The National Electrical Code is designed for the practical safeguarding of people and property from hazards arising from the use of electricity. The two major hazards are shock and fire, but the third, arc flash, is starting to find its way into installation requirements.
Overcurrent protection is the workhorse of meeting the purpose of the NEC. This is how to design the system to shut itself off when there is a problem. Many of those common practices are followed to help an overcurrent protective device (OCPD) do its job.
There are two types of overcurrent events, overload and fault, and they have some key differences.
In an overload condition, the load is drawing more current than intended. This might happen in a motor with worn bearings or an AC compressor motor when there is too much head pressure in the system. Either way, the motor has to work harder than it was designed for and is drawing more current than the system is intended to carry.
A fault is where there has been a physical alteration, intentional or not, that has bypassed the load. Faults can be short circuits, ground- or arcing-faults and each brings with it a unique set of problems. First, the short circuit, which is a fault that bypasses the load but stays on the intended current path. Second, the ground-fault, which is a fault in which current has bypassed the load by way of finding a grounded surface or conductive path that is connected to the ground. Third, the arcing fault, which usually has much lower current values than the other two because it uses air as a conductor, and this path has significantly more resistance in it than if it were a short circuit of ground-fault.
Exploring overcurrent protection
The NEC separates overcurrent protection into two categories: overcurrent equipment protection and overcurrent conductor protection. Article 240 contains the general overcurrent protection requirements, however, there are many equipment-specific overcurrent protection requirements throughout the Code. Section 240.3 lists where all the equipment-specific requirements can be found in the Code.
Looking at overcurrent equipment protection first might shed some light on how the NEC really uses this for safety. Take for instance a motor installation. Motors are an area in the NEC that utilizes multiple techniques to provide the full spectrum of overcurrent protection. Due to how a motor is constructed, it presents a challenge during starting because high starting currents can often trip an OCPD. To start the motor, the branch-circuit device is sized higher to allow the motor to start. A separate overload protection is then provided to protect the circuit components from too much current flowing over an extended period of time. This approach uses the combination of two separate devices to protect the overall circuit and is specific to the type of equipment that is being fed.
The other method is protection at the point where the circuit receives its supply of electricity. This is the more common method and uses a single device at the circuit origin to provide both overload and short circuit and ground-fault protection. This type of protection is still based on the expected load being supplied, but this also must be in accordance with the ampacity of the conductor, as opposed to equipment-specific rules being based solely on the expected load, as is the case with motors. This type of protection corresponds to the ampacity of the conductor in a way that prevents overheating from and provides the fault protection to de-energize the circuit in a short circuit or a ground-fault condition.
The rules for protecting conductors are generally less complicated than those for protecting equipment. Section 240.4 makes the general statement that conductors are protected in alignment with their allowable ampacities, and Section 240.21 makes the general statement that protection be located where the conductor gets its supply. There are a few exceptions such as when the conductor ampacity doesn’t work with a standard OCPD rating or when tapping off of a feeder. But with all of the exceptions to the general rule, there are conditions that limit when, where and how these unique scenarios are handled. However, the general rule for conductors is to protect them at their current-carrying capacity and put this protection where the conductor is connected to electricity.
Protecting against shock and arc flash
Let’s first take a look at how overcurrent is used to protect people from shock. To start, it is fair to say that OCPDs do not provide protection from shock on their own. They need help from another area of the NEC that gets a lot of attention, which is the grounding and bonding system. If overcurrent protection is the workhorse of safeguarding people and property, then grounding and bonding is the foundation that supports it. In fact, one part of the grounding and bonding system even mentions in the definition that it is installed to facilitate the operation of the OCPD, and that is the effective ground-fault current path. The effective ground-fault current path is also called out in the performance requirement in 250.4(A)(5). The combination of the performance requirements and the definition states that if something could become energized it must be connected in a way that provides an intentionally installed, low-impedance path back to the source to facilitate the operation of the overcurrent device.
However, this will not protect against shock that occurs when a person contacts an energized conductor 100% of the time. Often this appears to be just another load on the system and we rely on technology like ground-fault circuit interrupter protection to safeguard against these types of events. But OCPDs help to protect against shock by de-energizing the circuit before a person can come in contact with surfaces that are generally not supposed to be energized. This is where overcurrent protection needs a little help from the effective ground-fault current path. By connecting the normally nonenergized parts of equipment to this intentionally created low impedance path, we create a circuit that will open the OCPD in the event that these parts do become energized. This prevents shock by controlling the path that current takes in a way that tells the system that something is wrong and it should be addressed.
Lastly, how do we use overcurrent to protect people from arc flash? If you are familiar with NFPA 70E: Standard for Electrical Safety in the Workplace, then you already understand that the risk to employees is completely dependent on how much current the system is going to supply in an arc flash and how long is it going to take the upstream OCPD to open the circuit. The NECaddresses this in two spots specifically aimed at OCPDs in 240.67 for fuses and 240.87 for circuit breakers. These two sections require that the system take measures to reduce the arcing energy when we are dealing with large systems that can supply large amounts of incident energy. This is typically done with installing certain types of OCPDs or by adding features that allow workers to manipulate the parameters of an OCPD in order to reduce the risk during the work period. Energy-reducing maintenance switching and reducing the instantaneous trip setting are just a couple of the methods that the NEC requires for safeguarding workers from arc flash through the installation of overcurrent protection.
Understanding the role that overcurrent plays in our electrical safety ecosystem hopefully makes it clearer why we install electrical systems the way we do so we can all build, use and benefit from safe electrical installations.