• Let us understand what these three terms mean and how they may relate to the risk analysis that identifies the arc flash calculation parameters.
• VCB, HCB or VCBB refers to the conductors supplying power to the arc. It is the direction of power and location of the current path that is critical. Arcs move away from the source of power, they may, or may not turn a corner in the bus assembly. So, if there are corners you need to select the conductor location that represents the worst case.
• VCB, vertical electrodes within an enclosure, means the conductors are parallel to the standing worker. They may be at any angle, not just vertical, but what is important is that the ends of the conductors are not pointing at the worker. Whereas HCB means horizontal within a box and HCB does mean the conductors powering the arc are pointing towards the worker. If you think about the arc moving away from the source of power, HCB means the arc is moving “towards” the worker.
• VCBB means vertical, within a box but with a barrier near the ends of the conductors where the arc will settle. The main arcing fault characteristics this establishes is that the arc is somewhat constrained from moving and stretching. This means the plasma is more concentrated and that produces more arcing current and, in turn, means more energy per unit time. However, how much of that energy is projected towards the worker may not translate well from the test protocol to the real-life situation. The test protocol used vertical electrodes within a box so some of the energy is lost bouncing within the box. The real-world situation may be somewhat different. For example, a common VCBB scenario is the line side of an MCP within a starter unit. Upon examination of that space and comparing to the VCBB configuration used for the IEEE 1584 tests it will be evident that the starter unit is an even more constrained scenario. This might lead one to the conclusion that incident energy may be even more than the IEEE model predicts.
• The formulas in the IEEE model represent the various test protocols used in the testing for the project. The test protocol probably is not an exact replica of the real-world situation the worker will face. So, one must think about the parameters used for the calculation and assume there is some error and what the direction of that error may be. I would suggest that “too much PPE may be undesirable, but not enough is unacceptable”, is a good guiding principle. So, you must select the protocol that best matches your scenario, understand how you may be incorrect, what the impact of that error may be and if that is something that is acceptable. In some case you may need to estimate two scenarios, one to estimate the worst case arcing current (lowest) and another to estimate the highest energy. The arcing current is important because the protection speed will depend on that arcing current and time is an important determinant of incident energy.

This is a very good question, for which there is no formally provided answer within the IEEE guide. However, we would point to NFPA 70E Annex F. Where it states (underlines by this author); “The following risk management principles can readily be applied to electrical safety. Risk Management: (1) Is an integral part of all organizational process and decision making. (2) is systematic, structured, and timely. (3) is based on the best available information. (4) Takes human and cultural factors into account. (5) is dynamic, iterative, and responsive to change. (6) Facilitates continual improvement of the organization.“

Also, specifically see (NFPA 70E-2021 130.5 (G) Incident Energy Calculations. In the 3rd paragraph of that section it states that “The incident Energy analysis shall be updated when changes in the electrical distribution system that could affect the result of the analysis … I would submit that a change in the calculation method could be viewed as such a change especially in light of the Risk Management principles stated in the annex. Hence, new calculations may be advisable. It could stated the risk has changed and hence needs expedient attention particularly before any task is contemplated.

The new calculations can yield different arcing current and different incident energy than the previous calculations. The potential risk would suggest that prior to using values on a label, review of the arc flash study that was used to generate that label and review if the assumptions used for the arc flash study are applicable based on the new standard may be advisable. If there are potential differences one should determine if the difference could result in higher incident energy. If so, act accordingly to add the necessary precautions needed to control the risk. If arcing current is lower, interruption time may be significantly longer having serious impact on available incident energy. Higher energy is possible if arcing current is lower than previously calculated or if the electrode orientation (HCB of VCBB) is different than previously used (VCB).

There is no one number of calories that is too much. PPE exists up to 100 calories or more. The Canadian electrical safety standard, CSA Z462, now includes a PPE level 5 identified as 65 calories or higher. 40 Calories was once selected as a level above which other unestimated hazards such as pressure may present a risk than cannot be controlled. However, additional research has indicated that the impact of pressure is less than previously thought, though it is a hazard, maybe more associated with higher voltages and situations were a high levels of fault current are available. Exposure to high incident energy should be avoided if at all possible, but there is no one level that is defined as too much by standards as long as PPE is available.