RCD (Residual current device) will only trip on a fault between live conductors and the earth. It will not trip when there is a fault between live conductors or when there is an overload.
It is rare these days for RCDs to cause nuisance tripping. Usually, when an RCD trips it is because something is wrong. It is unlikely to be the RCD that is causing the problem.
RCD nuisance tripping solutions
RCD nuisance tripping causes and solutions are the following:
1. Check the neutral
Where the RCD is part of a split board and is a new connection, it is always worth checking that the neutrals of the circuits are all connected to the correct neutral terminal. In other words, check that all of the circuits that are protected by the RCD that keep tripping are connected to the neutral terminal corresponding to the side of the board that the RCD is protecting.
If a circuit neutral is connected wrongly, and a load is connected to the circuit, the RCD will detect more current in the neutral than the line conductor, as the neutral conductor and line conductor pass through it and trip.
2. Carry out a ramp test
Where everything seems to be connected correctly, it is worth carrying out what is called a ramp test before starting to take things apart. A ramp test will tell you how much current is passing through the test instrument before the RCD trips. Clearly, if a 30mA RCD trips at a current of, say, 10mA it could be prone to nuisance tripping.
The problem with this test is that if it is carried out on a circuit while all of the other circuits on the board are still connected, any small earth fault currents on other circuits will have an effect on the RCD. Although the test instrument is showing 10mA it is only measuring the current that is passing through the test instrument. It will not be able to measure any leakage current that is in the rest of the system.
Before you change the RCD, isolate it from all of the other circuits and test it on its own. If it still trips at 10mA, then change it. As a precaution I would always test the rest of the circuits on the board between live conductors and earth to check if there are any low readings, just to satisfy my own mind that I have cured the fault.
3. Ask the right questions to the users
Where the RCD is part of an older circuit that has been working for some time but has just started tripping, it is unlikely to be the RCD causing the problem. Once again, the very first thing I would do is gain as much information about the fault as possible. Ask whoever is using the installation questions like these will usually give you some clues as to what the problem is.
Always remember that many people have no idea at all of the ways any electrical systems work. Don’t be embarrassed about asking the obvious, even if the person seems to know what they are talking about.
- How often does it trip?
- Did it trip just once and won’t reset?
- Did it reset a few times before it began not resetting at all?
- Does it trip when you plug something in or switch a particular part of the circuit on?
- Has any work been done around the house: shelves put up, floorboards screwed down?
- Have any light fittings or switches been changed?
- Have you plugged in anything new recently?
- Has there been a water leak anywhere recently?
Once you have asked the questions, then it’s time to have a bit of a poke around.
4. Detect the circuit
The first objective is to try and detect the circuit that is causing the problem. Of course, if the board has RCBOs then the circuit causing the problem will be obvious. Always remember, though, that most RCBOs are single pole, and when you switch them off or they trip, the circuit may be off but if the fault is between neutral and earth the fault will still be there.
Where the circuit is part of a system that is protected by a single RCD or even part of a split board, then the problem circuit will not be obvious.
Switch off all of the circuits that are protected by the problem RCD. Now switch the RCD on. If it stays on, then turn on the circuit breaker nearest the RCD. If it stays on, leave it on and switch on to the next one. Keep doing this until the RCD trips. With a bit of luck, the circuit breaker that you switched on just before the RCD tripped will be for the problem circuit.
However, don’t get overexcited as the problem may be a build-up of small leakages over several circuits. This is simple to check – just turn all of the circuit breakers off and switch on the RCD. Now switch on the circuit breaker that caused the RCD to trip; if the RCD trips again you have found your problem circuit.
On the rare occasion when the RCD does not trip it may mean that more than one circuit is involved. Starting with the circuit breaker nearest to the one that you have switched on, switch the circuit breakers on one at a time until the RCD trips.
Now turn off all the circuit breakers other than the first and last ones that you switched on and try to reset the RCD. If it resets, then switch on another breaker. Keep repeating this process until you have found the combination of circuits that trips the RCD and those circuits which don’t.
Disconnect the live conductors of the problem circuits one circuit at a time (not the cpcs). Now carry out an insulation test between the live conductors joined together and the earth bar. This will show you which are the problematic circuits.
Where you find it is only one circuit causing the RCD to trip, things are slightly easier.
The RCD is acting as the main switch and it trips but will not reset.
- Switch off all circuit breakers or remove all fuses.
- Reset the RCD.
- If it resets, switch on the circuit breakers or replace fuses one at a time. When the RCD trips you have found the problem circuit.
- If the RCD does not reset, this could indicate an N to E fault.
- Remove the neutrals one at a time; each time one is removed, try and reset the RCD. If the RCD does not reset, do not reconnect the N but disconnect another one.
- Once the RCD resets, mark the last N disconnected for identification purposes.
- Reconnect the other neutrals one at a time, ensuring that each time you reconnect a neutral the RCD is switched off for isolation purposes.
- The fault will be on the circuit where the identified N is used.
