RCCB vs RCBO Explained | Working, Selection, Sizing & Calculation

[00:00:00]Hello everyone and welcome back to my channel Rashidikbal.

[00:00:04]In today's video we are going to understand RCCB and RCBO. Two very important protection device used in electrical installations.

[00:00:14]Now before we learn about these device, let understand why we actually need them. In almost every electrical installation, we use MCBs for protection. As we know MCB protect the circuit against overload and short circuit. But here is the important question. Can MCB protect a person from electric shock? The answer is no. It will not protect from electric shock.

[00:00:40]The problem is that this leakage current is usually too small to trip an MCB. So the question is here, how do we protect ourself from this type of fault? This is where the residual current device or RCD comes into the picture and the two most commonly used type of RCDs are RCCB and RCBO. Now let's understand how an RCCB actually work with a practical example.

[00:01:07]Assume we have four loads connected in a house. A motor, air condition, washing machine and a refrigerator. Each load is protected by its own MCB. Here you can see these all are protected with their own MCB while all of them are supplied through one common RCCB. This is the common RCCB. Under normal operating conditions, motor consume 5 A current, air condition 10 A, washing machine 3 A and refrigerator 2 A. I just assume this uh ampere rating of this uh equipment.

[00:01:43]So the total current flowing through the phase conductor is 20 A. After adding these all current rating it's become 20 aere. Now remember one important practical. Whatever amount of current goes through the phase wire the same amount of current must return to the neutral wire. So in this case phase current is 20 aere. So the neutral current will be 20 aere as well. Since both current are exactly equal the RCCB detects a balance condition and keep the circuit on. Here you can see 20 A is going from the phase wire and return 20 A through neutral wire. Now let's see what happened during a earth leakage fault. Suppose the insulation inside the refrigerator gets damaged and the phase wire touches the metal body of the refrigerator. Now the metal body becomes live and some currents start flowing through the earth instead of returning through the neutral wire. For this example, let assume the leakage current is uh 0.4 ampere which is equal to 400 milliamps. Now the current become through the phase wire is 20 aere and current returning through neutral is 20 minus 0.4 a equal to 19.6 aere. This is the current which is flowing through neutral after leakage. So why this 19.6 aere? Because uh 0.4 aere has taken an alternate path through the earth. This difference between outgoing phase current and the returning neutral current is called residual current or we can say leakage current. Actually the RCCB continuously compare these two currents. A standard RCCB used for human protection has a sensitivity of 30 miampere which means it can detect a leakage current as small as 0.03 aere.

[00:03:37]But in our example the leakage current is 0.4 4 ampere which is much higher than the RCCB limit. So when the RCCB detect this difference between phase and neutral current it trips within a milliseconds and disconnects the power supply helping to protect people from electric shock. Now let's understand what actually happens inside the RCCB.

[00:04:01]Inside the RCCB main component is toidal current transformer also called a core balance transformer. As you can see here it is a ringshaped core and both the phase wire and neutral wire passes through this core. Under normal condition the current flowing through the phase wire and neutral wire is the same. The phase current create a magnetic field in one direction while the neutral current create a equal magnetic field in opposite direction because both magnetic field are equal they cancel each other and the RCCB does not trip but when an earth leakage occurs this balance changes. Now the current in the phase wire is higher than the current returning through the neutral wire. Because of this difference the magnetic field are no longer canled.

[00:04:53]uh you know a small magnetic signal is created inside the core which generates a voltage in the sensing coil. This is a sensing coil. Once there is a leakage and imbalance of the current, this sensing coil they sense the leakage current. This signal activate the trip mechanism and the RCCB disconnect the power supply. So the complete working principle of uh the RCCB is simple. It continuously compare the current going out through the phase wire and the current coming back through the neutral wire. If there is any difference it will trip immediately it will trip. Now there is one more important things to understand. RCCB can protect against earth leakage and electric shock but it cannot protect against overload or short circuit. Generally we are using MCCB for overload and short circuit protection.

[00:05:43]But what if we need both protection in one device means uh RCCB protection plus MCB protection. This is where the RCBO comes into the picture. Actually RCCBO works on the same principle uh as a RCCB to detect the earth leakage but it also has MCB protection built into the same device. This means one RCBO can provide three type of protection. Earth leakage protection, overload protection and short circuit protection. Here you can see the internal component of RCCB and RCBO. If we compare both device, the leakage protection part is almost the same. Both have tooidal transformer which continuously check the balance between phase and neutral current. The main difference is the RCBO has additional component like thermal element which provide overload protection. Here you can see this is the thermal protection element and this magnetic mechanism provide short circuit protection. This this this provide short circuit protection. Now let's understand why RCBO is used in practical installations. Uh in important location like hospitals, airport and data centers continuously power supply is very important.

[00:07:01]If several circuit are connected under one common RCCB, a fault in one circuit can trip the RCCB and disconnect power to all connected circuit including the healthy one. So to avoid this problem, separate RCBO are installed for individual circuits. This why only the fault circuit will trip while the other circuits continue working normally. For example, if we redesign this panel and install a separate RCBO for each load circuit, every circuit get individual protection.

[00:07:36]Suppose a leakage fault occur in refrigerator circuit only the refrigerator RCBO will trip while the motor AC and washing machine circuit will continue operating. Similarly, during an overload condition, the thermal protection inside the RCBO will operate.

