Apologies if this has been discussed before. It is probably mostly a question for Jim but others who've actually done the testing can definitely chime in.

In all the arcing fault test videos that I've seen (I've only seen vids, never conducted a test myself) that I can make out the set up in I can see the small gauge jumper between all 3 phases. I never see tests with a jumper between only two phases or only one phase and the cabinet.

I have also read a number of times that "single phase and ground faults often escalate into 3 phase faults". How true is this really? I understand you must assume worse case when performing the studies. I get it and am in complete agreement with that approach. However, I will offer that I've seen the aftermath of a fault, small and large, a number of times and it's pretty much never involved all 3 phases when started as a ground fault primarily. At least not that was evident to me from burn marks and melted insulation or all 3 blown fuses.

Jim / others - when testing do you always set up the jumpers between all three phases or do you also test L-L and L-G faults and if so do they indeed "often escalate to 3 phase"?

Thanks.

I've seen it done both ways as far as the wire. The STANDARD test used for IEEE/NFPA testing at one time used a #12 wire going between all 3 phases. Since this doesn't always result in a successful arc at low voltages/currents, the wire size is reduced under those circumstances. EPRI has done a lot of tests where the fuse wire is an actual tool like vice grips simulating the case of a tool landing across two bus bars. The wire size is the reason that several early tests (PG&E testing) resulted in either no or little arc flash. One of Mike Lang's papers below talks about this as well.

Several of the white papers here from Mike Lang will answer most of your questions which documents the fact that in nearly all cases single phase arcing faults turn into 3 phase faults along with lots of pretty pictures:

http://ep-us.mersen.com/resources/arcfl ... ting-help/

Thanks for the response and providing the resources Paul. I appreciate it greatly. Looks like I have a lot of reading to do.

I'm aware of an event that occurred on some metal-clad 13.8 kV switch gear that was falsely assumed to be de-energized. Worker initiated a low current phase to ground fault that was limited by a grounding resistor. Fault evolved to go double line to ground prior to being cleared. Never went three phase. Workers were treated for sun burn and contract was terminated.

Great discussion! I have set up a few tests with phase-ground and phase-phase but almost all are three phase. The escalation issue is just as stated by everyone here. You never know. I had a client years ago that wanted to have all single phase tests assuming that was more real world (fair statement) but a) nothing escalated b) we could not get the arc to sustain, let along escalate. Don't draw any conclusion from that, just stating what was seen.

Certainly not all faults escalate but it can happen so that is what is used for the tests and equations for IEEE 1584. I've seen a few bizarre cases where we thought the event was over and it re-struck in another location from the plasma based on high speed video.

This is funny and not funny at all at the same time.

With overhead line systems, escalation doesn't happen very often. Usually instead you get some pretty significant traveling arcs.

Similarly in MCC's in the actual real world cases I'm aware of the arc travelled vertically down to the end of the bus bars then blew out the door at the bottom of the MCC, nowhere near the bucket where it started.

Here's the interesting twist though. If we simply insulate the bus then we won't have three phase escalation and not only that but arc propagation (moving arcs) goes away, right? This and more were claimed in this paper:
http://www.neiengineering.com/wp-conten ... s-2012.pdf

But interestingly when actually TESTED, these theories fell apart.

http://www.neiengineering.com/wp-conten ... esting.pdf

One other follow up...the first paper above says that arcs below 230 VAC are extremely unlikely and that there are no recorded fatalities. OSHA documented one in 2009 in Georgia so that kind of ends the "no fatalities" claim.

Medium voltage substations usually have newer relays that keep track of faults as well as provide reclosing on circuits. I have seen several examples of a single phase conductor fault on an underground system escalating into a three-phase fault with each reclose operation.

Here is one example:

Initial fault - single phase about 1,800 amperes
Reclose 1 - double line to ground around 2,000 amperes
Reclose 2 - three-phase fault around 2,500 amperes
Reclose 3 - three-phase fault around 2,500 amperes
Lockout

I have no knowledge of low voltage fault escalation. It might be possible at 480 volts, but I would not expect it at 240 volts and below.

Not the same thing. What you're referring to is when the arc path especially along a surface is contaminated to the point where it becomes a conductor or semiconductor which happens with non-renewable insulation (air), or when the delay between reclosing attempts is far too short so the air does not have time to cool down and it restrikes.

What "escalation" normally refers to occurs during arcing before a protective device attempts to clear the fault. It begins as say a single phase L-G arc and then escalates into a 3 phase L-L-L arc. This rarely happens in an overhead line system. It is more common in enclosed equipment where the arc, smoke, heat, and combustion products are all contained within an enclosure and the electrodes are much closer together.

In 2012 or 13, an incident occurred in the main electric room of a retirement home on the Pacific Coast. The attached incident photo shows a 208V 3Ph 4W 400A panel, fed by a 300A CB that sustained an arc that didn't extinguish primarily due to the condition of the breaker. The maintenance history is a common issue: No original testing and no subsequent testing or maintenance.

The facility manager wouldn't allow it to be shut down as it would be "inconvenient" to the residents. The contractor acquiesced and then this happened (the breaker seen leaning in the upper tight corner was the fault locus). It is likely that it was never torqued properly, when installed, and as I previously said was never shut down for maintenance.

The result was a lot of equipment damage, but due to the direction most of the arc energy took (up), the injuries were relatively minor (it did trigger a Cal/OSHA investigation). By the way, both the people involved in this weigh in at 260lb +. Both were knocked off their feet. The arc was stopped by manually shutting off the feed breaker. It didn't trip.

So, they can be sustained below 250V, and it really depends on the circumstances. It may have blown out at the end of the vertical buses and self-extinguished, but we'll never know.

Largely voltage, fault level and configuration dependent from the incidents that I have analysed. Faults in the closed box where the plasma is contained and clearances are smaller might appear to generate the required conductive path. As noted, the oscillographic records from more modern relays have aided in analysing post event. At 22kV with about 1kA restricted earth fault current, various cases that I have seen escalate within a couple of milli-seconds to 3 phase. On 415V that escalaction also occurs before the protection can operate, although triggered by the earth fault. Simply put, the bigger the arc and consequent plasma the more quickly this will happen.
IEEE 242 has made a distinction between low and high impedance earthed systems. in my experience, the high impedance earthed systems give you a chance of clearing the fault before escalation to 3 phase.