How to determine Arc Flash level?

Tis complex. Very complex.

But there are very simple ways to put an upper limit on how bad it could get when it is all stacked against you..

First you can metricate calories to joules with a factor of 4.2 joules per calorie to get a rating for the overalls in real units, as opposed to Diet numbers.

Around 5 joules per cm2 is the onset of something more permanent  than a bad sun-tan for bare skin, though to set a safe limit that is quite a bit less is better and leaves you with more hair on your arms.  (some US literature quotes 1.2cal/cm2 which is about the same, as the onset of permanent scarring.)

edit to set an expectation, in the test facility where I work (not a mains arc test facility, but we can certainly make some noise..)  a figure of 1 joule is used as the ‘action limit’  i.e. below that no special precautions need be taken, above that we start to think properly about it.

You can estimate the energy of the arc (volts times pssc amps times disconnection time) to get total joules of energy that could ever be in the arc between the time of onset and ADS doing it's thing. Note that actually you can safely quarter that, as when drawing the PSSC, the voltage at the fault has fallen to zero, in reality the power for making an arc - the I*V product -is maximum near to half voltage and half PSSC flowing.

Then you need a geometric factor - in free space the energy of the arc will spread out in all directions, and you can imagine those joules smeared out over the inside area of a spherical shell (like flattening an onion skin and measuring the area) This gets you so many joules per square cm or square m  at any given range like hand length or arm length..

Arcs in confined spaces are worse, as the energy that was going towards  the back and sides of the box may be redirected forwards, and  while the energy total is the same, we have robbed Peter to pay Paul and it is now safe to get closer  behind the box (duh well yes) but more dangerous to a greater distance in front of it.

A factor of 2 or 3 in the worst case direction, relative to spherical,  is typical.

By you have worked out a few you will find that a typical lighting circuit (PSSC of a few hundred amps disconnection less than 100msec ) needs no skin protection at all, unless you are within about 6 inches, while a bare bus bar off a 100A fuse deserves arms length treatment, or the fancy overalls.

There are far more accurate methods that allow for not all the arc energy being turned into flash, and the extra voltage loss in establishing the arc, but they all reduce the exposure compared to the outline method above which gives a very safe first boundary estimate.

Mike.

PS its not my method, it is a chap called lee.

Late EDIT and here is a very spotty scan of Lee's ground breaking paper from the 1980s. note that by assuming Zs was mostly inductive rather than resistance, and that the arc is resistive in nature, his peak of I_fauIt and associated V_fault is more like 70% of PSSC. This is probably true for faults in or very near large transformers. However for faults at the end of any reasonable length of submain, the ‘let's just pretend its two resistors in series' assumption is more applicable.

A more up to date view with a less cautious set of assumptions from a 230V country (ZA) is here

mapj1: 
 

Tis complex. Very complex.

But there are very simple ways to put an upper limit on how bad it could get when it is all stacked against you..

First you can metricate calories to joules with a factor of 4.2 joules per calorie to get a rating for the overalls in real units, as opposed to Diet numbers.

AFAIK the PPE to EU (and UK) standards is all rated in cal/sq cm so you won't be helping yourself doing that … stick to the diet ?

In the UK, our working practices and Forms of switchgear assemblies usually sort this kind of thing out, at least for LV AC. Notable exceptions include types of work done by DNO/meter operator.

DC batteries are another matter. If working with strings of batteries, I'd recommend a risk assessment similar to the process included in the IET Code of Practice for Electrical Energy Storage Systems, 2nd Ed (this is based on the approach in the Australia/New Zealand standard AS/NZS 5139). Hopefully, manufacturers of commercially-available packs have already done this risk assessment for you, and have some recommended practices and instructions, along with the anticipated energy levels for PPE etc.

I agree. All Arc Flash PPE we have bought in the UK, is sold and marked in cal/cm2. You won’t find anything marked in joules. 

Regards,

Alan. 

You can do it that way,  and to get back to simple formula, you just convert your PSSC to calories per electron-volt instead of amps.?

 You do however  end up with dazzlingly complex looking formulae with weird constants in if you mix units, and it rather obfuscates what is going on, especially in the improved formulae when folk use arc length in inches to estimate the fraction of the voltage drop that is not available for flash production.  Since my last posts on this a couple of months ago I have looked into it more deeply and the underlying ideas are becoming clearer.

But I stand by my comment that it is complex, and if in doubt use the Lee method and over- estimate the danger.

Mike

Edit 

Some metric numbers to conjure with - for the bare skin burn limit of 5j/cm2

at 10cm, (4 inches) from the event, the sphere area is 4pi r2 = 1256 cm2, so  a flash energy of 6000joules would be needed to give permanent skin damage, if omnidirectional. Perhaps 2000 joules if in a box and geometry unlucky.

If we have say a 0.1sec disconnection we need 60kW for the duration to illuminate that area to that intensity, and with an arc voltage of 120, and an arc current of  500A we get there. So under those conditions an open air arc from a  circuit PSSC of 1000A or so could be dangerous to unprotected skin at 4 inches or closer. In a box you should perhaps double that distance when in the firing line from the open side of the box.

If you wanted to be twice as far, perhaps 8 inches away, then you need 4 times the energy , as the sphere surface area has gone up by 4, to ~ 5000 square cm,  so 25000 joules. If again we assume 0.1 seconds and 120V arc voltage then arc current of   more like 2kA, so a PSSC of perhaps 4kA. Again double the distance for an arc in a box.

In practice then, down stream of MCBs that break in well under 0.1seconds,  and on PSSC of a few kA or less,  if you keep your head out of the boxes, then all that is needed is gloves and some forearm cover,, and maybe googles.

On the other hand, with a PSSC of 16kA and only a 100A cut out fuse to cover you,  the breaking time is more like 10msec, so  perhaps 8000A and 120V for 0.01 seconds is worst case ==   9600 joules - less than the let though of the MCB.  However, if there is a fault on the supply side of that incoming fuse, which is where the DNOs operate, it is a very different conclusion.

Low voltage ADS techniques on the load side of the company fuse in the UK and EU lead to disconnection times that are fast enough to limit the protection required to quite modest levels.

I must admit we cheated a bit, and used some software that added the feature to something we already had. We needed to classify over 1000 substations. 

Regards,

Alan