[00:07:54]So this individual protection provide better safety and selectivity compared to using one common RCCB. That is why in modern electrical installation RCBO are widely preferred for final circuits.

[00:08:08]Here you see the example a single fields distribution board where each outgoing circuit is connected through a separate RCBO. You can see here these all are RCBOs and it is connected to a separate circuits. Now the next question is how do we select the correct RCCB or RCBO rating? When selecting a RCCB or RCBO we mainly check three important parameters.

[00:08:31]First is current rating in ampere. This tell us how much current the device can safely carry continuously. Second is the leakage current rating in milliampere.

[00:08:43]This tell us the level of leakage current at which the device will trip and provide protection. And the third is braking capacity. This is mainly checked for RCBO because it can provide short circuit protection. Actually, it tells us uh whether the device can safely disconnect a high fault current. So, let's start with the current rating calculation. Suppose we have a 5 kW singlephase appliance connected to a kitchen socket outlet. This is switch socket outlet and the voltage is uh 230 volt. So as we know the load current equal to power divided by voltage. So the load current equal to power we have 5,000 divided by 230 volt. After calculating this value we get load current equal to 21.7 ampere. Since the calculated current is 21.7 aere we can select the next standard rating which is available in market is uh 25 ampere. Or if you want to add a s some safety factor so you can multiply with 1.25 based on that you can select uh circuit breaker means we need to select the higher size of RCCB or RCBO actually the same current calculation is required for both RCCB and RCBO selection. Now one important question arises here if RCCB does not provide overload protection then why do we select its current rating? Let understand this with a simple example. Here one motor is connected and the RCCB is carrying the load safely. Now we add a second motor and then a third motor.

[00:10:23]As the load increases beyond the RCCB rating, its internal contacts start heating but the RCCB will still not trip because it is not designed for overload protection. If this continues, the RCCB may get damaged. That is why the RCCB rating should be always be equal to or higher than the load current. And this is the reason RCCB is always installed together with MCB because MCB protect against overload and short circuit while the RCCB protect against earth leakage. So this is the reason why we calculate current rating for RCCB.

[00:11:03]Now let's understand leakage current rating. This rating tell us how much leakage current is required to trip the RCCB or RCBO. The most common leakage rating are 30 miampere, 100 miampere and 300 miampere.

[00:11:20]Each rating is used for different application. Let understand them one by one. 30 m RCCB or RCBO is mainly used for human protection. The reason is that when electric current passes through the human body, it can affect the muscle breathing and even a heart. So according to electrical safety standard, leakage current around 30 miampere can become dangerous if it flows through the body for enough time. Therefore, a 30 m RCCB is designed to detect this small leakage current and disconnect the supply very quickly. That is why 30 miampere RCCBs or RCBO are commonly installed on socket outlet bathrooms, kitchens, outdoor circuit and other final circuits where where people may come in direct contact with electrical equipment. Now you may have another question if uh if a 30 m provide human protection then why do we also have a 100 miampere and 300 mAh RCCB or RCBO? Why don't we use 30 mAh everywhere? So let's understand for 100 mAh RCCB it is mainly used for equipment protection and distribution circuits in large installations like uh many electrical equipment and uh long cable run can produce a small leakage current during normal operation. Always there is a leakage current but it is normal. For example, uh large motors like pumps or conveyor system can have small uh leakage current due to insulation and internal components. And second example we can say uh you maybe observe that computer computers or server or any other electronic uh equipment also produce a small leakage current through their filters. So when many devices are connected together these small leakage can add up. So if we use 30 mAh RCCB or RCBO for the complete system, it may trip frequently even when there is a no dangerous fault. So to avoid this, a 100 miampere RCCB is often used. So this is the reason why we use 100 mAh RCCB. Now let's understand 300 mAh RCCB. A 300 mAh RCCB is mainly used for fire protection.

[00:13:39]Suppose a leakage current may continuously flow through the building structure or cable tray or other materials. This continuous leakage generate heat. Over time, the heat can damage insulation further and may even start a fire. So 300 mia RCCB detect this higher leakage current and disconnect the supply before it become a serious fire hazard. That is why 300 mAh RCCB are commonly installed at the main incomer or uh main distribution board while 30 mAh RCCB are installed downstream on final circuit such as sockets, bathroom, kitchens and other outdoor circuits where people may come in direct contact with electrical equipment.

[00:14:24]Now let's discuss one more parameter required for RCBO selection which is braking capacity. This we use only in RCBO. Since RCBO also provide short circuit protection, it breaking capacity must be higher than the available fault current at the installation point. For example, if the fault current at the distribution board is 4 kilo, then we can select uh 6 kiloo RCBO. So the final selection become 25 aere this is the current rating 30 miampere this is the leakage current rating and 6 kilo this is the braking capacity this we use only for RCBO. So this rating of RCBO we we can select for our example for the kitchen uh switch socket load. I just clear one more thing maybe you have doubt why we use uh fault current only 4 kilo ampere when the transformer fault current is uh much higher. The reason is that as we move from the transformer, cable resistance and impedance increases which reduce the available fault current. That is why the fault current at the distribution board is usually lower than the transformer fault current. So if you want to learn how to calculate short circuit current at DB level step by step, I have already made a detailed video on this topic. You can find the link in the description or you can visit my channel. So this was all about RCCB and RCBO. I hope this video helped you understand their working differences and selection method. If you have any questions, feel free to ask in the comment section. If you found this video useful, please like and share it.

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