Selecting Proper Overcurrent Protection Devices

Electrical distribution systems can be complex, and no system is perfect. Accidentally damaging equipment, a tough environment, equipment deterioration, and many other factors can contribute to the occurrence of overcurrents. To avoid costly damage and exposure to serious safety hazards, it is important that proper overcurrent protection devices (OCPD) are installed.  Selecting the appropriate OCPD requires knowledge of the voltage rating of the device and correctly matching it to that of the system.

What Is an Overcurrent?

There are two different kinds of overcurrents:  overload currents and short circuit currents.  In an overload current, the current is excessive relative to the standard operating current and is isolated to the distribution system’s normal conductive path.  A short circuit current is a current that travels outside the standard conducting path.  Many OCPDs are designed to handle both types of currents.

Types of Overcurrent Protection Devices

In order to protect equipment and circuits from overcurrent, there are many different types of OCPDs that can be used.  The most common types include fuses and circuit breakers.  Both interrupt the flow of a current, but they do this in different ways:

Fuses

A fuse contains a metal wire that melts when the excessive current flows through it, effectively interrupting the current and preventing further damage to the system.  Fuses come in a variety of current and voltage ratings, and need to be replaced when they are blown.

Circuit Breakers

Circuit breakers can come in a variety of types and sizes, but the common characteristics are the same.  When the circuit breaker senses a fault condition, the trip unit in the circuit breaker opens its contacts to interrupt the current.  These contacts must then avoid excessive heating while carrying the load current.  When the electrical current is interrupted, an arc is generated and must be contained so that the space between the contacts can once more withstand voltage that flows through the circuit.  While fuses need to be replaced, circuit breakers can be reset and used again.

Ampere, Voltage, and Interrupting Rating

It is extremely important to know the different ratings for overcurrent protection devices so that proper rating can be chosen and the device will work.  The three types of ratings include ampere, voltage, and interrupting rating.

Ampere Rating

All fuses have an ampere rating that is chosen based on the type of load and certain code requirements.  In most cases, the amp rating of a fuse should not exceed the current carrying capacity of the circuit.

Voltage Rating

In order for the device to work, the voltage rating of the OCPD must be equal to or greater than the system voltage; otherwise the device may self-destruct.  The two types of overcurrent protective device voltage ratings include straight voltage rate and slash voltage rate.  While all fuses are straight voltage rated, some devices are slash voltage rated (such as some molded case circuit breakers) and require additional evaluation when being used.

Interrupting Rating

The interrupting rating is the rating that defines the amount of current that the OCPD can take without losing its integrity.  If the device can’t handle the fault current level, it could potentially rupture, and cause damage and injury.  Thus, it is essential that electrical workers use an OCPD that has an interrupting rating which is sufficient for all current fault levels.

Installation and Testing

To install the proper overcurrent device in the correct location, electrical workers should defer to Article 240 in the National Electric Code (NEC).  In this Article, the NEC states that all conductors (apart from flexible cords, flexible cables, and fixture wires) shall be protected with an OCPD in accordance to their ampere rating, which is specifically stated in Article 301.15, unless otherwise permitted or required in Article 240.4 A-G.  Depending on the installation, other Articles may apply as well.

Maintaining OCPDs is also an important part of proper circuit protection.  Electricians should look to NFPA 70B, which specifies the testing and maintenance of OCPDs for reliability and safety.

Overcurrent Protection Is Key to Overall Electrical Safety

Choosing the appropriate overcurrent protection device (OCPD) for a specific circuit is a fundamental requirement in any electrical system.  In addition to protecting the conductors and conductor insulation, OCPDs also protect the rest of the system by isolating the electrical fault.

If overcurrent is not corrected, it can create serious safety hazards and can cause expensive damage to electrical equipment and property.  In order to stay within the NEC and local electrical codes, and choose the right overprotection device, electrical workers need to get overprotection right and understand OCPD ratings.

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