RCD – Residual Current Device
RCD is a protection device designed to detect earth faults or leakage faults. RCDs operate by monitoring the current flowing into and out of a circuit. In a healthy circuit, the current supplied by the active wire should be equal to the current returning in the neutral wire. The magnetic fields produced are equal and opposite and cancel each other out. When there is an earth fault, the difference between active and neutral current is no longer zero. This difference in the currents is called residual current, which is also the leakage current flowing through the earthing conductor. The resultant magnetic field due to this residual current is called the residual magnetic field, which is detected by the ‘toroidal’ magnetic core of the RCD. When a residual magnetic field is detected, the RCD disconnects all live conductors from the supply.
RCBO – Residual Current Circuit Breaker with overcurrent protection
RCBO is a device that combines the functions of an RCD and MCB. That means it protects the circuit from earth fault, overload and short circuit currents. Bimetallic contact and short-circuit tripping coils are used for overload and short-circuit protection. For earth fault detection, CBCT or Core Balanced Current Transformer is used. The active and neutral are passed through CBCT.
In the above image, one can see that RCBO is rated for 16A and it has B-type characteristics. No more than 30mA residual current can pass through this type of RCBO. Like an MCB, RCBO also has characteristics of type A, B, C, and D. Type C MCB has a C – curve characteristics.
Difference between RCD and RCBO
An RCD is a device installed at the origin of the electrical installation and offers protection to the entire installation. There could be one RCD protecting 3 circuits as shown in the below image A.
On the other hand, an RCBO combines features of MCB and RCD. It provides individual circuit protection without affecting other circuits. RCBOs have come down in price and most electrical installations have RCBOs due to the added benefit of individual circuit protection. See image B. This is particularly handy because power failure in one part of the circuit does not affect another healthy circuit. In the case of an RCD-protected installation (image A) with multiple CBs protecting individual circuits an earth leakage fault in one circuit would trip the RCD which would disconnect power to the whole house.
Consider a scenario where you have 2 lighting circuits and one power circuit controlled by one RCD. A leakage fault in the power circuit would cause the RCD to trip, which in turn would disconnect both lighting circuits. If this event occurred at night, it would cause considerable inconvenience or safety concerns if older people were occupying the property.
Case Study –
In the above example, an Oven with a maximum demand of 34.8Amps is connected to a 40A RCD with a 6mm2 cable. Is this installation compliant with AS 3000:2018? See below image
The answer is no because it’s connected to an RCD by itself instead of an RCD and MCB combination. The oven circuit is protected from residual current against touch voltages but the cable is not protected as the RCD could allow a flow of over 40 A through the cable without tripping.
For residential electrical installation in Australia, electricians can refer to AS 3000:2018 clause 7.9.3
In the below image, a ‘3-phase RCBO’ has a rating of C20, where 20 is the current rating and the letter ‘C’ is a type of circuit breaker. This combination of RCD and CB is called an RCBO, which is compliant. On the other hand, it can be seen that the orange 40A RCD, by itself, is not an MCB or an RCBO. This means the orange 40A RCD is non-compliant based on the image. Further details on this RCD will be required to ascertain its compliance. This information can be found in the datasheet or user manual of the RCD.
MCBs usually have ratings with A, B, C, and D-type that identify the overload current based on the type of load. For example, a type C MCB is designed to trip at 7.5 times the rated current, which is 75A for a 10A MCB. For more information on the type of MCBs, read the blog on ‘Type of MCBs’
Another important point to note is that the RCD is of Type AC, which is non-compliant in Australia.
In the context of electrical safety, understanding the nuances between Residual Current Devices (RCDs) and Residual Current Circuit Breakers with Overcurrent Protection (RCBOs) is crucial. This blog post not only shed light on the differences between these protective devices but also emphasised the significance of having such in-depth knowledge for electrical safety. Tailored for those seeking excellence in the electrical industry, AUSINET, a specialised electrical RTO offers training with courses such as:
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1. Qualified Technical Person – QTP Course
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Published on: 08/01/2024