String inverter specialist Solis has analyzed in detail why a fuse box circuit breaker of a solar photovoltaic (PV) system must not be reversed (power line enters the load terminal of the circuit breaker), using an on-the-site example and demonstrated what harm it could cause.
A customer had reported that the circuit breaker in the fuse box of a 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 Solis technical engineer discovered that the circuit breakers were reverse fed. After replacing the circuit breaker and wiring it correctly on-site, the arc disappeared.
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 at the bottom.
When installing the PV system for grid connection, an additional circuit breaker must be added. To save material cost, 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 (as shown in the figure below).
Risks of reverse wiring
Principles of extinguishing circuit breaker arcs
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 a short circuit, the arc formed will be more intensive than a normal one. Metal plates in arc chutes are made of ferromagnetic material to ensure that 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 the figure below).
Potential damage due to reverse wiring
- Incomplete arc extinguishment will cause a longer arc time between moving contact and static contact, which cannot be extinguished quickly through the arc extinguishing chamber, causing part of the arc to escape.
- The braided wire, bimetallic sheet, and the common rotating shaft are always under the power supply voltage, which will cause them to age rapidly.
The input circuit contains components such as electromagnetic coils. So, 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, and a long-term switch 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 wirings are normal and not in a reverse wiring state for safe and stable operations of the system.
When installing a circuit breaker in a PV system, attention has to be paid to the reverse wiring of the circuit breaker. If needed, reverse wiring can be chosen, taking care of using the voltage and breaking current specified by reverse wiring.
If it is not explicitly marked that the device can use the reverse wiring method, it needs to be confirmed by the circuit breaker manufacturer and installed to ensure the long-term safe operation of the PV system.
This article is sponsored by Ginlong Technologies, solar inverter manufacturers.
Image credit: Solis