Conducting an arc flash incident energy analysis is becoming big business.  Unfortunately, this has lead to an abundance of providers, and a sea of information that can be quite difficult for a facility manager to understand.  Is the information provided accurate, appropriate, and complete?  There are a wide range of deliverables that are being offered and one of the biggest areas of concerns is regarding the accuracy of the information used in the calculations of the analysis.

The term “engineered assumptions” is pulled from several IEEE documents regarding arc flash studies and it was intended for the experienced engineer to determine what assumptions to make based on the intent of the analysis being performed.  Unfortunately, the demand and urgency of having an arc flash evaluation completed has led some firms to use assumptions as a means to complete studies in a quicker more cost effective manner – but there can be consequences for doing this unless very experienced and knowledgeable electrical safety professionals are involved.

Simply put; assumptions can tip the scale at any point in the system leading to a drastic change in clearing time for a protective device based on the arcing fault current plot on a time current curve.  Just as the 85% versus 100% evaluation of available fault current does in most software based analytical programs.  The fewer assumptions that are made the more accurate the analysis is likely to be.  A distance assumption of 10’ versus 15’ may not make a difference…but it may just as well – and the comfort in using more assumptions can grow from there. It is hard to predict ahead of time if a given assumption will affect the results of analysis at any given point in a system?  Only after the electrical system has been modeled and calculations have been run can you begin to see where assumptions might have been able to be made.  The insurmountable amount of interrelated calculations that are involved with an analysis make it impossible to know where an assumption will or will not change a result to a less conservative value.

The analysis processes in arc flash software programs themselves make assumptions. One core assumption is that the protective device will operate per the manufactures specifications – This assumes proper preventive maintenance has been adequately performed when in fact it may not have been.

It is not necessarily an acceptable reason to assume information just because no one wants to gather the data, pay someone to gather the data, or schedule an outage to be able to get data – much the same as a cost or inconvenience factor is not by itself considered a valid justification for “working” energized.  If the data is there is should be gathered and modeled.

Remembering the purpose for conducting the arc flash analysis in the first place is critical – To provide information to employees and outside contractors to use for determining safe work practices and PPE.  Failing to understand the effects that assumptions can have on your study may not be initially apparent; however, finding out that your system analysis was not based entirely on actual field values, and that “engineered assumptions” were used in order to get the study done cheaper or quicker may be a reality that is not worth the cost to begin with.


Stark Safety Consultants specializes in Arc flash hazard analysis and electrical safe work practices training as well as related consulting services to aid in the creation and updating of electrical safety policies. Stark Safety Consultants is a proud National Training Partner of the National Joint Apprenticeship Committee (NJATC) and associate member of the National Electrical Contractors association.