An RCD which keeps tripping is sometimes referred to as “nuisance tripping”, this is probably because to the end user the result of the power constantly tripping could be a nuisance to them but in reality an RCD tripping is likely preventing serious damage and or injury.
There are many possible causes of an RCD tripping and will require some investigating to find the root cause, sometimes the cause could be more than one issue.
What is an RCD ?
An RCD (Residual Current Device) is an electrical safety device which is normally installed within a consumer unit or other suitable enclosure and is available in many ratings (A) and sensitivities (mA), the rating (A) must be selected to be suitable to carry the required design current, the sensitivity (mA) of the RCD will depend on what the desired function of the RCD is to be used for, BS 7671 sets out some min/max mA ratings.
An RCD also incorporates an integral test button normally identified as “T”, the test button should be pressed at regular intervals in accordance with the manufactures instructions, the wiring regulations also include information on the frequency of testing the T test button of RCD’s. If the RCD does not trip when the test button is pressed seek expert advise from an Electrician without delay.
Why does an RCD trip?
An RCD will trip when an electrical fault is detected, the fault could be in the form of a faulty appliance, damaged cable, wiring fault or some thing else. RCD’s are sensitive safety devices which must NEVER be bypassed, if an RCD trips it is carrying out the function it is designed to do in the event of a fault to reduce the risk of fire, electric shock, serious injury or even death to persons, livestock or property.
A tripping RCD should never be ignored, RCD’s do not trip for no reason and could indicate a serious issue which needs urgent attention.
Intermittent Tripping RCD
An RCD which trips intermittently can be a difficult fault to diagnose and can be very time consuming to locate the cause(s) and it may take multiple fault finding sessions at varying times of the day over a number of days to catch the fault at a time when the RCD is actually tripping.
If at the time of carrying out the investigation and fault finding works the RCD is operating normally then it is unlikely the fault will be found at that time hence further investigation maybe required.
RCD trips at the same time every day
If an RCD trips at the same time every day this is a good indication of a faulty appliance or connected electrical equipment that is programmed to switch on at the same time each day, such as the central heating, an immersion heater or timed lighting.
Random RCD Tripping
An RCD that trips every 5 / 10 minutes or every so many hours can be an indication of damaged wiring caused by rodents such as rats or squirrels. If rodents have damaged any electrical wiring the damaged wiring will require replacing, it is worth noting that the rodents point of entry should be identified and rectified to prevent the rodent from damaging the new wiring.
Depending on the type of the installation the electrical system may require a complete overhaul if the current type of wiring and installation methods adopted are not suitable for the type and nature of the installation.
Electrical cables damaged by rodents is a serious immediate fire risk, if you find or suspect any electrical cables damaged by rodents isolate the electrical supply immediately and contact an Electrician.
Fault Finding Tripping RCD
- Identify the cause
- Rectify the cause
Identifying Circuit RCD Fault
An RCD within a consumer unit may cover a bank of circuits (more than one), in this case we need to identify which circuit is causing the RCD to trip.
To identify which circuit is at fault first turn off the RCD and all associated MCB’s (miniature circuit breakers), next turn on the RCD and assuming the RCD stays on then one by one turn back on each MCB in turn.
The RCD is likely to trip at the point when you turn on the faulty circuit, at that point repeat the above process but this time leave the MCB which caused the RCD to trip switched off, the reason for this is because there might be another fault on another circuit.
Once you have identified the faulty circuits move to step 1 “Faulty Appliance” below.
Possible Causes of RCD Tripping
As mentioned above there are multiple possible causes of RCD’s tripping, here we look at a number of the most common.
1) Faulty Appliance
The first thing to check when an RCD keeps tripping are the appliances and connected electrical equipment. Unplug every plug, switch off every isolator and turn off every switched fused spur, in some cases you may also need to turn of every light switch and extractor fan isolator, remember to check the immersion heater, central heating and switched fused spurs in the kitchen are all switched off.
Once you are certain every appliance is unplugged and all isolators are in the off position try to reset the RCD.
– RCD now stays on:
> Should the RCD reset and holds after a period of time it is likely that one of the pieces of electrical equipment was at fault.
– RCD still trips:
> Further investigation is required.
2) Wiring Fault
A wiring fault could be a damaged cable, loose connection or an incorrectly connected or wired electrical accessory.
To identify a circuit fault a series of electrical testing will be required, one of these tests would be an insulation resistance test, this test is best carried out with the electrical circuit completely disconnected from the consumer unit / distribution board with all appliances and electrical equipment switched off.
If the circuit is a ring main circuit an r1+r1, r2+r2 and rn+rn test should also be carried out, see the IET Guidance Note 3 for information on testing procedures, ensure this is the latest current version.
3) Excessive Earth Leakage
Most commonly installed RCD’s are generally rated at 30mA, this means that should a circuit earth leakage current reach the threshold of the RCD the RCD will likely trip, the exact mA that causes a 30mA RCD to trip may vary from manufacture to manufacture.
Electronic equipment such as computers or certain power supplies may have a small amount of earth leakage current by design, which on it’s own is not normally an issue, however if a lot of electronic equipment is covered by the same RCD then the cumulative earth leakage of all of the equipment or appliances added together may exceed the threshold of the RCD.
If excessive earth leakage current is caused by having too many appliances or electrical equipment covered by a single RCD or RCBO then additional circuits might be needed to shed some of the load to separate RCD’s, there are multiple solutions for excessive earth leakage current which would be worth considering.
4) Faulty RCD
A faulty RCD can be the cause of an RCD tripping, however this is generally the least likely cause. If an electrician has deemed an RCD to be faulty then in most cases the RCD can be replaced. In older consumer units or fuse boxes the correct RCD may be obsolete therefore may require a complete consumer unit replacement.