ARC FLASH BOUNDARIES AND SAFE DISTANCE BASICS FOR POWER PLANTS


Arc-flash boundaries need to be established around electrical equipment such as switchboards, panel boards, industrial control panels, motor control centers, and similar equipment if you plan to work on or in the proximity of exposed energized components.

Parts are considered exposed if they are energized and not enclosed, shielded, covered, or otherwise protected from contact. Work on these parts includes activities such as examinations, adjustment, servicing, maintenance, or troubleshooting.

Equipment energized below 240 V does not require arc-flash boundary calculation unless it is powered by a 112.5 KVA transformer or larger. The arc-flash boundary is the limit at which a person working on energized parts can be standing at the time of an arc-flash without risking permanent injury unless they are wearing flame-resistant clothing.

Permanent injury results from an arc-flash that causes an incident energy of 1.2 calories/centimeter2 (cal/cm2) or greater and causes a minimum of second-degree burns. This distance can only be effectively determined by calculating the destructive potential of an arc.

First you must determine the magnitude of the arc based on the available short circuit current, then estimate how long the arc will last based on the interrupting time of the fuse or circuit breaker. Finally, you will need to calculate how far away an individual must be to avoid being exposed to an incident energy of 1.2 cal/cm2.

It may sound like a lot of math and factoring in of potentials, but believe me the extra time you take to determine the arc flash boundary is well worth your safety and well-being.

Calculating flash protection boundaries for systems over 600 V requires performing a flash hazard analysis coupled with either the NFPA 70E Hazard Risk Category/PPE tables or the Incident Energy Formula.

Additionally, Section 4 of IEEE 1584 Guide for Arc Flash Hazard Calculations states that the results of the arc flash hazard analysis are used to identify the flash-protection boundary and the incident energy at assigned working distances throughout any position or level in the overall electrical system. The purpose is to establish safe work distances and the PPE required to protect workers from injury. A flash-hazard analysis is comprised of the following three different electrical system studies:

- A short circuit study
- A protective device time-current coordination study
- The flash-hazard analysis and application of the data

Arc flash hazard analysis
To perform an arc flash hazard analysis, you need to start by gathering information on the building’s power distribution system. This data should include the arrangement of components on a one-line drawing with nameplate specifications of every device on the system and the types and sizes of cables.

The local utility company should be contacted so that you can get the minimum and maximum fault currents entering the facility. Next you will want to perform a short circuit analysis and a coordination study. You will need this information to put into the equations provided in NFPA 70E or the IEEE Standard 1584.

These equations will give you the flash protection boundary distances and incident energy potentials you will need to determine your minimum PPE requirements. In many ways an arc fault analysis is actually a study in risk management.

You can be very conservative in your analysis and the results will almost always indicate the need for category 4 PPE. On the other hand, you can perform the analysis and make adjustments to reduce the arc fault conditions resulting in reduced PPE requirements.

However, use caution when adjusting your calculations. Reducing the bolted fault current can reduce the arc fault current, but it can actually result in a worse situation. For example, if you reduce the current applied to a motor from 4000 to 1800 A, the arc fault energy is increased from 0.6 to 78.8 cal/cm2. This is the exact opposite outcome that you might expect to achieve before doing the math.

Keep in mind that you are risking OSHA violations and fines if you choose nominal compliance. On the other hand, you can actually be increasing the risk of injury if you force workers to unnecessarily wear cumbersome PPE.

This can also result in little or no high voltage maintenance being performed, which will eventually compromise safety and proper equipment operation. It might prove beneficial to get a registered professional engineering firm to perform arc flash hazard calculations on your behalf and have them recommend appropriate actions and the lowest appropriate category of PPE.

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