- If all of the neutrals are disconnected and all of the circuit breakers are turned off the RCD will still not be set, it will indicate that the RCD is faulty. However, it is always a good idea to carry out a test on the RCD with nothing connected to it just to be sure.
Always be aware that it may not be one circuit that is causing the problem. If, for instance, the RCD has a trip rating of 30mA it could be that a number of circuits have a very small leakage. When all of the circuits are operational the accumulative value of the leakages may well be more than the trip rating of the RCD.
This is often the case where there are a lot of heating elements being used in an installation. It is not unusual for these elements to have very low insulation resistance values, particularly if they have not been used for a while. The metal tubes in which the element is contained can have very small hairline cracks that allow moisture to be absorbed by the insulating material, which is usually magnesium oxide. This is very hygroscopic and readily absorbs moisture.
Heating and cooking equipment with a rating ≥ 3kW are permitted to have an insulation resistance value of 0.3mΩ.
Where a number of heaters are being used, it is often beneficial to run them for a while to dry them out and then test them again.
RCBO trips and will not reset
- Check that neutrals are connected to the correct side of the board if it is a split board.
- If the RCD is protecting a power circuit isolate all loads. If it is protecting a lighting circuit, switch off all lights.
- Try and reset the RCD. If it resets, re-energize loads one at a time. When the RCD trips you will have found your problem. If the RCD is on a lighting circuit, switch the lights on one at a time until the RCD trips. Again, you will have found the problem.
- If the RCD will not set, disconnect the line and neutral of the circuit (do not disconnect the earth) and carry out an insulation resistance test between the joined live conductors and earth.
5. Check the power circuit
Check all socket outlets and unplug anything that is plugged in, switch off/isolate any fixed equipment such as electric fires, and make sure showers, cookers, and the like are isolated from the circuit.
Now switch the circuit back on and hope that the RCD remains on. If it does, then you are well on the way to solving the problem. If it is a socket outlet circuit, leave the circuit on and start plugging things back in. When the RCD trips you have found your problem.
If it is a radial circuit supplying a piece of equipment such as a cooker or shower, then isolate the equipment and test it with an insulation tester between live conductors and earth.
On circuits that trip when everything is disconnected, then clearly there is a problem with the circuit. Now visually inspect as much of the circuit and accessories as you can see to check for anything that looks like it could be a problem. Look for damaged cables/accessories.
Disconnect from the supply end the live conductors only and carry out an insulation test between the live conductors and the main earthing bar. Of course, you are expecting a low reading below, say, 0.008MΩ because this is the value of resistance that will cause a 30mA RCD to trip.
230V / 0,03 A = 7666 Ω
which, of course, is as near to 0.008MΩ as you can get.
But, of course, anything below 1MΩ will require attention.
Remove the accessory at the end of the circuit and check for damage to the conductor insulation/pinched cables, disconnect the accessory and separate the conductor ends. Now carry out the insulation test again.
If the resistance values have increased to an acceptable level, then you have found your problem.
If you have both ends of the cable disconnected, with the conductor ends separated, and are still getting a low reading, then, of course, it is showing you that the cable is damaged. If it is a radial circuit supplying only one item of equipment, the only option is to try and trace the circuit cable and perhaps cut into it around the halfway point and test each way to see if you can narrow down the point of the fault.
If it is a radial or ring final circuit, things become a little easier. Split the circuit somewhere around the center of it. Of course, you can only guess where the center is but anywhere will do to start with.
Make sure the ends of the circuit are separated at the supply point and at the point at which you are beginning your search.
Now test between live conductors and the cpc on all of the cables where you have split the circuit. One of them should provide you with good reading and the other with a low reading. At the supply point test between live conductors and earth. If the reading is still low, then the fault is between the separated point and the supply point. If the reading is clear, then the fault is between the separated point and the end of the circuit.
Now split the circuit again between the two ends of the circuit that has the fault and repeat the process as before. Keep doing this until you have pinpointed the section of cable that has the problem. Once you have found the section of cable it is just a matter of trying to find the problem or replacing the cable.
6. Check the lighting circuit
Where possible switch off all of the lights. When you have done that, try and reset the RCD. If it resets just turn the lights on one at a time until the RCD trips. When it trips you will have found the part of the circuit that the fault is on – now you just have to locate the problem.
If the RCD does not reset with all of the lights off, carry out a visual check on everything, paying particular attention to anything that is outside and could have moisture in. If a visual check does not produce a solution, then turn all of the light switches to the off position, disconnect the live conductors at the supply end of the circuit and carry out an insulation test between live conductors and earth. It is worth switching any two-way or intermediate switches just to make sure that they are in the off position. If the reading is satisfactory, say above 1MΩ, then turn each light on one at a time while carrying out the test. When you get a low reading you will have found the problem area.
When the resistance value is low with all of the light switches in the off position, then you must follow the same procedure as for the power circuit. Lighting is often easier to work with than power as splitting the circuit is a simple process.
Split the circuit somewhere near the middle, test both ways, and continue doing that until you have found the part of the cable that is giving the problem.