Arc Flash Studies

For starters, the zero impedance is improbable, but at least it errs on the safe side.

Wow. So far four anonymous cowards obviously don't know what they are voting about.

Four anonymous cowards beating up on one anonymous poster.

Has a nice ring to it.

They never heard about and certainly cannot understand L'Hopital's Rule.

Looking at your link, while I'm sure Mr. Wolfram is a very clever boy, I take issue with him over his example limit_u→∞u/√(u²+1). If you substitute u = ∞ it's clear the limit = 1. u/√(u²+1) is not an indeterminate form so no need to invoke L'Hospital's Rule, with the apparent complication referred to.

What I've done is changed the cable (that represents the bus connections) to 0.00001. Yes you are right but this is the way I was told to do it.

But the questions remains, should the line sides be close to main bus calculations?

What about asking the local individual who gave advice earlier?

You have to be trying to use a simulator to understand basic electricity.

The resistivity of annealed copper is 1.72 *10^-8 ohm meters at 25 °C. How long would a 5 mm by 20 mm piece of annealed copper buss bar be that has 1*10^-5 ohms of resistance?

What do you think the contact resistance of these circuit breakers will be?

What do you think the source impedance of this generator will be?

As others here have implied, assuming unrealistic component values can produce a safer fault design when one understands what is happening. It can also reveal how much one understands, too.

Arc flash protection and analysis should not be done by somebody just pushing buttons on a simulator. This can happen.

You have quite the dilemma.

On the one hand, you've got some undisclosed, unknown individual telling you how to do it, and you are asking a group of anonymous, unknown individuals how it should be done.

NFPA/NEC is the bible here in the USA. My fair city directly references the NEC and says "do it that way".

I'd do it per the NEC and any local regulatory agencies which may have authority over your work.

Doing it wrong is very exciting, for a flash in time.

Zero was that exactly what I meant. The rules in this program are having a breaker connected directly to the bus with almost zero impendance.
So I entered .00001 ohms/1k feet. It would appear that all induction/synchronous motors would certainly contribute to the arc flash total of the main bus. Then all line side studies would be very close to the total contribution.

Sorry I confused you.

I had a motor test panel designed to be as near to safe as possible, considering the motors had to be connected and disconnected manually, even with the methods for connection interlocked, light curtain interlocks, and the expenditure of a large amount of money for a professional arc flash study, It still came down to a confirmed function test of the meter used to confirm that the circuits where de-energized, each and every time.This was for a low voltage panel.

<...It still came down to a confirmed function test of the meter used to confirm that the circuits where de-energized...> Such a thing is a requirement under the UK's various Electricity at Work Regulations.

As it should be anywhere, This make the invisible visible.

I'm very surprised at the array of comments exihbiting negative attitudes. It's clear to me that none of you understand Arc Flash Studies under IEEE 1584 and NFPA 70E! I know there's and abundance of brilliant engineers among you but I don't understand your sarcasm. That being said I am terminating my subscription and will go elsewhere for brainstorming.

Now that's funny. You come to this public engineering forum looking for help on how to calculate one of the most critical electrical hazard information for any installation. This analysis establishes what level of PPE and hazard zones for power distribution that only properly trained professionals should ever approach, let alone enter. Technical questions are asked by some but most here dismiss your lack of knowledge with mild humor. Instead of even attempting to answer any of the technical questions you paint us all as beneath you.

Try not to kill anybody.

Better yet, don't work with electricity. You do not even understand the questions, let alone how to answer them.

I'm trying to learn an Arc Flash program. I simply wanted a simple yes or no answer. Of all people (as I've read many of your comments) I don't find any humor.

Fair enough. You want a simple yes or no answer about how to properly calculate the safe distance around an arc flash.

NO! You appear to not understand what you are doing. Any numbers you produce from a program will be meaningless because you appear to not understand the meaning of the numbers you put in.

Now if you really want to gain some meaning, calculate the buss bar length for the resistance you selected for your cables. I gave you a link there that not only references the value I stated but also provides the formula to obtain a result. (Hint: Be very careful with your units.)

The second link in that reply is a video of a deliberate arc flash condition. You should know how dangerous is an arc flash condition. The calculations one makes using IEEE 1584 will assign how large of an area must all occupants wear very cumbersome protective gear.

This is not child's play!! If you calculate things wrong people can get seriously hurt or you will put such overly harsh safety limits that people will ignore you.

Bon voyage.

I find his signature line to now be the most ironic signature.

If I wanted some free information that I might be able to use as justification for a specific test, I'd do a search,

• Arc Flash Energy Calculations for Circuit Breakers ... - Graybar
• Energy-Based Tripping and Its Affects on Selective ...
• Arc Flash Hazard Options - EasyPower
• 2014 National Electrical Code: Changes in ... - Mersen

At least these sources can be traced.

All you've gotten here are anonymous opinions from anonymous people.

And we know what opinions are like.

Friend,

There is a slight difference in the available short circuit or fault current as one moves down the bus. However, the difference is going to be smaller than the probable error in the approximations that have fit real world results into formulae. In my experience, a probable error from ignoring the bus impedance is going to be only a couple percent, which is negligible.

Run your calculations at the full available fault current and again at 85% of this amount. For each, determine the time it will take for any upstream protective device to fully open and clear. The generators are going to act like a transformer with a given kVA output at a known voltage and with a (hopefully) known impedance. This clearing time for any protective device is going to be the most important approximation you face.

You want any errors to be on the high side of available fault energy. Sometimes these will be at the lower current because of a significantly longer clearing time!

I hope that you are only working on the circuit when it is "in an electrically safe working condition". If this is impossible, document carefully and plan carefully what you will do and how you will protect yourself and everyone else and then choose the proper level of protective equipment, insulated tools, etc.

As others have implied or clearly stated, arc flash incidents are extremely nasty and will maim or kill you before you even know they are happening. The upper limits of an arc flash can be 35,000 deg-F, shrapnel flying at 700 mph, and blast pressures well over 100 psf. So, whatever you do, please avoid any situation that can cause an arc.

--JMM