|
The hazards and risks associated with arc flash have been on my mind for the past couple of years. It would seem that the issue is pretty simple and straightforward, but the more I consider it, the less obvious it becomes.
The topic was completely off my radar until about three years ago, and at first, I ignored it because it didn't seem to apply to projects I was working on. Wikipedia has the following definition for the term: "An arc flash is a voltage breakdown of the resistance of air resulting in an arc which can occur where there is sufficient voltage in an electrical system and a path to ground or lower voltage. An arc flash with 1000 amps or more can cause substantial damage, fire or injury. The massive energy released in the fault rapidly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extreme force. The result of the violent event can cause destruction of equipment involved, fire and injury not only to the worker but also to nearby people.
"In addition to the explosive blast of such a fault, destruction also arises from the intense radiant heat produced by the arc. The metal plasma arc produces tremendous amounts of light energy from far infrared to ultraviolet. Surfaces of nearby people and objects absorb this energy and are instantly heated to vaporizing temperatures. The effects of this can be seen on adjacent walls and equipment — they are often ablated and eroded from the radiant effects." I believe the Wikipedia definition has one rather large flaw, and that is the reference to more than 1000 amps. This problem can exist at currents far below that.
If this sounds pretty dramatic, it is. NFPA 70E has a long section and an appendix devoted to this topic. Unfortunately, this standard seems to raise more questions than it answers, leaving me wondering just how to properly address the issues involved.
The first question that comes up is, "When can this problem exist?" It's my understanding that it can happen whenever an arc is created during "hot work" (work on energized conductors) if sufficient current is available. It can start as a small spark, which creates a low resistance path through the air (an arc), which rapidly grows to an intense arc — causing the explosion and flash described in the Wikipedia definition. Since most industrial operations use Lockout Tagout (LOTO) procedures to ensure that work is not performed on high-current conductors, it might seem that the issue does not exist. However, part of the procedure is to make a voltage measurement to verify that the circuit is actually dead. This implies that it might not be dead; therefore, this might, in fact, be hot work and might draw an arc, which could precipitate a lethal arc flash explosion! So, the very act of implementing a LOTO procedure might well be creating the hazard. It is also possible for this to occur at random because of the failure of components or insulation, or accidental shorting between conductors. Any time there are exposed high-current carrying conductors, it is a possibility, especially when associated with switchboards and panel boards.
Some, but not all, electrical panels are designed to withstand the forces created by an arc flash explosion. In some instances, protection is provided if the panels are closed, but that protection does not exist if the panel doors are open. Therefore, it is a possible hazard whenever a person is near a high-current panel board, electrical panel or switch board. This is a bit of a problem in that it is ubiquitous in many industrial environments. NFPA 70E implies that the hazard exists for all electrical circuits above about 100 volts and unspecified amperage — presumably quite low. (This whole discussion only applies to "power circuits," not to circuits that are inherently current-limited such as in the old automobile coils, where there were tens of thousands of volts but very little current capabilities.)
next page »
|
