Fault Calculations & Labeling: An Introduction

Have you ever blown a fuse in your home and then had to go searching through the basement for your circuit breaker with a flashlight to restore power? If so, you’ve already dealt with the concept of a “fault” current, or the short circuiting of your electrical system. Your circuit breaker is doing its job of protecting the rest of your electrical system when it shuts off power when you “blow a fuse.”

In the industrial world, short circuits can be brought about by a number of factors, ranging from physical disturbances (earthquakes, heavy equipment/vehicle movement etc.) to aging equipment to another stressed fault nearby. The consequences for your business and production can also be exponentially worse. Rapid equipment failure and even workplace fires are major risks.

Equipment and Labels

Circuit protection usually comes down to the fault current rating of any fail-safes or electrical protection devices in your system. Individual pieces of equipment have a fault current rating, and these combined ratings must not be so high as to overwhelm your protective circuit devices in the event of an abnormality. For this reason, the National Electric Code specifies how equipment must be labeled with regards to its fault current potential. Labeling can be done by using an industrial label printer. An example of this type of printer would be the LabelTac 4 PRO. These types of printers can print labels that are UV, chemical, weather, water, and wear resistant.

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What You Need to Know When Labeling

The last update to the NEC requirements for fault current labeling was published in September of 2010, but depending on the state or part of a state you work in, you may fall under one of several versions published in the past decade (there is no universally required adoption of the latest standards). In fact, only about 15 states are currently using the version set forth in 2010 (which are known as the 2011 standards – confusing, I know).

One of your first responsibilities is to mark any piece of equipment with its maximum available fault current. Because you are also required to update these labels when any changes occur within your electrical setup, you also need to include the date of calculation. It is also very important that your label can withstand the work environment it is in and maintain legibility. Consider what liquids, dirt, or abrasion might occur over time.

When labeling, it is also important to know the difference between an overload and a short circuit. A short circuit occurs when more than ten times the rated current occurs, whereas a short circuit means more than ten times. Overloads are far more common but also less severe, and therefore you should be more concerned about short circuits (indeed, many electrical systems can endure minor overloads without any interruptions or blown fuses).

LabelTac 4 PRO Industrial Label Printer

While not required, it is also a good idea to include the X/R rating at the point of the equipment (X/R is a calculation of the asymmetry of a current at a given point). This is because testing for fault current is generally conducted under the assumption of a certain X/R rating. If that assumption is too far off from the actual rating, short circuit behavior may be intensified or different from what would be expected based on just the fault current labeling.

One difference in the recent versions of NEC standards is who the responsibility falls upon, this article is vastly more important to those who fall under the jurisdiction of states following the most recent updates. Before, the calculation of fault current ratings lay with the installer and was enforced by officials from the jurisdiction. Now, each workplace is expected to either be able to do the studies of their equipment themselves (to ensure accurate labeling) or hire someone who can do so. The owner of the factory or plant is also now expected to take a more active role in enforcement.

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