Understanding the consequences of accepting "engineered assumptions" when performing an arc flash study
Conducting an arc flash energy analysis has become big business. Unfortunately, this has led to an abundance of providers in the marketplace and a sea of information that can be quite difficult for a facility manager to navigate. Is the information provided accurate, appropriate, and complete? There are a wide range of deliverables being offered, and one of the biggest areas of concern relates to the accuracy of the information used in the calculations.
The term “engineered assumptions” is pulled from several IEEE documents regarding arc flash studies. 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. Unless very experienced and knowledgeable electrical safety professionals are involved in this process, there can be adverse consequences for going this route.
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 ft versus 15 ft may not make a difference, but it may just as well — and the comfort in using more assumptions can grow from there.
It’s difficult 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 makes 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 manufacturer’s 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 obtain 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, it 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 appropriate levels of personal protective equipment (PPE). Failing to understand the effects 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’s not worth the cost to begin with.
Abbott is the president of Stark Safety Consultants in Canton, Ohio. He can be reached at email@example.com.