RCDs or Residual Current Devices
- RCD stands for
Residual Current Device
- RCDs are protective devices designed to monitor the balance of current flow in the line and neutral wires (brown and blue, or in older systems red and black).
- RCDs are used to protect one or more circuits. They are designed to
trip when something causes an imbalance between the live conductors (wires) in one of the circuits the RCD is protecting. This could be due to a loose live wire making contact with the earth wire, accidental damage, moisture, etc.
- A faulty appliance such as kettle or iron could be a cause of nuisance tripping too, or there may be water getting into an outdoor security light and causing a short circuit.
- An RCD should trip within a split second of the fault occurring, and disconnect the electric supply to protect you from a potentially fatal electric shock. Your electrician should be able to test the disconnection times on your RCDs in conjunction with the resistance values of each circuit to ensure that in the event of a fault, automatic disconnection of supply (ADS) occurs within the times specified in the wiring regulations (BS 7671)
- RCD are not designed to protect against overload. That is the job of the MCBs.
- RCDs are typically found protecting a number of circuits which themselves are protected by MCBs. However, there are many domestic properties which still have circuits protected by MCBs but not RCDs.
- An example is shown below, where there are several MCBs to the left which are are protected by the RCD to their right.
- You will also notice from the illustration above that the RDC has a little blue test button which should be tested at least once every 6 months.
I have a separate page to explain how this RCD test button should be operated.
What is the difference between a Type AC and Type A RCD?
- I am only going to explain this at high level with a view that a customer may be told by their electrician that their Type AC RCD needs to be replaced with a Type A RCD, and this might help you to understand why.
- Referring to the image below which shows a Type AC RCD on the left and a Type A RCD on the right, most domestic properties which had RCDs installed until just a few years ago are likely to have had a Type AC RCD installed.
These were perfectly adequate for most applications, and worked by monitoring the Line and Neutral paths, and responding to any imbalance due to a fault by tripping and disconnecting the circuit.
- However, with more appliances and equipment such as some washing machines, electric vehicle chargers, solar panels, etc., being prone to leaking DC current into the AC circuit, this can effectively disable a Type AC RCD.
- What does that mean in real life?
Well as RCDs are there to protect against faults such as short circuits and other potentially dangerous instances, if such a fault occurs and the RCD doesn't work, then the potential for an electric shock (or worse) will still be there. That is dangerous and potentially lethal.
- So what can we do about it? For the above reason, more electricians are fitting Type A RCDs instead of the older Type AC RCD. A Type A RCD is designed to handle faults even when there is pulsating DC current present which would cause a Type AC RCD to stop working.
- My recommendation to you if you are having a new consumer unit installed with RCDs is to insist that the electrical installs Type A RCDs. There is hardly any difference in the cost, so it makes complete sense to use the safer of the two options.
Portable RCDs - Plugs and Adaptors
- Whilst the illustrations above and many of the comments are aimed at RCDs which are found in the consumer unit, you can buy other kinds of plug-in RCDs, or you might find that you have one already fitted to your mains extension lead.
- Two off the shelf options illustrated below cater for either attachment to the mains lead of an appliance, in which case it replaces the standard mains plug which would normally be fitted. The other option serves as an adaptor, which is designed to be plugged into a mains socket outlet, and then your appliance can be plugged into the front of the RCD body.
- Obviously the fuse ratings which I have described elsewhere need to be observed. So if these RCDs are rated at 13A maximum, they must not be overloaded.
- For more information and prices, I have provided these links to RCD Adaptors and RCD Plugs
which are available at Screwfix and other outlets.
- Other devices typically found in a domestic consumer unit include MCBs and RCBOs.