RCD Research

RCDs pass the test of time … but are some consumers still at risk?

Since its inception, the Council has been raising awareness of the benefits of residual current devices (RCDs), and encouraging their use in the home. In parallel with this campaign, we have been reporting on the progress of our research into the long term reliability of RCDs installed in domestic premises.

Now that ERA has completed its study and analysis of the results of the in-situ tests, we take a closer look at their final report and how the key findings of the research will steer our consumer awareness activities.

The need for research

Although it is widely accepted that RCDs reduce the risk of death and injury in particular circumstances, lingering doubts remained in some quarters as to their long term in-service reliability.

The reliability of RCDs, particularly after prolonged inactivity, had been brought into question by studies undertaken in other countries. However, until now, there has been no published research carried out in the UK. In order to provide meaningful data for the UK and to help understand why RCDs might fail to operate in service, we commissioned ERA Technology to undertake a thorough investigation for us.

Initial studies of available literature and research published in other countries revealed that there had been a number of research projects involving the testing of RCDs, all with findings similar to those first published in Italy. Details of those findings were published in the autumn 2006 issue of Switched On, and can still be viewed on our website.

Whilst the research conducted in other countries helped us to better understand the mechanisms by which RCDs may fail to operate, it is important to recognize that the results were predominantly based on tests carried out in the early 1990s.
Consequently, the findings are unlikely to reflect improvements in RCD design and manufacture that have been introduced through subsequent revisions of the relevant product standards. For devices manufactured since 1996, these include a 28 day environmental type test to simulate ageing, and EMC tests.

On-site testing

Our tests were carried out on a total of 607 RCDs in properties owned by four Housing Associations and Local Housing Authorities in the UK. All the devices tested were of the electromechanical (voltage independent) type.

The tests were made on the load side of the RCD, as close as practicable to the device. Occupiers were asked to turn off all appliances such as washing machines, TVs and computers for the duration of the test.

Although it could not be guaranteed that all loads had been disconnected prior to the RCD tests, there was no indication from subsequent analysis of the results that the tests would have been affected by any capacitive loads that had inadvertently remained energized.
The initial test for the first 172 devices was carried out in alternate properties at one times (30 mA) and five times (150 mA) the rated operating current. This was to determine whether or not the magnitude of the initial test current had any significant effect on the failure rate.

For the remainder of the tests, each RCD was tested at the rated operating current of the RCD (30 mA) three times during the positive and negative half cycles of the supply waveform. The tests were then repeated at five times the rated operating current (150 mA).
Functional testing of the integral push button was carried out last.

Failure methodology

For the purpose of the research, RCDs that did not disconnect the supply within the maximum permitted operating time, irrespective of the likely cause, were deemed to have failed. This included those that failed to operate at all.
This enabled the analysis to focus on the most important function of the RCDs, being the provision of additional protection against electric shock.

During the on-site testing, a number of RCDs were found to have been deliberately shorted out by the installation of additional wiring across their terminals, rendering them totally ineffective. These RCDs were also judged as a test failure but, for clarity of analysis, the results have been presented with and without the shorted RCDs included in the failure statistics.
In some cases it was found that RCDs could not be reset after a test. These RCDs were replaced but were not included in the failure statistics.

Similarly, in some cases, RCDs operated correctly when tested at 30mA and 150mA but the test buttons did not function. Again, although these RCDs were replaced, they were not included in the failure analysis because they did not present an electric shock risk.

Results and analysis

Of the 607 RCDs tested, a total of 23 failures were recorded, giving an overall failure rate of:

• 3.8% - including the six RCDs that had been shorted out.
• 2.8% - excluding the shorted out RCDs.

From discussions with occupiers, the RCDs that had been shorted out had been tripping for no apparent reason. An electrician, or electricians, from the Housing Authority had stopped this “nuisance tripping” by shorting out the problem RCDs – a dangerous practice. These RCDs were not returned to ERA for examination.

Of the ten RCDs returned to ERA, investigation in most cases suggested a possible cause of the on-site failures, but the cause could not always be determined with a high degree of certainty.

This was because, in some cases, the RCDs operated correctly when tested at ERA, perhaps due to the mechanical effects of their removal and transportation.

Also, the action of opening the RCD enclosures to enable areas such as the main contact surfaces and the moving components to be examined could have disturbed dust or debris, potentially obscuring some of the visual evidence.

