Recently, a customer reported that the circuit breaker in the fuse box of the newly installed solar PV system was experiencing an abnormal arc when it was disconnected. The customer suspected that it was caused by the inverter; however, a waaree technical engineer discovered that the circuit breakers were reversing fed. After replacing the circuit breaker and wiring it correctly on-site, the arc disappeared.
In this waaree Seminar, we will analyze in detail why the circuit breaker must not be reversed (the power line enters the load terminal of the circuit breaker) using this example and what harm it will cause.
Reverse Feeding of AC Breaker
Normal wiring and reverse wiring
In the fuse box, the power interface is on the top and the load interface is on the bottom.
When installing the PV system for grid connection, you will add an additional circuit breaker. In order to save material costs, and installation space and reduce installation workload, some installers choose to use the bottom-in and top-out method (reverse wiring).
However, many miniature circuit breakers generally do not allow such wiring because of the limitation of current direction, which is shown in the figure below.
1. Principles of Extinguishing Circuit Breaker Arcs
Risks of Reverse Wiring
An electric arc is formed between the contacts of a circuit breaker when it interrupts the current flow to loads during normal operation. In the event of short circuits, the arc formed will be more intensive than a normal one. Metal plates in arc chutes are made of ferromagnetic material to ensure any arc formed is attracted towards the plates and moves towards the arc chutes through the arc runners.
As it reaches the arc chute, the structure of the arc chutes widens the arc further and splits it into small segments. This increases the length of the arc. It then cools down and eventually gets extinguished (as shown in Figure below).
2. Potential Damage Due to Reverse Wiring
① Incomplete arc extinguishment. This will cause longer arc time between moving contact and static contact, which cannot be quickly extinguished through the arc extinguishing chamber, causing part of the arc to escape.
② The braided wire, bimetallic sheet and the common rotating shaft are always under the power supply voltage, which will cause them to age rapidly.
③ The input circuit contains components such as electromagnetic coils, therefore the switching impedance will be higher than that of normal wiring. The transient recovery voltage will be higher when the circuit is disconnected. It is more difficult to extinguish the arc and this will cause the contact heat to accelerate aging. A long-term switch action will cause the circuit breaker to be damaged (shown in the figure below).
Solution
When installing a circuit breaker in a PV system, pay close attention to the circuit breaker’s reverse wiring. If necessary, use reverse wiring and pay close attention to the voltage and breaking current specifications.
Conclusion
If it is not clearly marked the device can be used in reverse wiring mode. You must confirm with the circuit breaker manufacturer and install it to ensure the PV system’s long-term safety.