Electrical Safety and Arc flash considerations for Armco Infrared
The Reality is that most facilities have not had arc-flash studies completed that would provide specifications for Personal Protective Equipment (PPE) and equipment approach boundaries. Worse, many plant personnel are unaware of or minimize the danger presented by live 480 volt switchgear. Scroll down to the Stoneridge Engineering center video below to get an idea what may be missing
“In the right demonstration, arcs were intentionally initiated by bridging #28 AWG(!!) wires across three 1 inch copper bus bars in a testing laboratory. When power is applied, the wires immediately explode, forming conductive arcs between the bus bars. In this example, three one inch copper bus bars were separated by one inch and connected to a 480 volt source. During the 0.8 second event, the average short circuit current was 17,000 amperes, and the peak current exceeded 30,000 amperes!!” ”Once initiated, the resulting arc(s) can bridge significant distances even though the voltage is relatively low.”
“The energy dissipated within a power arc is limited only by the fault current capability of the upstream power source and the duration before protective hardware “clears” (interrupts) the short circuit. In many low voltage (480 – 600 volt) electrical power distribution systems, fault currents can exceed 70,000 amps. The thermal energy liberated within a high current arc can be many tens of megawatts – equivalent to several sticks of dynamite. The arc core may reach 35,000 degrees F (four times that of the surface of the sun!). As the arc “roots” vaporize portions of the copper bus bars, the copper vapor explosively expands to over 60,000 times its solid volume. The incandescent copper vapor then combines with oxygen in the atmosphere, forming dense clouds of cupric oxide, blackening the air and covering nearby objects with black “soot”. Globules of molten copper are also violently ejected, showering the immediate vicinity with 2,000 degree droplets that can approach speeds of 700 miles per hour.” – that will ruin our day!
To address the dangers of arc-flash as well as the concerns around shock injury, the NFPA and OSHA have created the comprehensive NFPA 70E Standard for Safety in the Workplace. This document provides clear guidelines that, if followed, do minimize the chances that an electrical shock or arc-flash incident will occur in your facility or your home.
The actual level of arc-flash risk is dependent more on the current capacity and clearing times of the upstream circuit breaker/protection device than operating voltage. As an example: A 480v fused disconnect can have the potential for a much larger arc-flash/blast than a similar device operating at 1kv. The following chart is taken from the NFPA 70E standard and forms the basis of Armco policy:
1. For all exposed and fixed equipment rated at 600 volts and below we will utilize PPE to a Category 2 or 8/10 cal/cmsq. For all equipment rated at above 600 volts we will dress to a Category 4 or 50 cal/cmsq. We will also observe an assumed arc-flash boundary of 42 inches and stay outside of that distance when possible.
Category 2 – 10 cal/cmsq Less than 600 v to gnd
Category 4 dress – 50 cal/cmsq Greater than 600v to gnd
2. Approach limits for shock protection are as outlined in the NFPA-70E Table 130.2(C) Approach Boundaries to Energized Electrical Conductors or Circuit Parts for Shock Protection and are not dependent on arc-flash analysis. This table suggests a Restricted Approach Boundary of 12 inches for phase to ground voltages of 750 and below. For above 750 and up to 15kv the Restricted Approach Boundary is listed at approximately 30 inches. Armco Infrared will utilize these approach limit guidelines in your facility.
A bad day
Arc flash Engineering Studies
Arc-flash engineering studies provide information about the burn injury potential for a given incident at a particular point (or piece of equipment) in the electrical distribution system. This arc-blast potential or incident energy is dependent on many factors including the bolted and/or arcing multi-source short circuit current that may flow as well as the clearing or interrupt times of the upstream and parallel protective devices. Protective device clearing times typically get longer at lower currents that may occur during an arcing short circuit or at lower voltages (480) or both. Ultimately, the data and complex analysis will result in labels indicating proper distances and levels of dress to minimize burn injury to second degree or less (recoverable without permanent injury).
Because the engineering analysis is complex and laborious it is typically accomplished with modeling software that will provide consistent and current system documentation and appropriate working distance and PPE labeling. The arc-flash engineering evaluation process will also provide:
- Short-circuit current reduction possibilities including improved device co-ordination,
- Identification of inadequate (damaged or improperly sized) protective devices,
- A path for cost-effective system modification/upgrade engineering
- Possibilities for facility operating cost reduction including insurance premiums
- Injury reduction through significantly increased electrical safety awareness
For facilities desiring an improved plant electrical safety environment through compliance with the NFPA 70E Workplace Safety guidelines, Armco Infrared has teamed with several electrical engineering firms to arrange the necessary field and engineering support for this work. Generally, the most time consuming part of the project will be data collection and it may be time efficient to utilize local personnel to minimize travel expense. For facilities where Armco Infrared provides infrared/ultrasound electrical equipment inspections, combination of the two projects may be feasible.
Various levels of Armco Infrared collaboration include:
- Co-ordination and management of the project while also providing the facility data collection for the engineering analysis
- Providing local data collection for the study with total project management and engineering analysis by the engineering firm
- Entire project referred to the engineering firm (data collection, analysis, presentation, equipment labeling, employee training, safety program design assistance, etc)
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