National Electrical Code 110.9 Interrupting Ratings states that:
Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that is available at the line terminals of the equipment.
To comply with this requirement, a short circuit studies is typically performed to determine the available fault current for comparison to the protective devices interrupting rating. The results of a short circuit study are also a critical component for other studies such as an arc flash study. Requesting the available short-circuit data from the electric utility company should be one of the first tasks in performing the study. This information is very important because it defines the magnitude of current that could flow from the utility and is used as a starting point for arc flash calculations.
In addition to requesting this data for normal operating conditions, for an arc flash study the request should also include minimum short-circuit current conditions, if available. The minimum condition could be for a utility transformer or transmission line out of service or similar scenario. The minimum value can then be used to determine if the lower current could result in a protective device operating more slowly, which may increase the total incident energy during an arc flash.
Having been in charge of the Short Circuit Studies group for a very large electric utility company in a past life, the accuracy of the Continue reading
How Does Everyone Else Do This?
There are many frequently asked questions about performing an arc flash study (risk assessment) and understanding electrical safety requirements. A careful read of standards such as NPFA 70E or IEEE 1584 can answer some questions. Yet, other questions can be more complex, gray areas can lead to confusion, second-guessing and wondering how everyone else does it. Continue reading
Hi. I currently work as an Engineer in the energy management field. I would like to branch off in to doing Arc Flash, Short Circuit and Coordination Studies. How would I go about doing this. I have been exposed to these reports but have never done them before. Would any of you happen to have any training material on how to get started? From what I’ve read I should learn how to do these reports manually then look in to getting software to aid with the reports. Not sure if it matters or not but I’m located in Canada. Thanks. READ MORE
Would a Harmonic Correction Unit impact the incident energy during an arc flash in an MCC line-up? READ ANSWER
With the 2015 Edition of NFPA 70E being published and all of the changes that it brings, it is time to review your arc flash study, labels and overall practices. There are many key areas that should be evaluated. Here ten of the more important areas to look at to give your site a check up. Continue reading
An arc flash study can be a bit complicated if you are new to this field. Knowing where to begin, what to include, how far to go, how to use the software etc. can seem like an insurmountable undertaking. WORSE – you are going to contract the study and don’t know what to ask for. The good news, there are many well qualified consultants that can help guide you through the process. The bad news – there are plenty of people ready to take advantage of the situation once they realize this might be your first study. Continue reading
The term “working distance” appears 20 times in the 2012 Edition of NFPA 70E, the Standard for Electrical Safety in the Workplace. It appears 12 more times in the annexes. The working distance is an important component of the arc flash hazard analysis and is frequently listed on arc flash warning labels and in the arc flash report.
IEEE 1584—IEEE Guide for Performing Arc Flash Hazard Calculations 2002 defines the working distance as “the dimension between the possible arc point and the head and body of the worker positioned in place to perform the assigned task.” Continue reading
Although beginning with an erratic schedule with revisions to NFPA 70E being spaced anywhere from 2 to 5 years apart, this very important electrical safety standard is now on a regular 3 year revision cycle. In early 2011, I wrote an article about the significant changes that were about to be part of the 9th Edition, the 2012 Edition of NFPA 70E Standard for Electrical Safety in the Workplace. This article will take us a little further into the standard and address some changes that I was not able to include in the previous article. Continue reading
One of the first steps in performing an arc flash calculation study is to request short-circuit data from the electric utility company. This kind of request is pretty routine, and utilities have been providing this type of data for short-circuit studies for years. The problem is the data used for a short-circuit study may not be suitable for an arc flash study. Continue reading
One sentence in the IEEE 1584 Standard, IEEE Guide for Performing Arc-Flash Hazard Calculations, frequently has people scratching their heads: Equipment below 240V need not be considered unless it involves at least one 125 kVA or larger low-impedance transformer in its immediate power supply. What does this sentence mean? What is so significant about 240 volts and 125 kilovolt-amperes?
Download: 125 kVA / 240 Volt Exception