by , on December 18, 2014
red background with burning question

Q.  We have a synchronous generator feeding a fault as a source where there is no circuit breaker to disconnect, on the generator feed side of the breaker, for example, and are relying on the excitation system of the generator to trip.

As we have no details of individual generator excitation system configuration, we have tried to apply a simplified and conservative approach, which is,  assessment of the collapse of the generator fault current dictated by the sub-transient and transient time constants only.

For simplicity, a duration of one time constant of the transient response has been considered (i.e., a collapse to 36.8% of the transient contribution).

In some cases one time constant may be as long as 5.6 seconds.  When we refer to IEEE 1584 there is a guidance note within  Annex B that states:

If the time is longer than two seconds, consider how long a person is likely to remain in the location of the arc flash. It is likely that a person exposed to an arc flash will move away quickly if it is physically possible and two seconds is a reasonable maximum time for calculations. A person in a bucket truck or a person who has crawled into equipment will need more time to move away.

We have checked a couple of scenarios, and using two seconds at a higher fault current has brought the arc incident energy down, suggesting that we may be being overly conservative.

Do you agree with our approach, or should we be assessing a fault duration of no more than two seconds based on the guidance in 1584, providing there is sufficient space, etc., for a person to back away from the arc?

A.  Our opinion is that the two seconds recommended in IEEE 1584 is usually quite conservative for the worker unless there is a confined space like an underground vault

Most high fault current arcs will have a substantial pressure wave in the direction of the arc ejection and I have seen mannequins move in less than two cycles on many occasions.  To the side, the mannequin can stay the full time of the event, but little energy typically goes to the side.  My position is that in the future we will find the two seconds is too long when you account for human movement, arc ejection directional and other factors such as equipment configurations.  But this is the standard and I would not go beyond the standard unless there are strong reasons as outlined in IEEE 1584.

I think the IEEE 1584 standard is a good engineering achievement but could be a little too conservative. The next version will be even better. We recently saw an event in which the fault cleared in 4 seconds but the worker in a 40 cal system was protected.  He did remember being moved back. The only other place where IEEE 1584 is not conservative enough is in a tracking arc bu this problem can only be solved with work practices and insulating cover up at this time.

Hugh Hoagland
About author:
Hugh Hoagland is the foremost tester of clothing and PPE exposed to electrical arcs and is an arc flash expert. Read more about Hugh.

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