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Belt and braces – plant safety and Ex protection

Safety switching devices for niche applications
Belt and braces – plant safety and Ex protection

Machine safety in explosive environments: when switchgear is required for this combined application field, two different and complex sets of rules and standards have to be complied with, both of which make high demands on the products in question. Even though this field is a niche within a niche, users still have diverse options in their choice of safety switchgear and sensors, including new position switch series with a safety function.

Both machine safety and explosion protection are subject to comprehensive normative sets of rules and standards, listed in Europe as the Machinery Directive and Atex regulations. In addition, international regulations (e.g. IECEx) and national standards or directives (UL/CSA, EAC, etc.) sometimes apply in both areas. This poses high demands on the switching devices used to monitor the position of guard doors on machines in Ex zones because here both sets of rules apply equally. In some types of application, such as shipbuilding or oil exploration, plants and manufacturers have to meet yet more additional standards, and for good reason: the risk of explosion is ever-present, and the physical work involved is hard and prone to accidents. Safety measures must be particularly effective.

The Ex AZ 16 was especially developed for such niche applications: a safety switch with a separate actuator, a “classic” of machine safety in its original basic design (without Ex protection). The Ex variant has the same dimensions as the original version, making the switching devices absolutely compatible for mounting. Users can also expect the same high standards regarding reliability and durability. The separate terminal compartment with the Ex e type of protection meets the demands for Ex protection; if desired, the Ex AZ 16 is optionally available with a cable assembly. It is suitable for use in Ex Zones 1 and 21 (gas) as well as zone 22 (dust).

Alternatively, the machine builder can opt for non-contact safety sensors – for example the Ex HS Si 4 series which, in combination with an actuator, monitors the position of guard doors. Its features include particularly high shock resistance. Another alternative are the cylindrical safety sensors in the Ex RC Si M 30 series with a separate actuator, also available as an Extreme version with IP 69K protection.

Emergency pull-wire switches are mainly used in dust Ex zones, e.g. conveyor plants for bulk goods. They serve as an “extended” emergency stop button and enable the emergency stop function to be activated across distances of up to 2 x 100 m. The Steute Ex range incorporates several ZS series which are able to fulfil their safety-related task even in adverse environments.

Safety position switches

One of the new features of the Ex range is two series of position switches which are also suited to functional safety applications. The switches in the Ex 97 and Ex 99 series can be used universally thanks to standard dimensions according to DIN EN 50047 and 50041 plus a variety of different actuators (rolling plunger, rolling lever, turning lever, etc.). Atex and IECEx have approved both series for use in Ex zones 1 and 2 (gas) as well as Ex zones 21 and 22 (dust). In addition, they are suitable for temperatures down to -60 °C, making high demands on the housing design and sealing. For instance, the high protection class of the switchgear (IP 66) must still be guaranteed at these temperatures following a 7-joule impact test.

The rugged housing of the position switches with a safety function is manufactured from high-quality fibreglass-reinforced plastic for this reason. The seal of the housing cover on the Ex 97 is vulcanised across the entire surface. The plunger is
sealed using a redundant system. The sealing materials and lubricants are approved by the manufacturers for temperatures down to -95 and -75 °C respectively. This means a sufficient safety net to the approved temperature of the overall switch, which is -60 °C. The insulated plastic design simultaneously means that earthing and equipotential bonding are no longer necessary.

The applications for the Ex 97 and Ex 99 include safety monitoring of flaps and doors in the oil and gas industry, as well as shipbuilding. Depending on the actuator – there are several options to choose from – the switches in this series can also be used as “normal” position switches without a safety function.

Metal alternative

Users who prefer a safety position switch with a metal housing can opt for the Ex 98 series, featuring a robust, anti-corrosive aluminium housing with a stainless steel cover, high protection classes and standard dimensions to DIN EN 50041. Staggered mounting of the various actuators is possible (4 x 90°) and approval for gas and dust Ex zones goes without saying. This series can be used down to -40 °C and is thus likewise suitable for extremely cold environments. Several series are available with approvals for explosive environments, even in applications requiring a solenoid interlock, e. g. because dangerous coasting movements are to be expected.

Wide selection even in the niche

This brief overview shows that even with complex niche applications like those combining explosion protection and functional safety, machine builders and plant operators still have different options when selecting switchgear. And this is equally true if yet more requirements are added to the mix, such as an extremely robust design or a suitability for sub-zero temperatures.


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Author: Rainer Lumme

Product Manager Extreme,
Steute Schaltgeräte

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