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Arc Flash is the re­sult of a rapid re­lease of en­ergy due to an arc­ing fault be­tween a phase bus bar and an­other phase bus bar, neu­tral or a ground. Dur­ing an arc fault, the air is the con­duc­tor. Arc faults are gen­er­ally limited to sys­tems where the bus volt­age is in ex­cess of 120 volts. Lower volt­age lev­els nor­mally will not sus­tain an arc. An arc fault is sim­i­lar to the arc ob­tained dur­ing elec­tric weld­ing and the fault has to be man­u­ally started by some­thing cre­at­ing the path of con­duc­tion or a fail­ure such as a break­down in in­su­la­tion.

The cause of the short nor­mally burns away dur­ing the ini­tial f lash and the arc fault is then sus­tained by the estab­lish­ment of a highly-con­duc­tive plasma. The plasma will con­duct as much en­ergy as is avail­able and is only limited by the im­ped­ance of the arc. This mas­sive en­ergy dis­charge burns the bus bars, va­por­is­ing the cop­per and thus caus­ing an ex­plo­sive vol­u­met­ric in­crease, the arc blast, con­ser­va­tively es­ti­mated, has an ex­pan­sion ra­tio of 40,000 to 1. This fiery ex­plo­sion dev­as­tates ev­ery­thing in its path, cre­at­ing deadly shrap­nel as it dis­si­pates. This ex­plo­sion process is ex­tremely fast, and is mea­sured in tens of mil­lisec­onds.

As an ex­am­ple, a 50kA arc would gen­er­ate a pres­sure of around 37 MPa at the source and pro­vide enough force to pro­pel a per­son weigh­ing 75kg stand­ing 600 mm from the source with ac­cel­er­a­tion of ap­prox­i­mately 100 m/s2. At one me­tre from the source, the pres­sure would be around 13 kPa and since the con­struc­tion strength of most switch rooms is only in the or­der of 0.5 kPa, the switch room struc­ture is also sub­ject to cat­a­strophic dam­age. The arc fault cur­rent is usu­ally much less than the avail­able bolted fault cur­rent and be­low the rat­ing of cir­cuit break­ers. Un­less th­ese de­vices have been se­lected to han­dle the arc fault con­di­tion, they will not trip and the full force of an arc f lash will oc­cur. The elec­tri­cal equa­tion for en­ergy is volts x cur­rent x time. The tran­si­tion from arc fault to arc f lash takes a fi­nite time, in­creas­ing in in­ten­sity as the pres­sure wave de­vel­ops. The chal­lenge is to sense the arc fault cur­rent and shut off the volt­age in a timely man­ner be­fore it de­vel­ops into a se­ri­ous arc f lash con­di­tion. Pro­tec­tion of per­son­nel is vi­tal but so is pro­tec­tion of the struc­tural in­tegrity of the switch room it­self. This re­quires a spe­cial type of pres­sure re­lief vent to be in­stalled in the switch room wall that will not only pro­vide the re­quired free vent­ing area, but also re­act within mil­lisec­onds be­fore cat­a­strophic dam­age can oc­cur. In ad­di­tion, the pres­sure re­lief vent needs to be weath­er­proof and be able to be fit­ted with ap­pro­pri­ate se­cu­rity de­vices.

One such pres­sure re­lief vent is the HIGH-X-100 SHX Blast Re­lief vent from AFP Air Tech­nolo­gies, dis­trib­uted through­out Australia, New Zealand and South East Asia. Th­ese Pres­sure Re­lief Vents are 100 per­cent ef­fi­cient at 95.5 Pas­cals and are fully open in 15 mil­lisec­onds. This re­ac­tion time will en­sure that the over­pres­sure gen­er­ated by an arc f lash is safely dis­si­pated into the sur­round­ing at­mos­phere with­out dam­age oc­cur­ring to the switch room struc­ture.

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