How To Choose An Electrical Circuit Breaker

This is an introductory article on the types of electrical circuit breakers used in solar photovoltaic installations.

What is a circuit breaker?

Outside of the UL listed enclosures of the PV modules and inverters, there are many variables in system design which can have dramatic effects on personal and property safety. Circuit Breakers, a type of Overcurrent Protection Device, are the virtual  'safety valves' which protect a system from end to end.

An excessive current through an electrical circuit could be hazardous and could damage the components on an AC or DC circuit. To prevent excessive current, installers use circuit breakers. A circuit breaker is a switch that can automatically interrupt the flow of current in a circuit when the circuit is overloaded, stressed or under abnormal condition.

What are the different types of circuit breakers?

We can divide the circuit breakers in 3 categories in terms of electrical specifications:

  1. Standard circuit breakers
  2. Ground Fault Circuit Interrupter (GFCI) circuit breakers
  3. Arc Fault Circuit Interrupter (AFCI) circuit breakers

1) Standard circuit breakers
When a hot wire accidentally makes contact with a normal wire, a ground wire or another hot wire, the Standard circuit breaker stops the current in order to protect these wires. Standard circuit breakers come in main varieties, AC and DC rated. The difference between AC and DC breakers is essentially that an electrical current arc inside a DC breaker is harder to extinguish than inside an AC breaker due to the cyclical nature of AC current. Many breakers are rated for AC and DC use, but will permit a much lower maximum voltage for DC use than AC use. A breaker's label or it's specification sheet will indicate AC and DC ratings.

2) Ground fault circuit interrupter (GFCI) circuit breakers

The power is cut by a ground fault circuit interrupter (GFCI) circuit breaker when there is an overload current, a short circuit or a line-to-ground fault in the circuit. It occurs when we have an undesired path that forms between a grounded element and an electrical current. GFCI breakers are available in AC and DC versions and usually have test and reset buttons, as well as indicator lights.

3) Arc Fault Circuit Interrupter (AFCI) breakers
These kinds of circuit breakers are focused on un-intentional discharge in an electrical wire or a cord. The breaker detects the sudden electrical jump and stops the current which circulates in the circuit in order to prevent arcing which could produce enough heat to start a fire. Generally, these abrupt electrical jumps come from damaged wires and cords.
Just like the GFCI circuit breakers, the AFCI breakers have test buttons.

How does one choose a circuit breaker?

There are three basic criteria for choosing a circuit breaker:

  1. Type of Mount

  2. Current Rating

  3. Voltage Rating

1- Mounting: DIN or Panel mount

DIN rail mount circuit breaker

DIN rail breakers
A circuit breaker can be mounted to a metal rail of a standard type which is named “DIN rail”. (DIN comes from “Deutsche Institute Norm” a German standard). DIN rail mount circuit breakers are specifically designed to be mounted to this DIN rail strip. A side of the circuit breaker adopts the shape of the DIN rail to be attached to it. A clip mechanism represented in the following picture allows quick mounting and removal of the breaker.

Panel mount circuit breaker

Panel mount breakers
Panel mount circuit breakers are, as the name indicates, circuit breakers which are designed to be directly mounted on a standard electrical service panel. DIN and Panel Mount breakers are not interchangeable.

 

2- Current Rating: Circuit breakers are categorized by the maximum current that they expect to carry (Ampere rating). The US National Electrical Code requires circuit breakers to be rated for 125% of the circuit current on the circuit they protect or the closest larger standard rating.

3- Voltage Rating: The voltage rating of the circuit is crucially important. The voltage rating should always be higher than the AC or DC system voltage. Otherwise it may result in electrical arcs which are among the major reasons for fire.

Other Considerations

The guidlines above are only an introduction to fascinating world of Over Current Protection - and not to be considered adequate for OCPD design and installation. For actual NEC rules, check out 2011 NEC section 690.8 or 690.17 for useful formulas and guidelines for OCPD use in PV systems.