Comment: Reducing hazard potentials
Birger Diekmann, Process Technology Product manager - Business Unit ICC of Germany-based Phoenix Contact GmbH & Co. has his say on terminal blocks in explosive atmospheres
Many production plants process flammable gases, fumes, dusts, or liquids – and this quite often has the side effect of generating explosive atmospheres.
Electrical devices such as terminal blocks, which are often installed throughout the plant in the thousands, could potentially act as ignition sources. In order to keep the risk of explosion as small as possible, numerous points need to be observed.
For an explosion to actually occur, these three things generally have to coincide: an explosive substance, oxygen, and an ignition source. In many cases, it is impossible to avoid handling explosive substances.
Examples include flour dust in mills, flammable liquids such as gasoline, and gases such as hydrogen. Whether or not these explosive substances generate an explosive atmosphere mainly depends on the ratio of oxygen – for example, from ambient air – and the explosive substance. Consequently, ‘primary explosion protection’ refers to preventing such a dangerous constellation from arising.
One way to go about preventing potentially explosive substances from mixing is to provide ample ventilation or to add nitrogen (inertization).
If the measures taken are inadequate or if none are taken at all, the key objective is to prevent the explosive atmosphere from igniting. This is referred to as secondary explosion protection. Production environments are replete with factors that act as potential ignition sources.
This includes hot surfaces, electrostatic discharge, ultrasound, and sparks, which need to be considered when selecting the components to be used in such an environment. Particular attention should be paid to electrical devices, and among these the terminal block.
Terminal blocks in potentially explosive zones
The application range of terminal blocks is wide: These are found in control cabinets in the field and the control center, where they connect small and large wires in order to distribute power or transfer signals.
Terminal blocks are highly reliable, easy to handle, modular, and flexible, and bring clarity and structure to any switching task, making them indispensable even in the age of bus technology. When selecting suitable terminal blocks, operators need to carefully gauge the probability of having an explosive atmosphere in place as well as the risk of the terminal block acting as an ignition source under certain conditions.
The operator is responsible for making this hazard assessment. The procedure involves dividing the plant into multiple zones. The selected ignition protection class for a component depends on the zone where it is to be used.
Free choice of connection technology
The ‘increased safety’ ignition protection class is one of the most common terminal block types, which is suitable for use in zones 1 and 2. This level of protection prevents ignition behaviour during operation and requires the exclusion of malfunctions based on specific measures.
Terminal blocks therefore need to fulfill the following requirements, among other things: vibration resistance and corrosion protection, proof against self-triggering, and safe contacting in environments with fluctuating temperatures. In addition, during production the terminal blocks are already subjected to a 100% individual dielectric strength inspection. Suitability is certified by an independent test centre.
In addition to the common screw connection technology, nowadays there are other technical options for meeting these requirements, which opens the door for new applications. Pierce contact technology, for instance, supports wires of up to 2.5 mm² – without the need for previous stripping and crimping. This makes it possible to reduce the wiring effort by up to 60 percent, thus greatly reducing the cost of new plants or retrofitting existing ones.
Another connection technology that is particularly popular in the field of process technology is push-in technology. Here, too, the tool-free, direct insertion of crimped and rigid wires delivers time savings. And the compact design allows operators to save up to 30% installation space as compared with common solutions. Featuring connection capacities of up to 150 mm², even zone 1 applications with currents beyond 250 A are viable.
Safe disconnecting and measuring in Ex environments
Intrinsic safety ‘Ex i’ represents another explosion protection concept. Electric power provided to the device is limited to ensure it cannot become an ignition source. The same principle can be applied to all three zones.
Most signals in these areas feature a low voltage and cover the current range from 4 to 20 mA, which is why measurement and control technology mainly draws on this ignition protection class. Modular terminal blocks used in intrinsically safe circuits are simple electrical devices.
The manufacturer guarantees compliance with the required air and creepage paths as well as spacing through solid insulation for circuits of up to 60 V. Terminal blocks certified for increased safety may also be used in intrinsically safe circuits, and most feed-through terminal blocks have this certification.
However, this certification is generally only required for disconnectable or fuse terminal blocks. If more electric power were used, an ignition spark could occur when opening or closing the sectioning point, or a blown fuse might reach a temperature beyond the permitted limit. For this reason, such function terminal blocks are not suitable for Ex e certification.
In order to make currents above the intrinsically safe range measurable or to protect such currents using common glass fuses, knife disconnect and fuse terminal blocks are available in the Ex nA ignition protection class.
This requires all disconnectable components to have, for example, a release force exceeding 15 N or 100 times their own weight to prevent accidental opening. The fuse carriers in zone 2 are equipped with G-type sand-filled fuse inserts.
Terminal blocks can be used to set up safe electrical connections in all zones of plants with an explosion hazard. The different characteristics of the connection technologies allow operators to address individual application specifics. Terminal blocks deliver functions above and beyond directly connecting two wires – they further support measuring, disconnecting, and limiting currents in Ex zones.