The mere thought of such a power failure is enough to make logistics managers break out in a cold sweat. It would no longer be possible to place goods in storage, and customers would be left waiting in vain for their packages. The standard next-day delivery promise of online traders would be at serious risk. In many cases, the cause of such an outage is trivial. A tool that’s been left behind or a screw is all it takes to trigger an arc flash in switchgear, with dire consequences. Depending on the type of arc fault protection installed, parts of the system may be destroyed or at the very least badly damaged. In the worst-case scenario, it can take several weeks to repair – a period during which the logistics centre is restricted to emergency operations.

But what can be done to prevent problems of this kind or at least limit their duration? “That’s the question plaguing many managers,” says Jan Kretzschmar who, in his role as head of the Low/Medium-Voltage business unit at Elektro Vieweg, planned the power supply systems for two logistics centres operated by one of Germany’s largest online mail order businesses. The energy consumption of these centres is extremely high. Four low voltage main distribution boards (LVDBs), each of which is supplied via two separate medium-voltage transformers, provide the electrical energy for the two centres. Together, these installations have a total rated current of 17,000 A. One of the key requirements when planning the LVDBs was Class C active arc fault protection. According to Kretzschmar, Elektro Vieweg is increasingly encountering these kinds of requirements: “The demand for arc fault protection has been growing strongly for a number of years.”

ACTIVE ARC FAULT PROTECTION

If the kind of active arc fault protection demanded by logistics centres is installed, an LVDB can quickly be taken back into operation after an arc fault accident. Elektro Vieweg used the VX25 Ri4Power system from Rittal to provide such equipment in the logistics centres. When planning and costing the solution in close collaboration with Rittal, Class C arc fault protection was achieved by installing a DEHNshort system. A combination of optical sensors and current converters can very quickly detect an arc fault as it is occurring. This triggers a connected ultrafast switching device that acts as a short circuiter and, within a millisecond, creates a parallel current path with lower resistance to the arc flash.

The whole process – from detection through to the arc flash being extinguished – takes just a few milliseconds. The extent of the arc flash is thus so limited that the switchgear is ready to turn back on as soon as the cause of the fault has been rectified and the extinguishing devices have been replaced. The DEHNshort system’s sensors can identify the switchgear panel where the fault occurred. The system can also be installed in such a way that the switchgear is split into several protection areas, meaning the fault can be located very quickly and rectified immediately. The power supply can be restored in no time at all, and operations can continue.

EXTENSIVELY TESTED SOLUTION

The combination of VX25 Ri4Power and DEHNshort has been tested at the IPH in Berlin, an institute that specialises in testing high-performance electrical technology. Besides reliably extinguishing arc flashes, the system must also be resistant to false tripping – in the event of an arc being emitted by circuit-breakers when disconnecting short-circuit currents, for example. The connections between the switchgear and the extinguishing devices must withstand the mechanical forces of the current following activation. Proof of compliance with all these requirements is provided in tests to DIN VDE 0660-600-2-1 (IEC TS 63107). Subsequent evaluation of the test results takes place in accordance with DIN EN 61439-2 Supplement 1 and IEC TR 6164.

COOPERATION WITH EVERYONE INVOLVED

“The fact that the process of implementing the switchgear in line with the requirements went so smoothly was partly due to the excellent cooperation between everyone involved in the project,” insists Kretzschmar. Besides obtaining certification for the Ri4Power and DEHNshort combination, extensive training at DEHN also played a particularly important role. The team is now planning more, similar solutions for logistics centres. “And we’ll be using the same – now proven – technology for these, too,” confirms Kretzschmar.

• Arc fault protection

  What are arc fault classes A, B and C?

  An arc fault accident in low-voltage switchgear can have dire consequences. If no precautionary measures have been taken, the switchgear can be damaged – or even completely destroyed – by the resulting high pressures and extreme temperatures of over 10,000 degrees Celsius.

  People located in the vicinity are also at acute risk. DIN EN 61439-2 Supplement 1 defines various arc
  fault protection classes.

  Class A relates exclusively to the protection of people, who must not be put at risk by the high arc temperatures. Furthermore, the high pressures generated must not lead to risks resulting from parts of enclosures being hurled through the air in an uncontrolled manner.

  This is normally prevented using pressure-relief components that dissipate the pressure by directing it upwards. Class B additionally requires the arc flash to remain confined to one part of the switchgear. This means that, although the switchgear is damaged, it is not completely destroyed.

  Class C – the highest class – calls for either an appropriate design that confines the arc flash to its point of origin or a suitable device, such as the one used in this case, that extinguishes the arc flash before it can cause any damage. The switchgear is then ready to operate again as soon as the fault that caused the arc flash has been rectified and the short-circuiters have been replaced.