2 Second Maximum Clearing Time?

I am reviewing a Study based on 70E-2004. One assumption used was that the maximum clearing time would be no more than 2 seconds in duration [assuming that personnel would be clear the arc in that time].

This seems a very short interval for large systems, especially during arcing faults.

Is the 2 second max a valid assumption under 70E-2009?

Your are confusing clearing time with exposure time. The 2s comes from IEEE, and assumes an exposed worker will react to an arc event by moving away.

Whether or not it is valid depends on where the event is assumed to occur, and whether worker has a clear path. Longer times are recommended if worker must crawl away or operate bucket controls to get away.

As I have said here before, I for one believe that a bucket in itself provides an escape route and a certain measure of protection.....because for 8 years I was a lineman and spent many hours/days/years in a bucket. On one occasion I had to dive to the bottom of the bucket for an arc flash event and was not injured (pretty scary though). Had a friend that also did the same thing. So it may depend on the particular circumstance but I do not see a bucket as creating an "inescapable" hazard. I see it as a refuge.

Hence I do not think that longer times should be recommended for bucket work.

I WAS burned on a pole when I was bound with a safety and could not get away, so I do undersatnd the difference.

Yes, I do use and recommend the 2 sec max time.
I have also modified this for locations and equipment where the aisle between equipment was too small, and there was only one exit.

The full arcing time time is important when estimating the damage caused to equipment, not just personnel. I always allow the software's maximum arcing time.

Yes, I also use and would recommend the 2 sec time base >208v & <1000v
<208v I use .5 seconds, >1000v = 3 sec.

As WDN says above, this value is subjective to the hazard and conditions. More or less an engineering determined assumption.

This is for arc flash hazard HRC not for equipment withstand ability. I suppose if you were concerned about the equipment you may look at it differently.

Very uncomfortable with the two second rule

I'm really grappling with this. When I first suggested this to some of our field guys, they thought it was totally absurd. I am very uncomfortable with this indoors on low and medium voltage panels/metalclad I see no way of guaranteeing that someone won't somehow get stuck in a position where they can't get up and run away. Is there any data to sopport that people in these situations have been able to get away in two seconds or less? Does anyone have a reference?

Jody Levine
Hydro One
Station Maintenance, Technical Services

Don't forget that the calculaitons in most cases are based upon an 18" working distance. I understand in some extreme cases when a worker cannot physically move that the 2 second rule would not apply, but in most cases it is perfectly valid. Actually in most cases it would be very hard force yourself to stay there for 2 seconds. Reaction time is one thing but the other is the explosive action of the arc itself. When you calculate IE at 18" with a 2 second clearing time the value that is calculated is saying that the worker is planting there feet like a baseball catcher with someone charging homeplate and taking the brunt of the arc flash/blast for 2 seconds. That is a long time to force your self to stay put.

Thats my 2 cents.

Jody,
This arc flash video < http://www.youtube.com/watch?v=PHs14ZTo96M> shows the arc going for at least five seconds, with nowhere for the electrician to escape. We've had two squirrel incidents that went on for longer than two seconds.
The two seconds limit seems to be a special case, where personnel are able to run away. Most arc flash victims are stunned, blinded, deafened. It's not clear that they would run in the right direction.
I still maintain that two seconds is an unrealistic limit of arcing time. We probably should be looking at each situation (enclosed area, outside in switchyard, in a bucket, on a ladder) and assigning individual maximum arcing times. Unlimited arcing times can be used to estimate equipment damage.
If the calculated arcing times are more than one or two seconds, either the overcurrent protective relays should be adjusted or an alternative protective device (optical sensor, differential current) should be considered.

I think a review of the 2 second "rule" is in order here. Not really a "rule"

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. (Hence Alan's bucket example).

I don't read this as a rule, I read this as something that can be considered and if the 2 second "max" is used the person doing the study had better be able to justify it. Nothing in writing here I see that would hold up in a court as "following IEEE 1584" to bail you out.

I believe 2 seconds is a key time in protective relaying, I cannot recall where I read that, maybe someone else knows where this 2 seconds is referenced, another IEEE standard maybe?

long clearing times - fuses

What I'm running up against is that we're just about to start a labelling program of all our station service low voltage panels. These are 600 V and 208 V. The problem is that many of these services were sized to accommodate the very occasional use of transformer oil degasifiers and the transformers may have 1000 A ratings. And even though the normal peak load (without degasifier) is well below 300A, engineering has always fused so that the maximum capacity would be available if needed, so we'll see 600A or 800 A or 1000 A fuses on station service transformer secondaries (chosen to protect the transformer!). With short circuit levels around 5 kA (often with high transformer impedance as part of the spec) this makes for loooooong clearing times.

These then feed some sort of transfer scheme that may have circuit breakers to do the switching, but very rarely are there trip units on them. So the first set of distribution panels has these monstrous arc flash energy levels. Now I'm finding out that all the energy calculations were truncated at 2 seconds, with no consideration for physical layout, and I'm not happy about it. The way I see it, head office doesn't get to decide if the guys can get out of the way, the field staff should be assessing that themselves and documenting it in their job plan.

We will have better designs for arc flash going forward, but for now I'm facing quite a mess in terms of providing guidance on how to manage the existing problem.

Our equipment folks have done a great job putting in medium voltage metalclad with the highest possible level of arc resistance since the lat 80s, but low voltage has not had that level of attention.

Jody

Agreed! I believe to call it a rule is a mistake. It has to be a judgement call based on the circumstances and the experience of the user. But I also think that is the case with all of the arc flash hazard issues. Many times it is not so "cut and dried".

I had a professor tell me once that if I could not think of a good reason to do something, then it was probably wrong....still think that was (and is) good advice.

Another issue to consider, is the way the IEEE 1584 calculations assume that the arc will be at a constant magnitude for the entire duration of the clearing time.

The arc is probably constant for the 2 sec period but after that, the arc path becomes longer as material is vaporized, which also affects the plasma of the arc. At some point enough material will be consumed that the arc may not re-strike. The arc also tends to move away from its point of origin, in fact it may not confine itself to being an 'arc-in-a-box'. It is unlikely that an arc will maintain its intensity at a constant distance for 1000sec.

Will the public 'trust' an arc flash boundary of 156231 inches (2.4 miles) for a fault initiated in enclosed switchgear in an outdoor 13.8kV substation?

Is it reasonable to think about replacing the breakers?

Or retrofitting them?

Oh absolutely reasonable to replace anything, but it's just not going to start tomorrow or be done in ten years. In the mean time I have to tell people how to live with it.

It's not funny but I had to laugh at the 2 miles. If the energy is that high, that's how far the porcelain bits will fly.... We're using ArcPro for MV and it's not quite so bad.

I think it's quite clear the radiated heat approximation has only a certain range for which the calculations match the actual PPE required. I already have a problem with reasonable flash protection boundaries (4-10 feet) where I'm pretty damn sure that plasma and molten metal are being spewed farther, and I wouldn't want to be there without a face shield.

Jody

2 miles? Our remote operators only work to around 450 ft, looks like we need a stronger remote

Here's where the two seconds comes from:
IEEE 1584-2004 page 75

B.1 IEEE Std 1584-2002 Arc-Flash Hazard Calculator
B.1.1 Basic information tab
B.1.2 Data-normal tab
...
"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."
....