RCDs -- Everything you need to know about

Residual Current Device is a type of a circuit breaker that cuts off current when it detects an earth fault from live back to the neutral. Ideally, it works as an equalizer that tries to ensure that the current flow is similar across the two pathways.

The wiring installation is monitored permanently by the residual current device to ensure that there is no current leakage. It does this by continuously monitoring the current that passes through the wire in a particular and its opposite direction. The idea is to ensure that it is measured and balanced.


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When the detected current is greater than 30mA the circuit is broken thus ensuring that it is safe. If it detects no current, then it means that the missing current is actually a current leakage, and it might be going through a person’s body and could be fatal.

You need to make sure that every electrical job you do has an RCD as it required by law. And you’d also have to mention this fact on your safety certificate thus ensuring that your customer is safe and out of harm of electricity’s way. You will also be doing your job as a licensed electrical contractor.

The residual current device (RCD) is used to detect earth fault currents throughout your wiring system and it will interrupt in case of a current leakage. Thus it will prevent a fatal electrical accident. In this way, electrical equipment can also be protected against fire. The earth fault currents can range from 5mA to any amperes. The RCDs that are installed in a home typically trip when they detect a current of 30mA. It can be opened and closed manually so that you can switch it back to normal load currents.

The toroidal transformer is an important component of RCD because of the live current (load) and the neutral current (return) conductors are coiled  in different directions. It tends to carry out the detecting winding. In absence of any earth current, the live and neutral currents are equal. The mmfs produced by the two currents are equal. So there is no change in the toroidal and the detecting winding does not generate any current.

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In contrast, when an earth fault current flows, it creates a difference between the live and the neutral currents leading to a change in the toroidal. It ends up producing a current in the detecting winding and leads  to current leakage.

If no earth fault current is flowing, then the load and return currents are equal. If the mmfs generated by the load and return current windings are similar then there is no resultant change in the toroidal and the detecting winding does not generate any current.

The current produced in the detecting winding runs a relay which will open the main contacts of the RCD. It has to produce from a very small output. Which should be sufficient power to operate the tripping mechanism. Two alternative methods can be used here. In the first method, the detecting coil’s output signal is electronically amplified and in the second method, a polarised relay is operated on a sensitive mechanical trip mechanism. This operation is based on the magnetic output generated by a small coil which nullifies the field from a permanent magnet and as a result armature is released.  

What we discussed above is a single-phase operation of an RCD, but it can also function in a three-phase way in a light industrial unit for providing protection against fires. It can be arranged in two ways for a three-phase model. First one is where the three phases are all coiled around a current transformer. The second one is where the three phases, as well as the neutral, are all coiled onto the balancing transformer.

It is important to note here that the RCD comes with a limited breaking capacity and is in no way a replacement for a device like MCB which protects against overcurrents. To fuse the circuit breakers and the RCD together, a new device called RCBO (residual current breaker with overcurrent) is available in the market. It comes with enhanced contacts so that it can cope with the interruptions caused because of fault conditions. It comes with an overcurrent tripping mechanism, too.

IEC 1008 and IEC 1009 set the design and test regulations for the RCDs.

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As per the law introduced in 2009, all the home sellers and lessors need to install RCDs thus protecting all the power and lighting circuits in the property.

RCD is to be installed on the switchboard so that it will monitor the flow of electricity to the complete circuit from the main switchboard. Usually, two RCDs are recommended to be installed - one for the lights and one for the power. However, if the electrical consumption is high, then it is recommended to have multiple RCDs, different ones for different circuits.

RCDs prevent electrocutions and fatal accidents by cutting off the electrical supply if it detects an imbalance in the current. Say, if you were cutting a hole in the wall and accidentally cut into a cable. The RCD will detect this and realise that the current flow between the neutral and the live is not well balanced. So instead of you getting electrocuted by the live wire, the RCD will trip and cut off the current.

It is required by the Australian Standards that the RCDs operate within 300 meters of the domestic property. Thus the heat would be prevented from going into ventricular fibrillation.

All power and lighting circuits of the rental and sale properties in Australia should have RCDs installed. So every time you go on site for an electrical job, make sure to educate your customers about the RCDs and their importance. Check that they have the RCDs installed on their main switchboard. But, if they don’t, then it is your duty as an electrician to make sure that they get one as soon as possible. If they do not then let them know that they are putting themselves and their family at a serious risk of death by electrocution.

All properties constructed since the year 2009 are required to have RCDs installed by the law. Even though older properties don’t have RCDs installed and if they do, it’s limited to one. So again, if you visit an older property for electrical work then you can let them know about the importance of RCDs and ask them if you could install one for them.

When you install two RCDs on your customer’s switchboard, then you are able to divide the property’s circuit in an even fashion. It will ensure that some amount of light and power will remain when one malfunction or fails. It will also minimise the fault operation which results because of the low-level leakage current from the faulty appliances.

If you think that your client is unsure whether their property is in sync with the Western Australia laws, then you can try to book them in for an electrical inspection or an electrical health report. It would ensure that they and their family are safe and that your work is compliant with the law.


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RCDs -- Everything you need to know about