I work with DC circuit breakers quite a lot, and had the urge to make a quick video primer about choosing DC circuit breakers and the various ins and outs of the process.
This presentation tries to be beginner friendly while encouraging further investigation and independent research – which is the core part of DIY. Hope this video helps someone out!
Researching and Expanding Knowledge Is Time Well Spent
Searching for circuit breakers for your solar power board? Make sure you choose the correct TYPE of circuit breaker for each use case, for example: PV solar feed, charge controller, inverter. Making the effort to get these details right and understand them is time well spent.
There are so many DC 12/24V and solar PV circuit breaker options on the market, it can be hard to distinguish which ones work well and which ones don’t. Reading honest reviews and performing a bit of research can save you a lot of trouble in the future.
Watch out for dishonest sellers listing AC circuit breakers as DC PV solar rated. I have personally seen this done, perhaps the seller did not know or they just figured it didn’t matter (it really does matter A LOT).
Resistance of Protective Devices
Using an ohmmeter to check new and existing circuit breakers for high resistance is a good idea. There should be very little resistance across the breaker. Otherwise, the breaker is suspect.
If you are concerned about the resistance of the protective device (circuit breaker for example), consider the following:
The hotter a circuit breaker gets, generally, the greater it’s potential resistance and so the lower its efficiency. Believe it or not, a circuit breaker should be kept as cool as possible.
Sometimes it’s better to use an appropriate fuse (such as an ANL fuse) instead of a circuit breaker, as they generally have less resistance – although they too should be kept as cool as possible for the same reasons as circuit breakers.
The reason is, depending on its design, if the fuse or circuit breaker is in an unusually hot environment, it may not perform as intended and may trip at a lower rating than it’s supposed to.
Quality circuit breakers usually have detailed specifications listed, and you should read them before buying.
Avoid cheap circuit breakers that make false claims and/or don’t have proper specifications listed.
A circuit breaker can’t protect your investment of wiring and equipment if it doesn’t work as expected.
Research is Better Than Assumptions
Assumptions can be deadly. It’s important to do research and ensure you have the right circuit breaker for the job, and that it will really protect the system if there is a fault.
It is important to pick a breaker with the right amperage rating for the application.
Too low and the breaker will activate when there is a surge draw under normal operating conditions, for example when an inverter starts an air conditioner.
Too high and it won’t activate when there is a short circuit or fault, leading to damage and possibly a fire.
Circuit Breaker Markings
Learn to read and understand the labels on the front. Can you tell the difference between AC and DC markings?
A symbol of a straight line over a dashed line is the symbol for DC or direct current. Breakers can be rated for 500V DC or more, but I would not operate a breaker at its maximum.
AC breakers usually have a symbol that looks like a wavy line – this is an AC breaker and not suitable for DC, well perhaps it could turn a 5V USB light bulb on and off if you had no other use for it.
A very common type of circuit breaker is called a mini circuit breaker or MCB. But there are a lot of classes and variations of each type, so it’s important to pay attention to the details.
A type or class A circuit breaker is rated for fast trip, meaning it’s sensitive to overcurrent and easily trips. I prefer this type, but sometimes, you would want a brief delay before the breaker trips.
Circuit breakers are rated by amperage, such as 15A, but sometimes that rating is not the whole story. Again, it’s important to read the specifications and choose the right breaker for the job, because there are different versions which behave differently by design.
Circuit Breaker Polarization
Some circuit breakers are polarized, meaning they have a positive and negative polarity. Be sure to check the markings and specifications to ensure the breaker works correctly.
Warning – if the DC circuit breaker is polarized, and you install it incorrectly, it will lose its ability to protect the circuit and break arcs. It can be rather confusing to understand, but think of it this way: the polarized circuit breaker must interrupt the DC arc on disconnect or trip, and in order to do that, the voltage must be flowing in the correct direction. Markings on breakers can be confusing and vary amongst manufacturers. If you see positive and negative markings on the breaker, slow down and take the time to examine the specifications and installation instructions. Otherwise, this type of circuit breaker can be dangerous and won’t protect your installation.
Thermal Expansion and Contraction Effects on Connections
When installing a circuit breaker, make sure and torque the terminals properly
Thermal expansion and contraction makes the wires want to move. The breaker will naturally warm up and cool down over the course of normal use.
This can cause loose wires which leads to arcing, heat and fire.
Wires should also be clamped down near the breaker so they don’t move at all.
Stay away from the cheapest circuit breakers if you really want to protect your system.
Don’t cheap out! Saving 10 dollars on a circuit breaker is not worth causing a fire or burning out wires and equipment!
Consider testing a circuit breaker if you’re not sure it will work, but only if you have the ability to do the test safely.
Double Pole DC circuit breakers can switch both the positive and negative side of the PV feed simultaneously.
Circuit breakers are riveted together at the factory, and thus cannot be serviced. Genuine high quality DC circuit breakers are so complex inside, reassembling them would be pointless and educational only.
Hope this presentation helps you out. Thanks for reading!