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Arc-Flash Trip Times

Tuesday, January 19th, 2016

When using Design Master Electrical to calculate arc-flash values, you have to specify the trip time for the breakers. These values are based upon the time-current curve for the specific breakers that will be installed in your project.

Often, you don’t know what breakers are going to be used or are unable to obtain the curves for them. Here is a general method you can use for determining the trip time in those situations.

For each piece of equipment, locate the upstream overcurrent protector (OCP). This OCP needs to be outside the equipment. For example, do not use the main breaker for the panel. Instead, use the breaker in the panel upstream.

Note the size of the OCP and the rating of the OCP.

For a breaker, multiply the rating by 10. For a fuse, multiply the rating by 5. Compare that number to the arcing current value calculated by Design Master Electrical.

If the arcing current is greater than that value, set the arcing time to 0.0167. The OCP will trip immediately in this case, which requires 1 cycle.

If the arcing current is less than that value, set the arcing time to 2. The OCP will effectively never trip. A maximum of 2 seconds is used because we assume that the individual will move out of the arc in that time. It is possible that the PPE listed will be X at this point. If you want a numeric PPE listed, reduce the arcing time 0.1 seconds at a time until it changes to 4.

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