Bas Bouman Eaton Electric B.V Hengelo The Netherlands
Jan Verstraten Dow Benelux B.V. Terneuzen The Netherlands
Rudy Vervaet (retired) Dow Benelux B.V. Terneuzen The Netherlands
William Oomen Eaton Electric B.V. Hengelo The Netherlands

ARC FREE FAULT DESIGN FOR ELECTRICAL ASSEMBLIES

All electrical assemblies according to the standards IEC 60439-1 and ANSI/NEMA, main assemblies and Motor Control Centres (MCC’s) are required to withstand a short circuit current that is liable to occur. Part of the type tests that these standards require is the verification of the withstand capability of the through going short-circuit current. This short circuit current occurs for instance during an external short circuit in an outgoing feeder, which is not cleared by the outgoing short circuits protection device SCPD. According to these standards this test is performed on the main, distribution busbars, and incoming unit, if any, that are short-circuited with a bolted link on the end. The short circuit withstand current capability test is carried out for the maximum peak and RMS value of the prospective current and duration as specified in the standards. The ANSI/Nema standard requires the short-circuit current duration to be 3 cycles, the IEC requires in general 1 s, and optional 3 s. The assembly passes the test when it is still intact afterwards, thus suitable for further operation. The practical relevance of this test must not be over rated. It is very unlikely that during the lifetime the main and distribution busbar system have to withstand these rated short-circuit currents. Normally the short circuit protective devices in the outgoing shall clear these through going short-circuit currents, so it is unlikely that the main and distribution bus bars systems are faced to these short circuit currents. If the busbar is subjected to these short circuit currents, it is most likely caused by an internal arcing fault. At such an event, the assembly is subjected to very different stresses and threats for its surroundings which outcome in not defined in the IEC 60439-1.
In the 70’s, internal arc fault accidents occurred in assemblies operated at petrochemical plants, one of them unfortunately with fatalities. This was the inducement to find a new approach for the development of a new design LV MCC. In modern process industries where in general a high short-circuit capacity is present, the internal faults in the switchgear are a potential risk for operating personnel, the equipment and for the process. National and international standardization institutes already incorporated tests for conditions under arcing in product standards for medium voltage switchgear. It became clear that with the increasing short-circuit capacity of LV distribution grids such a rule also applied to LV switchgear.
The phenomenon of an arc caused by an internal fault from ignition until the interruption of the energy supply is a very rapid and destroying event. The inducement of the ignition can be caused by various errors that will be discussed later in this paper. An internal fault often starts with an unintended (short) circuit within the assembly that is blown away within milliseconds. A mixture of ionised gases developed due to vaporizing of metals takes over the current path. This metal vapour reaches high temperatures of 5000 – 10,000 degrees C. At the same time, a high pressure is built-up within the enclosure, due to the increasingly heating up of the developed gasses not able to expand within the enclosure. If the current is not interrupted through a SCPD, an explosion may be the result. The seriousness of this explosion depends on the amplitude of the fault current, the duration of the arc and on how well the enclosure is able to contain the explosion. However, there are more dangerous effects of this phenomenon occurring. Like for example poisonous gases that are developed due to vaporizing of metals and insulation materials. These gases may have short-term and long-term effects on the person(s) inhaling them. The gasses are so hot that the clothing of persons in the vicinity of the assembly may start burning and exposed skin can suffer severe burning due to radiation of the arc. Note that severe burns are often caused by ignited clothing and not by the original arc flash or the fire. The explosion can be so loud that damage to the hearing has to be expected. Besides the direct danger of arc faults, secondary effects like those of the occurrence of dazzling are often worsened by panic movements. The number of injuries from arc flash accidents in the presence of peoples is likely very high, however reliable records do not exist.
II. SHORT CIRCUIT THROUGH INTERNAL ARCING
...poisonous gases that are developed due to vaporizing of metals and insulation materials. These gases may have short-term and long-term effects on the person(s) inhaling them. The gasses are so hot that the clothing of persons in the vicinity of the assembly may start burning and exposed skin can suffer severe burning due to radiation of the arc. Note that severe burns are often caused by ignited clothing and not by the original arc flash or the fire. The explosion can be so loud that damage to the hearing has to be expected. Besides the direct danger of arc faults, secondary effects like those of the occurrence of dazzling are often worsened by panic movements. The number of injuries from arc flash accidents in the presence of peoples is likely very high, however reliable records do not exist.
Besides the very serious consequences for the operating personnel or technician working on or standing close to the assembly, arcing faults can lead to considerable economic losses. Production loss and long downtimes through repair works have to be taken into account. An arc-flash in the main or distribution busbars most likely result in loss of equipment, consequently loss of supply, i.e. loss of product.
III. HOW DO INTERNAL SHORT CIRCUITS OCCUR
Internal short circuits are most likely a result of hot spots, tracking across insulation material, animals or vermin entering into the assembly, loose parts or human errors. Typically,
1) Tracking across insulation material Comparative tracking index (CTI) values categorized in material groups are very relevant in this perspective. The applied insulating materials play an important role avoiding the chance of an internal fault due to deposits of dirt and dust or condensation of water vapour when ambient temperatures varies during the day.
2) Hot spots A possible cause of short circuit can be a bad connection. A bad connection will cause the generation of heat and/or coaled insulation material, which in the end likely can lead to an arcing accident. Reasons for bad connections may be: 1. False tightening torques at connections caused by poor construction methods or by lack of maintenance; 2. Excessive vibrations or atmospheric conditions. Provisions should be taken to avoid the risk of corroded or dirty contact surfaces. All connections are preferably to be silver plated, provided with multiple contact spots and provisions for contact pressure with sufficient capacity to compensate for any shrinkage, vibration or material flow; 3. Contacts not remaining closed under short circuit conditions. If a contact opens due to the repulsion forces of the short circuit, arcing occurs on the contact spot and a copper-oxide or aluminium oxide film remains. After such occurrence, the electrical resistance of that contact or connection is likely substantially higher and more threatening, probably not stable;
3) Whiskers Another cause for a short circuit in an assembly, which we found in the petrochemical industry as well as in the food industry, is whisker formation. Silver whiskers may be the cause of unexpected short-circuits. A silver whisker likes to grow on sharp edges of silver coated parts in a sulphur rich, warm and moisture environment;
4) Animals and vermin Large openings in the enclosure allow small animals and insects to enter the assembly with the risk of them touching live parts and consequently introducing an unintended circuit that develops to an arcing fault;
5) Fungus Moisture may cause fungus to grow within the enclosure that may flourish on the hotter spots, i.e. connection points.
6) Human error Wrongly operated switchgear and carelessness during maintenance works and cleaning the equipment. These may introduce errors with destructive faults leading to internal arcing. Notorious are the accidents caused through a forgotten tool or loose laying bolts and nuts in the assembly. Note that loose ferro metallic objects may move under de influence of magnetic fields caused by inrush currents.