What is an RCBO and How Does it Work? 1. Introduction 2. When to use an RCBO Introduction RCBO stands for 'Residual Current Breaker with Over-Current'. As the name suggests it protects against two types of fault and in essence combines the functionality of an MCB and RCD. Let us first remind ourselves of those two faults: 1. 2. Residual Current, or Earth Leakage - Occurs when there is an accidental break in a circuit through poor electrical wiring or DIY accidents such as drilling through a cable when mounting a picture hook or cutting through a cable with the lawn mower. In this instance the electricity must go somewhere and choosing the easiest route travels through the lawnmower or drill to the human causing electric shock. Over-Current takes two forms: 1. Overload - Occurs when too many devices are in use on the circuit, drawing an amount of power which exceeds the capacity of the cable. 2. Short Circuit - Occurs when there is a direct connection between the live and neutral conductors. Without the resistance provided by the normal circuit integrity, electrical current rushes around the circuit in a loop and multiplies the amperage by many thousand times in just milliseconds and is considerably more dangerous than Overload. Whereas an RCD is designed solely to protect against earth leakage and an MCB protects only against overcurrent, an RCBO protects against both types of fault. When to use an RCBO In order to reduce the number of injuries caused by electric shock, the 17th Edition Wiring Regulations state that all circuits must be protected from earth leakage. In many domestic situations, this is achieved by the use of an RCD which protects a bank of circuits simultaneously. Our image shows two banks of green MCBs (which protect circuits from over-current), under the earth leakage protection of the 2 RCDs, coloured blue. Whilst this is a cost effective and still popular solution it has a considerable drawback as an earth leakage fault on one of the circuits will knock out the power to all the other circuits on that RCD. An example of 'nuisance tripping', this is a serious problem if the other circuits distribute power to your fridge freezer, security system, fire alarm panel or tropical fish tank for example. Such circuits are often referred to as 'mission critical circuits' though what constitutes 'mission critical' of course depends on the home owner. An RCBO, which does the job of both an RCD and MCB, thus prevents this type of nuisance tripping and should be used on mission critical circuits. Our image shows a main switch consumer unit, populated exclusively with RCBOs. Many electricians consider this the best circuit protection solution as every single circuit is independently protected from both over-current and earth leakage. This image shows a high integrity consumer unit which has three neutral bars and thus gives three banks of circuits. Allowing for two RCD protected banks of MCBs and a bank of RCBOs they offer total circuit separation for mission critical circuits and cost effective protection for standard circuits. High Integrity consumer units are becoming increasingly popular with electricians because they offer highly flexible circuit design - and homeowners because they offer excellent protection without breaking the bank. They are particularly good in projects requiring 12 or more circuits. MCB, MCCB, RCCB, and ELCB are circuit breakers but all of them are designed for serving a specific purpose. Before knowing about the difference among all, let’s get to know about them a little bit. MCB MCB stands for Miniature Circuit Breaker. It automatically switches OFF electrical circuit during any abnormal condition in the electrical network such as overload & short circuitconditions. However, fuse may sense these conditions but it has to be replaced though MCB can be reset. The MCB is an electromechanical device which guards the electric wires &electrical load from overcurrent so as to avoid any kind of fire or electrical hazards. Handling MCB is quite safer and it quickly restores the supply.When it comes to house applications, MCB is the most preferred choice for overload and short circuit protection. MCB can be reset very fast & don’t have any maintenance cost. MCB works on bi-metal respective principle which provides protection against overload current&solenoidshort circuit current. MCCB MCCB stands for Molded Case Circuit Breaker. It is another type of electrical protection device which is used when load current exceeds the limit of a miniature circuit breaker. The MCCB provides protection against overload, short circuit faults and is also used for switching the circuits. It can be used for higher current rating and fault level even in domestic applications. The wide current ratings and high breaking capacity in MCCB find their use in industrial applications. MCCB can be used for protection of capacitor bank, generator protection and main electric feeder distribution. It offers adequate protection whenever an application requires discrimination, adjustable overload setting or earth fault protection. Difference between MCB and MCCB S.No MCB MCCB 1 It stands for Miniature Circuit Breaker. It stands for Molded Case Circuit Breaker. 2 Rated current not more than 125 Ampere. Rated Current up to 1600A 3 Its interrupting current rating is under 10KA Their interrupting current ranges from around 10KA -85KA 4 Judging from their power capacities, MCB is mainly used for low Breaking capacity requirement mainly domestic. MCCB is mainly used for both low and high Breaking capacity requirements mainly industrial. 5 Its trip characteristics are normally not adjustable since they basically cater to low circuits. Its trip current may be fixed as well as adjustable for overload and magnetic setting. RCCB RCCB stands for Residual Current Circuit Breaker. This residual current device is basically an electrical wiring device that disconnects the circuit whenever there is leakage of current flow through the Human body or the current is not balanced between the phaseconductor. It is the safest device to detect and trip against electrical leakage currents, thus ensure protection against electric shock caused by direct contacts. RCCB is generally used in series with an MCB which protects them from over current and short circuit current. Both phase and neutral wires are connected through a RCCB device. These are an extremely effective form of shock protection&widely used for protection from a leakage current of 30,100 & 300mA. It is essential lifesaving equipment used to protect the human body from electrical and is mandatory in many states for domestic installation. ELCB ELCB stands for Earth Leakage Circuit Breaker. They have the same function as RCCB but are voltage sensor devise. However, this is an old technology & is not in common use. RCCB being a current sensitive device have a better advantage over ELCB as under: Difference between RCCB and ELCB S.No RCCB ELCB 1. RCCB refers to ear stands for Residual Current Circuit Breaker. ELCB stands for Electric Leakage Circuit Breaker. 2. It is a new name and refers to current operated devices. ELCB refers to voltage operated earth leakage device. 3 It ensures 100% detection of leakage current& is available to sense the AC as well as DC leakage current. It is not preferable as it can only detect current that flow back through the main earth wire. 4 RCCB has no connection with the earth wire and that’s why it can trip when both currents (phase and neutral) are different and it withstands up to both the currents are same. ELCB is working based on Earth leakage current. These devices measured the voltage on the earth conductor; if this voltage was not zero this indicated a current leakage to earth.
