Arc Sustainability

I ran into some data about Pacific Gas and Electric conducting several hundred tests a while back to determine how long a 480V and 208V arc could actually sustain itself. At 208 Volts they found the arc could sustain itself for less than 1 cycle if the arc gap was ½ inch. The longest duration at 208V was around 10 cycles.

At 480V they conducted tests from 10 kA to 40 kA with arc gaps ranging from ½ inch to 6 inches. There were 5 tests conducted for each condition. The maximum arc duration was between 80 and 90 cycles and seemed to be with gap spacing around 1 inch. Many of the arc durations were lower than the maximums.

I have heard about other tests that had longer durations but I do not have data about them. I understand this is also dependent on the orientation of the bus bars during the test.

Don't use this for your studies since I don't believe this has appeared anywhere in a standard but it is very good info to show how the arcs behave.

I am sure the next phase of testing with IEEE 1584 and NFPA will help better define the arc duration limits.

Catcher, can you tell me where to find this?

WDN
I have a copy of the data but can't seem to get approval to post it. Probably liability issues. This was just one of many sets of tests that have been conducted independently by various companies. At the same time I received this data, I also saw a presentation from an east coast fuse manufacturer that conducted tests with a different bus bar configuration and the arc faults could sustain themselves for quite a while - I don't remember the exact duration and did not receive a copy of the data on that one.

I think what this all means is arc flash science and testing is still in its early stages and there is a lot to learn.

2 second rule and arc duration testing

Is this type of data what the 2 second rule is based on? i.e. IEEE 1584 has the wording about cutting the duration off at 2 seconds if a low short circuit current gives a long clearing time according to the time current curve?

mHannah,
The 2 second rule was based on the ability of the "victim" to escape the vicinity of the arc flash. Two seconds was thought to be a reasonable reaction time for most people in an open area. If there is nowhere to go (or you are dealing with "slow" people ) , you should not use the 2 second rule.

I would be concerned that during an arc incident, the spark gap might not match what the study supplied.

I have heard of one accident where the 480V panel arc continued until the primary of the transformer was de-energized. Not sure what the circumstances were but it does give pause for concern.

Also, all 480 receptacles are not rated as disconnect devices (except when equipped with special load break mechanism), apparently due to concern with interrupting the 480V arc.

This would be very interesting information if some reliabilty could be granted.