Solis, a string inverter expert, analyzed in detail why the fuse box circuit breaker of a solar photovoltaic (PV) system cannot be reversed (the power cord enters the load terminal of the circuit breaker) and demonstrated the possible damage through field examples.

A customer reported that the circuit breaker in the fuse box of the newly installed solar photovoltaic system had an abnormal arc when it opened. The customer suspected that it was caused by the inverter. However, Solis technical engineers found that the circuit breaker was back-fed. After replacing the circuit breaker and wiring correctly on site, the arc disappeared.

AC circuit breaker back feed 

Normal wiring and reverse wiring 

In the fuse box, the power interface is at the top and the load interface is at the bottom.

When the photovoltaic system is installed and connected to the grid, an additional circuit breaker must be added. In order to save material cost, installation space and reduce installation workload, some installers choose to use bottom-up and top-down methods (reverse wiring).

However, many miniature circuit breakers are generally not allowed to be wired in this way due to the limitation of the current direction (as shown in the figure below).

Circuit breaker arc extinguishing principle 

When the circuit breaker interrupts the current flowing to the load during normal operation, an arc is formed between the contacts of the circuit breaker. If a short circuit occurs, the arc formed will be stronger than the normal arc. The metal plate in the arc extinguishing chamber is made of ferromagnetic material to ensure that any arc formed is attracted to the plate and moved to the arc extinguishing chamber through the arc slide.

When it reaches the arc extinguishing chamber, the structure of the arc extinguishing chamber further widens the arc and divides it into smaller segments. This increases the length of the arc. Then it cools down and finally extinguishes (as shown in the picture below).

Possible damage caused by reverse wiring

The input circuit contains components such as electromagnetic coils. Therefore, the switch impedance will be higher than ordinary wiring. When the circuit is disconnected, the transient recovery voltage will be higher. It is difficult to extinguish the arc, which will cause the contacts to heat and accelerate the aging, and the long-term switching action will damage the circuit breaker (as shown in the figure below).

There are many circuit breakers with a margin of breaking capacity. It is often seen that some circuit breakers are nominally capable of reverse wiring. Solis recommends that all AC wiring is normal and not in reverse wiring to ensure safe and stable operation of the system.

When installing a circuit breaker in a photovoltaic system, you must pay attention to the reverse wiring of the circuit breaker. If necessary, you can choose reverse wiring. Pay attention to the voltage and breaking current specified for reverse wiring.

If it is not clearly marked that the equipment can be reversed wiring, it needs to be confirmed and installed by the circuit breaker manufacturer to ensure the long-term safe operation of the photovoltaic system.

This article is sponsored by the solar inverter manufacturer Jinlong Technology.