Arc angel, arc flash prevention measure increases safety

This article comes from ABB and is authored by Pavlo Tkachenko and Andreas von Lako.

An arc flash is one of the most serious incidents that can occur in an electrical installation. Arc-flash temperatures can reach 20,000ºC and the energy and shrapnel produced in the arc blast can cause death, injury and serious damage. Normal short-circuit measures are too slow to protect against arcing events like this.

ABB’s Arc Guard System TVOC-2-48 extends arc flash protection to the 24 to 48 V DC range. Electrical cabinet safety is further enhanced by following the guidelines contained the IEC 61641 Technical Report.

ABB has released a new version of its well-established 110 to 240 Vac and Vdc  Arc Guard System that has been protecting people and electrical equipment from dangerous electrical arcs for over 35 years. The TVOC-2-48 now makes the Arc Guard System available for equipment in the 24 to 48 Vdc range. In addition to Arc Guard, rigorous testing according to the guidelines contained in the IEC 61641 Technical Report further ensures the safety of ABB electrical cabinets under conditions of internal arcing.

Though rare, an arc flash is an extremely serious and dramatic event. An arc flash is the heat and light produced by an arc caused by a fault in electrical equipment. Most arc flashes arise from human error – eg, conducting materials inadvertently left in assemblies during manufacture, installation or maintenance; faults in materials or workmanship; failure to put measures in place to exclude the entry of small animals such as mice and snakes; use of an incorrect assembly for the application, resulting in overheating and, subsequently, an internal arcing fault; inappropriate operating conditions; incorrect operation, or lack of maintenance.

Arc flashes can produce earsplitting noises that surpass 160 dB. The United States OSHA (Occupational Safety and Health Administration) limit is 115 dB for a maximum of 15 mins. Copper expands by a factor of 67,000:1 when changing from a solid to a vapor (water: 40,000:1) and the molten metal and shrapnel produced by the arc can reach velocities of 1,600 km/h. An intense, high-energy radiation with a temperature of up to 20,000C is produced that is capable of vaporizing nearby materials.

This radiation can also be absorbed by metal objects worn by people in the vicinity, heating the objects and causing severe burns to the wearer. A full 65% of switchgear arc flash incidents occur with an operator working on the equipment.

                  Usual short-circuit protection methods are much too slow to prevent arcing events

Because fatalities and serious injury are often classified as burn victims by hospitals, statistics on arc flash casualties are hard to come by.

However, even one arc flash incident is one too many. In incidents that lead to arc flash, the usual methods of short-circuit protection are inadequate. This is because the arc itself acts as a resistor that limits the overcurrent. This, in many cases, will lead to such a delay that by the time the load protection kicks in, it is far too late.

1. Arc Guard ABB has long recognized the hazards of arcing accidents and, nearly 40 years ago, developed its Arc Guard System – a product that significantly reduces the damage resulting from an arcing accident by quickly disconnecting the switchgear, with the help of the installed breaker, after an arcing fault.
The latest ABB Arc Guard System TVOC-2-48 extends the operating regime of the product from 110 to 240 Vac and Vdc to the 24 to 48-Vdc range. Arc Guard employs light sensors that detect the start of the arc flash

2. On detection, a signal is sent directly to the breaker trip mechanism. The total time from detection to trip signal being sent is less than 1 ms and, with a modern breaker, the entire disconnect sequence can be reduced to under 50 ms.

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