What is an RCD
RCD stands for “residual current device”, in the most basic terms it is a device that will automatically disconnect the supply from a circuit thus preventing electrocution & fires as a result of a fault or by direct contact of a person or an animal to the source of supply.
It is a misconception that if you were to touch a live terminal or cable protected by an RCD that you wouldn’t receive a shock, you can receive a slight shock but only for an instantaneous duration and of limited current.
Types of RCD
There have been over the last 15 years or so many developments in RCD protective devices, today however there are really only Two that you will probably encounter, these are the straight RCD’s and getting more common place, RCBO’s
Below are some early examples of straight RCD’s
Today’s modern equivalent
As you can see they are basically the same & operate in the same way, just more modern looking & more compact in design.
What is a RCBO
To keep it simple an RCBO is a combination or amalgamation of an RCD and a circuit breaker in One package, it has the functionality of an RCD and a circuit breaker.
A circuit breaker will only trip on overload, either too much current is being drawn from the circuit or there is a fault causing a short circuit, it will NOT automatically disconnect if you or an animal comes into contact with a live conductor or terminal.
The RCBO works just the same as a circuit breaker, will not trip unless the current drawn exceeds its current rating or a short circuit is present. However the RCBO will trip if contact of a live part is made by a person or an animal, so in essence it does the job of both a circuit breaker & an RCD in One unit.
Examples of RCBO’s
How do I test these devices
On all RCD’s & RCBO’s there is a small button normally labelled up as “Test”, if you press this the RDC or RCBO will trip out to the OFF position, this however is only a mechanical test and does not simulate a person or animal coming into contact with a live conductor.
You guessed it there is a tester for this very purpose, not surprisingly called an “RCD Tester”
If you look on the front of most if not all RCD’s & RCBO’s used in domestic/commercial systems it will have printed on the front 30 mA.
This is the rated current that will trip an RCD or RCBO, so in simplistic terms, if the device detects 30 milliamps (That’s 0.03 of an Amp) as there are 1000 mA in 1 Ampere or 1 Amp flowing to Earth then this will cause the RCD or RCBO to trip to the off position, there is also a time constant to consider as the device needs to operate as fast as possible to limit the exposure of contact to a live part, thus reducing the shock received by the person or animal in contact.
What are the tripping times & how to test
The British Standards & the IEE regulations have done all the hard work for us and provided us lucky folks with these values & are part of the regulations that have to be adhered to when commissioning or inspecting installations. These test results are required to be documented on handover certificates to ensure they are functioning correctly & will disconnect as the regulations require.
- 40 ms at 150 mA
- Within 300 ms at rated tripping current (30 mA)
- There are 0.001 seconds in a millisecond
As you can see its quite defined and simple as this is how the IEE regulations have worded the parameters above.
Above are just a couple of examples of RCD testers, again most if not all Multi function Testers (MFT’s) have the RCD test built in, so if you have an MFT you probably wont require a dedicated RCD tester as it will undertake all the same tests required.
There are a few ways to test, some have a dedicated lead with a Three pin plug molded on One end and Three leads that connect to the meter on the other, these are used to test power outlet socket circuits that are protected by either RDC or RCBO.
I again personally don’t like this approach and as mentioned in previous posts have lost this lead along time ago.
Be aware this is a LIVE test and you will be connecting in a consumer unit or distribution board
My Method for undertaking these tests
I will pick an RCD for simplicity, but the RCBO is tested in exactly the same way but possibly harder to describe or illustrate so I will reference to the RCD.
Please ensure the RCD is switched OFF before making the below connections
- One lead is connected to the Earth terminal (normally GREEN test lead) or main Earth bar or Main Earth Terminal (MET)
- The RED lead to the LIVE outgoing terminal of the RCD under test
- The BLACK lead to the Neutral outgoing terminal of the RCD under test
- The first test to perform is the NO TRIP test, this injects a fault current of 15 mA (half the rated trip current of the RCD or RCBO)
- To undertake this test set the tester to 15 mA and switch on the RCD, press the test button on the tester and this test will terminate after 2 seconds, the RCD should not trip to off during this test. This is ensuring that some current can and does naturally leak back to Earth and wont cause the RCD to trip to off, the reason is some appliances like fridges naturally leak a few mA to Earth and you don’t want this to turn the power off and in turn stop the fridge from working, this is called nuisance tripping.
- Set the test meter to 30 mA and press the test button, the RCD should trip to off, the test meter will indicate a reading in Milliseconds, the reading indicates the time it took the RCD to trip to off. From the above this figure has to be below 300 ms
- Set the test meter to 150 mA (5 times its rated tripping current) again test as above and note the reading in ms, this has to trip the RCD to off within 40 ms. The reason for the 5 X test is that 100 mA has the potential to stop a human heart so the tripping times are greatly reduced thus limiting exposure in the event of contact.
- There is One other test I always perform & is not a requirement of BS 7671 but can really help in identifying RCD’s or RCBO’s that do nuisance trip. This test is called a ramp test, basically it ramps up the current in mA incrementally until the RCD or RCBO trips to off, this time the reading is in mA & not a time value. From this you can see what current level actually trips the RCD or RCBO, this is handy to know. From the 1st test at 1/2 the rated test current (15 mA) the RCD or RCBO should not trip, and if it didn’t then this fulfills the requirements of BS 7671, but on a ramp test it may be tripping the RCD or RCBO to the off position at 16 mA, that’s 1 mA over the rated no trip value, not much of a difference but this will lead to nuisance tripping for sure, and this is a sure fire way of finding little potential issues such as nuisance tripping RCD’s or RCBO’s
There is just one other point to make, all RCD testers will have a function button to select which part of the AC waveform is used for testing the RCD. The above Three tests should be tested at both points on the AC supply, usually 0 degrees & 180 degrees of the AC waveform, this ensures that the RCD/RCBO will trip anywhere on the AC cycle.
Your manual will give more information on this and how to select this functionality, it really is not that difficult as you have to repeat each step twice, meaning 6 tests in all excluding the ramp test, again I also do this for both points on the AC waveform when undertaking the ramp tests, so I do this twice also.
There are also socket outlets that have RCD’s built in, usually easy to spot as they have a test button & a reset button on the face plate, these can be tested with the 3 pin socket lead that normally comes with the tester, for me its about the only time I would have a use for this, but I use a plug top tester that I connect my test leads to instead, does the same job !
Please be safe people, if you are in any doubt about working on electrical apparatus then please DONT ! Please obtain some expert training & guidance before attempting any work or testing on any electrical apparatus or systems.
Remember 100 mA can kill that’s a tenth of One Amp, when put in perspective your average domestic lighting circuit is protected by a 6 Amp breaker or 5 Amp fuse and can run cables of 1.0 mm in diameter (the smallest one in the board usually).
I have over the years cut through these, it does go with a loud bang, usually accompanied by a change of underwear and a bloody big chunk missing from your cable cutters, most are made of high quality hardened steel so if you are unlucky enough to experience this look at the damage it can do to hardened steel afterwards, then you will realize just what 6 Amps is capable of. Humans are not made of steel and will fair far worse.