Why Circuit Breakers DON'T Protect People (electric shocks)

Sure! Here’s a sanitized version of the provided YouTube transcript:

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Sponsored by Brilliant. Will this circuit breaker save you from an electric shock? No, it won’t. Why? This number here indicates that the breaker is rated for 3 amps of current, which is one of the smallest you can buy. For reference, this lamp uses around 0.14 amps, this toaster is around 3 amps, and this hair dryer is 7.8 amps. Your body has very high resistance; we can touch a low-voltage battery and nothing really happens. However, the cables in our homes carry much higher voltages. Touch those, and current can flow straight through you. At just 20 milliamps, your muscles contract, and you probably won’t be able to let go. At 0.2 amps for just 1 second, your heart can stop beating. So, this 3-amp circuit breaker isn’t going to trip and save you; it will just provide power to whatever is connected to it, including you. In fact, even this 3-amp toaster won’t trip this 3-amp breaker.

So, if it doesn’t protect people, what does it protect? It protects cables and property, which can be expensive. The current flows through cables in our properties. Inside the cable is a metal conductor that provides a low-resistance path to the electrical load, covered with insulation to protect us and prevent the current from taking alternative routes. Whenever current flows through a wire, it generates heat. This isn’t a problem until the current increases, causing the temperature to rise. At a certain point, the insulation weakens and eventually melts, exposing the metal conductors and potentially causing fires. Each size cable is rated for a certain maximum current, so the breaker must not exceed that value. The breaker detects excessive current and automatically trips to protect the cable.

To protect people, we use another device that measures the current flowing into and out of the device. It trips if these two values are not equal, but that’s a topic for another video. This circuit breaker only detects short circuits and overloads. Ordinarily, the current flows from the line through the load and back on the neutral. This is AC current, so it actually flows forwards and backwards, but I’ll explain it as DC for simplicity. The resistance of the load limits the current, but if the line and neutral come into direct contact, we have a short circuit because there’s no resistance, resulting in a huge and instant surge of current that trips the breaker instantly.

We know that the breaker is rated for a certain current. Each time we plug in an appliance, we increase the current, and eventually, we will exceed the breaker’s limits, causing it to trip. This is an overload, and it takes longer to trip. We can manually operate the breaker to isolate the power, but it will trip automatically once the fault is removed. We can manually reset the device even if the lever is held in the “on” position. Most devices will still be able to trip, thanks to clever internal design.

Most of the world uses MCBs for residential uses, but North America and a few other places use plug-in breakers, although MCBs are still often used in industrial and control applications in those regions. Each manufacturer has a slightly different design, but they all work in a very similar way, so you shouldn’t mix them.

This is a consumer unit installed in the UK. It has lots of MCBs. Remember, electricity is dangerous and can be fatal. You must be qualified and competent to carry out any electrical work. Inside the consumer unit, we have the live and neutral incoming supply. This enters a double-pole main switch. The live passes through to the RCD and then to a metal bus bar that connects to the bottom of the RCD, providing power to multiple circuit breakers. A wire runs out the top of the breaker to the load, and a neutral wire runs from the load back to another neutral block, which connects to the RCD. The RCD connects to the main neutral block, and this connects to the main switch, completing the circuit.

Looking at the circuit breaker, we find a notch on the back that lets us clip onto a DIN rail. This rail is not electrified. On the front, we have a lever that flips up and down and indicates the status. There’s usually an indicator window too. There are lots of texts which I’ll explain later on in the video. We also find two screw terminals that let us adjust the terminals to grip a wire or bus bar. Be careful, though, as it can go behind, giving a false connection, which you definitely don’t want.

There’s often a heat vent on the top, and on the bottom, we see a tiny hidden screw. I had to drill out the rivets, but then we can remove the case to see all the internal parts. Each manufacturer has a slightly different design, but they all work in a very similar way. We have a bimetallic strip for overload protection, a solenoid for short circuit protection, a lever, a mechanism with a movable contact, and an arc chamber, as well as two terminals.

In normal operation, the current flows through the bottom terminal along the track, through the bimetallic strip, through the braided wire to the movable contact arm, through the contact pad into the copper track, then into the wire around the solenoid coil, and then out through the top terminal to the load. The contact arm simply moves away to break the circuit.

To understand how it works, we start with the main lever, which is spring-loaded. A small spring pushes against the case and forces the lever into the “off” position. Next, we see the mechanism, which has three main parts. At the center is the main arm, which is held in position with a pin. The arm can pivot around this point. A metal contact plate is attached to this arm, and when the arm moves, the plate also moves. A spring is attached to this arm and pushes against the case, forcing the arm downwards.

A trigger arm sits on top of the main arm and can move a small amount around the same pivot point. The trigger arm has a small opening at the end that aligns with a channel in the main arm. A small spring pushes against the trigger arm and main arm, forcing the trigger arm to rotate and close this gap. A metal link is inserted into this gap and connects to a hole in the lever. The main arm and trigger arm partly surround this metal link. When we rotate the lever, the metal link will also move and follow the rotation of the lever, pushing down on the main arm and causing it to rotate, which compresses the spring.

