by Thomas Appleyard MIET
Garbage in Garbage out.
Coined by the IBM programmer and instructor George Fuechel – Garbage in Garbage out or GIGI – basically speaking in computer logic programming refers to no matter how good the program is if the input data is incorrect the end result will be invalid.
So, what does that mean with regard to Arc Flash calculations?
You might think that if the input data is about right then the output data will be about right and sometimes it is…. Sometimes. But often the results will be way out, and this will have a knock-on effect to how your company manages risk for Arc Flash.
2 circuits side by side, same Prospective fault current, same cable same bus types same MCCBs.
The only difference is the overload settings of the breakers BL-1 (left) is set to min BL1-A (right) is set to max.
Not a huge difference so one might not expect a massive difference in calculated Incident energy value. (incident energy is a measure of thermal and ballistic energy at a working distance from an arc fault in this case the distance is 455mm so basically how big is the boom and how bad will it hurt)
So… let’s blow up the system!!!
Fig. 2 shows 3 phase arc flash values after the protection.
At BUS-2 the value is 1.3 Cal/cm² at BUS-2A the value is 32.7 Cal/cm².
A huge difference.
What would the effect of these different incident energy values be without appropriate PPE?
At 1.2 Cal/cm² an Arc flash incident can cause 2nd degree burns, pain, redness, swelling and blistering.
At 8 Cal/cm² an event can cause 3rd degree burns in which the damage has progressed to the point of skin death.
So, at 32.7 Cal/cm²? Well, you might leave a cool pattern on the wall behind you…. Not that you’ll be able to see it.
‘’It’s too hard to get the info so can we make a best guess.’’
The issue here is that if we are not able to model the data accurately due to a lack of quality information then we may have to make conservative/ worst case estimates, which means you may be going overboard on your risk management, why make all staff wear 40 Cal/cm² PPE if it’s not needed? Why pay specialist companies to perform upstream isolations if not necessary? These may have massive cost implications on you or your company. (forgetting the fact that a 40 Cal/cm² suit is hot and heavy, think beekeeper on mars).
So, what can we and you do as a company to make sure the data is accurate?
The phrase ‘’a picture paints a thousand words’’ really does apply here, when looking at an MCCB we need the make, model, kA rating, frame size, short time overload setting, long time overload settings, instantaneous settings, I²t in or out? Or…. A picture. A picture of the breaker including all the dials, is normally all we need to work out the breaker properties. We look at this stuff all day so for us working out what’s what is pretty straight forward, send us an accurate picture and where it is on a single line diagram and away, we go.
The same goes for Transformer name plates. Picture. Distribution boards. Picture. Motors. Picture. You get the point.
Some things can be harder to get for instance the cross sectional area of a cable, if you don’t have accurate up to date installation or periodic test and maintenance records (if you do we want them, we’re greedy like that) then give us the cable type and total outside diameter we have tables to work out the internal cross sectional area.
Oh, we have all that and more we’ll just send you everything!!!
If possible, please don’t do this.
This is the other end of the spectrum, I know we’re engineers who go to bed every night reading through technical documents and manuals and re-reading all the regulations and codes front to back so we are at the peak of engineering, don’t we?
Realistically, no we don’t so if we request a specific piece of data and you know where and what it is send us the answer or a page from a document that gives the answer, sending 2000 pages over 100 separate pdfs is just going to delay the project to a point where beekeeping on mars may actually be a thing.
Pick up the phone.
If you’re not sure on what the info were asking for is, or where to find us give us a call, by now (in these difficult times, sigh) we should all be great at video chat we can get drawings up and show you what we need.
And lastly don’t use a potato to take the picture. We need to be able to read the fine print on the protective device, so quality high def pictures please.
View our Arc flash case studies for Honda Racing and more.
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