Overcurrent protection device

Overcurrent Protection Device examples:
1. Breaker 2. Fuse

All components of an electrical system have a maximum amount of current that they are rated to continuous handle, if this rating is exceeded excess heat will be generated which can result in a fire. Overcurrent protection devices (OCPDs) are used to automatically disconnect a circuit if a certain current is reached for a certain period of time. The size of the required overcurrent protection device is determined by the maximum amount of current that a circuit is anticipated to carry and the ampacity of the circuit wiring. The OCPD must be smaller than the rated ampacity of the wire. The most common types of overcurrent protection device that are used in electrical systems are fuses and breakers.

Characteristics

Graph of different trip curves. The breaker has the same current rating, but comes with different ratings B (most sensitive), C (less sensitive), D (most sensitive).

There are innumerable different OCPDs in the market with each one designed to fit a specific purpose. Many OCPDs are similar in appearance, therefore it is very important to revise the fine print on the side of any OCPD being considered to make sure that it is appropriate for the conditions of use.

Current type=

OCPDs may be designed to work with AC, DC or both types of current. If a breaker is rated for both AC and DC, it is likely that the DC voltage rating will be lower as DC circuits are more difficult to interrupt.

Current

OCPDs will be rated for a specific continous current rating. Standard international OCPD sizes: 1 A, 2 A, 4 A, 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A, 100 A and 125 A. Standard North American OCPD sizes per US NEC: 15, 20,25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250,300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000,4000 5000, and 6000 amps. Additional standard fuse sizes are 1, 3, 6, 10, and 601 amps.

Trip curve

OCPDs will have a trip curve which specifies how long the device can sustain current above its continuous duty rating. This is desirable because in electrical systems there are often surge loads that require additional current for a very brief period of time when starting. If an OCPD does not have a slight time delay, it will trip every time a surge load is connected.