ERA concluded that the most likely causes of failure were:

• Deliberate shorting out
• Ingress of moisture and contaminants
• Component misalignment
• Disruption of contact surfaces causing contact welding

One of the principal conclusions for this research is that the regular operation of the test button does have a positive effect on the overall reliability of an RCD when subjected to earth fault conditions.

This is because failures that had not been evident until the research was carried out would have been identified earlier had the integral test button been regularly operated by the householder. If these defective RCDs had been replaced, it is logical to expect that the overall reliability rate would have been improved.

Furthermore, from published papers and anecdotal evidence, there is a belief that RCD operation can be effected by ingress of contamination causing “sticktion” between the moving parts of an RCD.

If this was the case, it would be expected that if the RCD was tested three times in succession, it would operate slowest on the first test and fastest on the last test. However, ERA’s analysis of average operating times found no evidence to suggest that repeated operation improves the operating time of RCDs.

Therefore, it appears unlikely that the performance of RCDs that are correctly selected and installed are adversely affected by prolonged inactivity.

Manufacturers of those RCDs for which the cause of failure was considered most likely to be a result of design or manufacturing defects have been invited to comment on the findings of the final report.

ERA’s full report detailing the results of their research and their conclusions can be viewed on our website: www.esc.org.uk.

Our conclusions

Our research confirms that, overall, RCDs in domestic installations are reliable long term, provided they have been selected and installed correctly, and that the integral test button is operated regularly to confirm whether or not the devices continue to be capable of disconnecting the circuits they are intended to protect.

However, the research also confirmed that consumers are failing to follow manufacturers’ and installers instructions to test the devices regularly. As a result, faults that occur in the devices are likely to remain undetected.

Whilst it is reassuring to note that over 96% of RCDs tested were found to be fully functional and would have provided valuable protection from electric shock caused by electrical faults, it is clear that there is still considerable scope for raising the awareness of householders of the importance of testing their RCDs on a regular basis. We are considering the most effective way this can be achieved.
Operating the test button would have picked up most, if not all, of the failed devices in our investigation, but there may be a number of reasons why householders would be unlikely to test an RCD.

Our research found that, particularly for older installations, RCDs are likely to be installed out of sight in a cupboard or similar location and, consequently, many people will simply forget to test them. Indeed, they may not even be aware that they have RCD protection or that the devices should be tested regularly.

Those having responsibility for the installation of consumer units containing RCDs should ensure that they are readily accessible, and that clear instructions are provided, to encourage consumers to use the test button on a regular basis.

Of the RCDs that failed, six had been deliberately shorted out to prevent so-called ‘nuisance tripping’; a practice strongly condemned by the Council. This is a dangerous practice as a shorted out RCD will not provide the intended protection against electric shock from direct contact.

Part of our ongoing work will be to encourage homeowners experiencing unwanted tripping to seek the advice of a registered electrical contractor to investigate and deal with the underlying cause of the problem, rather than treating the symptom by shorting out the RCD.
Guidance is available to electrical contractors on this issue, such as that in the BEAMA Installation RCD Handbook: Guide to the Selection and Application of Residual Current Devices. A copy of the handbook is included in the Council’s Technical Manual.

We know that the availability of RCD protection is linked to many factors, not only being related to the performance of RCDs themselves but also to the quality of the mains supply, the condition and use of the electrical installation, and the environmental conditions in which RCDs are installed. As a consequence, the underlying causes of failure could be attributed to any one, or a combination of several, of these factors.

We are therefore looking to carry out further research into the installation and environmental factors that may affect the performance of RCDs. It is hoped that this research may also be able to confirm (or otherwise) the premise that certain loads connected to RCDs whilst under test have an adverse effect on test instrument readings rather than the availability of protection.

The revised requirements for electrical installations (BS 7671: 2008, 17th edition) coming into effect this year put significantly greater emphasis on the use of RCDs for additional protection against electric shock, a move we welcome.

We are pleased that, on the whole, the results of our research are very positive. We will therefore be continuing to promote the message that RCDs, appropriately installed and used, can and do save lives.

However, we will also be looking to manufacturers to continue to improve the reliability of their RCDs. A longer term aim will be to encourage them to design out the need for regular testing by users, which experience indicates is rarely carried out in practice.