system. The stand alone version is connected over CC-Link to the host PLC system. Data from these relays may be linked into higher level control systems such as MES (manufacturing enterprise systems), SCADA (supervisory control and data acquisition) and various management information generating systems.

The QS monitors eight variables per connected safety circuit, safety input status (on/off), safety output status (on/off), safety relay coil status (on/off), and safety relay contact status (on/off). The various possible combinations of these are the basis of a strong diagnostic capability developed from the control system’s intelligent assessment of the overall status.

“Most importantly the QS is powered independently from the PLC,” notes Shinton. “So if the PLC fails it does not affect the safety circuit, this stays independent protecting man and machine come what may.” The key advantage however of either integrating the QS with the PLC rack data bus, or by connecting the stand alone version via CC Link, is that detailed safety circuit status can be easily transmitted to the control system. This feature cannot be replicated using traditional safety relays, where basic tripped status is all that can be derived.

Multiple QS relays can be included in a single PLC rack, each of which can support up to three extension relays, connecting directly to individual field devices such as drives, switches, light curtains, interlocks and temperature monitors. Networking of PLCs effectively allows large safety systems to be configured. The networked CC-Link version further enhances system adaptability with the ability to create small stand alone groups.

The CC Link version also supports up to three extension relays and multiple CC relay stations can be configured. The flexibility of this architecture allows the safety circuits to be ‘zoned’ so that a safety trip only shuts down the relevant part of the plant.

The QS is certified by TUV, and covers EN954-1, Category 3 and Category 4 installations. It has also been certified to the new EN ISO13849-1 standard and meets performance level “e”

The QS Safety Relay is the first in a series of safety products launched by Mitsubishi, which will take this control giant into several new fields. In the UK the available safety market is thought to be valued at around £20m, and that technically there has been a large gap from the standalone (dumb) relays to the costly specialised safety PLCs.

The QS Safety Relay is easy to retrofit and reduces the need for panel space, so will find favour in both new build and project upgrades.

“We developed QS from listening to what our customers told us in that they wanted something with more intelligence than a stand alone safety relay but that a dedicated safety PLC was too specialised and costly. The fact that installation is as simple as clipping a standard module onto a PLC rack or a DIN rail and that no programming is involved means there is no learning curve to slow take up.”

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