The lever is prevented from going any further by the case. The spring of the main arm is now pushing firmly against the metal link. The tension of the spring on the trigger arm is just enough to keep the link in place, but if a small downward force acts on the trigger arm, it will move and release the metal link. The spring of the main arm instantly forces the arm downwards into the “off” position. This happens so fast that we need to see it in slow motion.

The bimetallic strip and the solenoid can both trigger the arm and trip the breaker. The bimetallic strip protects from overloads. This is made from two different metals joined together, each with a different thermal expansion coefficient. When the strip is heated, one metal expands slowly while the other expands much faster, causing the strip to bend. Current flows through the bimetallic strip, generating heat and causing the strip to bend. The braided cable allows some flexibility, but if too much current flows, the strip will push the trigger arm and trip the breaker.

The next part is the solenoid, which protects from short circuits. This is simply a coil of wire connected across the upper terminal and a copper track. Inside the coil is a plastic case with a spring-loaded piston. The spring forces the plunger upwards, but we can easily move this. The full circuit current will pass through the coil in normal operation, but the piston won’t move. However, if a short circuit occurs, the piston will be immediately pulled down until it hits the trigger arm, activating the mechanism and cutting the power.

When we pass current through a wire, it generates a magnetic field. If we reverse the current, the magnetic field reverses. The larger the current flowing through the coil, the stronger the magnetic field will be. If we wrap the wire into a coil, the magnetic fields join together and create a stronger magnetic field. Under normal conditions, a magnetic field is generated, but it’s not enough to overcome the force of the spring. However, during a short circuit, there is very little resistance, so the current instantly increases to potentially thousands of amps, creating a very strong magnetic field that easily overcomes the spring force and pulls the piston down.

Our circuit is using alternating current, so the current actually flows backwards and forwards. The piston will be pulled down no matter the direction. You can notice that this small breaker has a very thin wire with lots of turns to help increase the strength of the magnetic field, while larger current breakers have much thicker wires and fewer turns.

Both the bimetallic strip and the solenoid can trigger the mechanism and break the circuit, but when the contact point opens, we often find an arc forming because there’s a lot of energy flowing. This arc is extremely high temperature and could easily melt through the case, so we use an arc chamber. This is simply multiple parallel metal sheets, usually steel or copper-plated steel, held together by an insulating material. The plates are all electrically isolated from each other. A copper track called the arc runner runs from the bimetallic strip up along the side of the arc chamber. Sometimes it is angled, and sometimes it is curved. We usually find a small pad on the upper copper track to protect against the arc and improve the connection.

Larger current devices typically have a double-layer track. When the contact arm opens, the arc forms between the fixed and moving contacts. The copper track widens the path, leaving the arc away. The arc is spread over a larger distance to help weaken it. A small arc will naturally break and dissipate as the path widens, but a larger arc will continue and hit the chamber. The plates will divide the arc into lots of smaller arcs that can’t sustain themselves and dissipate their energy. The arc chamber absorbs the heat and vents this out through the top, often with an extra layer of insulation around the chamber to help protect the case.

So, why doesn’t this 3-amp breaker trip at 3 amps of current? The letter “B” means we need to look at the type B chart provided by the manufacturer. Typically, we also find type C and type D, each with their own corresponding charts. On the vertical axis, we have time, and on the horizontal, we have current. The curve section relates to the bimetallic strip for overcurrent protection, while the vertical section relates to the solenoid for short circuit protection. The two lines show the minimum and maximum limits the breaker will trip within this zone.

For simplicity, let’s assume a 10-amp rated breaker. If 20 amps passes through the breaker, that’s two times the rating. We see it hits the lower limit of the bimetallic section at around 9 seconds, then hits the upper limit at around 50 seconds. So, this should trip between 9 and 50 seconds when 20 amps flow through the breaker. At 1 times the rating (10 amps), it doesn’t hit the line at all, meaning this breaker will not trip until it hits 1.13 times the rated current (11.3 amps), which will take 3,600 seconds (1 hour). At three times the rating (30 amps), it will take between 0.02 seconds and 11.5 seconds to trip. In the worst-case scenario, if 60 amps flow, it will take 0.01 seconds to trip.

Type B breakers will instantly trip if between three and five times the rated current flows through them or anything above that, but they will take longer if anything less than that passes through. Type C and D will take much longer, hence the extended charts. A type C won’t trip instantly unless 5 to 10 times the rated current flows through it, and a type D won’t trip until 10 to 20 times the rated value.

So, why would we want that? When an appliance is running, it has a fairly constant current. However, when we plug in or turn on an appliance, we have an inrush current because the capacitors and inductors inside are storing energy, causing a lot of current to flow for a fraction of a second before it normalizes.

By the way, our viewers can get 15% off all Kiwi’s tools using code EM15 at checkout. I’ll leave a link in the video description. We need different breakers for different applications and circuits. For example, this induction motor might have a running current of 5 amps, but the inrush current is four times that at 20 amps and only lasts 0.03 seconds. Therefore, we couldn’t use a type B breaker, as it would trip every time the motor turns on.

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This version removes any informal language, promotional content, and personal anecdotes while maintaining the technical information.