General - Overcurrent protection (OCP) devices are designed to protect conductors and electrical equipment from dangerous amounts of current flowing through them when something goes wrong. They do this by “tripping”, opening up the electrical circuit so there is no longer a conductive pathway for current to flow. There are two main types of overcurrent protection devices, circuit breakers and fuses. The big difference is this: fuses are used up after one overcurrent event and breakers are reusable. Article 240 of the National Electrical Code is all about overcurrent protection devices. The schematic symbols for breakers and fuses are here for reference:
Overcurrent Protection Device Electrical Symbols
Low Voltage - Breakers and fuses are rated according to three key parameters: the voltage rating, the ampere rating, and the interrupting rating.
The voltage rating is the system voltage the equipment is designed for. For low voltage equipment, OCP devices can be used in systems with voltages at or below the OCP device rating.
The ampere rating describes the amount of current that an OCP device can safely carry. For low voltage overcurrent protection devices, the ampere rating is usually the same as the trip rating. For example, a low voltage 30A breaker is designed to open up (trip) at no more than 30A flowing through it.
The interrupting rating is the maximum value of current that the OCP device can safely interrupt. This value is usually on the order of kiloAmps. The interrupting rating must be coordinated with the available short circuit current to ensure that the system is adequately protected.
The interrupting rating and the trip rating both show up on the overcurrent protection device’s trip curve. A trip curve is a graph showing the clearing time vs. current flowing through the OCP device. The long-time trip rating is the value of current that just barely is high enough to trip the OCP device. The instantaneous trip rating is the value of current which leads to the minimum time delay for clearing a fault (nothing can really happen instantaneously).
Conceptual Trip Curve Diagram
Medium Voltage - At higher voltages, the OCP device ratings mentioned above take on a slightly different meaning.
Medium Voltage Circuit Breaker, Photo Courtesy Eaton
Medium Voltage Fuse, Photo Courtesy Eaton
Per NEC 240.100, medium voltage breakers must be controlled with a overcurrent relay elements and current transformers. Medium voltage breakers usually have an ampere rating that corresponds to their maximum allowable current that can pass through continuously. However, the trip setting of the relay, the value of current which the breaker opens at, could be adjusted down much lower than the device rating. In fact, the entire trip curve of a medium voltage breaker can be customized, including the long-time and instantaneous trip settings.
Medium voltage fuses also often behave differently than their low voltage counterparts. MV fuses possess an ampere rating like medium voltage breakers. The rating corresponds to the allowable continuous current the fuse can carry safely, but does not correspond to the trip rating of the fuse. Medium voltage fuses usually trip at values over 200% of the ampere rating. As a result, overload protection is almost never included in medium voltage fuses.
Withstand - Overcurrent protection devices don’t operate instantaneously, as seen in the trip curves above. In the time between a fault occurring and the OCP device operating, damage can occur to equipment and conductors in the path of the fault. The amount of fault current that can safely pass through for a defined period of time is known as withstand.
Electrical equipment withstand ratings are provided by the manufacturer, usually for a specified period of time per an industry standard. Conductor withstand can be calculated in accordance with Table 240.92(B) in the 2020 NEC. A good design will ensure that equipment and conductors don’t have their withstand ratings exceeded during a short circuit event.
Withstand is different from ampacity. Ampacity describes how much current a conductor can carry indefinitely without exceeding a particular temperature. Withstand describes how much current a conductor can carry for a brief period of time without exceeding the insulation’s maximum temperature rating.
Sizing Devices and Conductors - Coordinating the trip rating of overcurrent protection devices, the expected load current, and the ampacity of conductors is necessary to achieve installations that are safe and user-friendly. If the trip rating is too close to the expected load current, OCP devices will frequently activate unnecessarily, a problem known as “nuisance tripping”. If the trip rating exceeds the conductor ampacity under normal operation, then the design may not be safe.
For low voltage systems, the standard relation required to ensure protection is:
A' > O > e I
Where:
A’ is the ampacity of the conductors in Amperes (after all derating)
O is the ampere rating of the overcurrent protection device
e is a unitless multiplying factor
I is the expected load current in Amperes
The value of e varies throughout the code. For most loads that run continuously (3 hours or more at their expected value), e = 1.25. For most loads that do not run continuously, e = 1. For special equipment referenced by name in the NEC, the relevant article should be checked to determine the correct value of e.
The NEC actually gives a little bit of leniency on the definition above. For OCP devices rated less than 800A, the ampere rating of the overcurrent device is actually allowed to be larger than the ampacity of the conductors by up to 1 standard size. If the OCP is rated greater than 800A, then the conductors need to follow the relation above strictly.
On medium voltage systems, the relationship above is not applicable. Instead, conductors and equipment just have to be protected against short circuit and overload per engineering supervision. Conductors still need to have an ampacity greater than their load current to ensure that they won't overheat. However, the ampacity of the conductors can be less than the trip setting of the OCP device. The design conditions for medium voltage can be summarized as:
A'> e I and d A' > O > e I
Where d is a multiplier on the conductor ampacity as described in NEC 240.100.