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Forget the Zs test, just hope your RCD works!


Ok, I mentioned a little while ago that the NICEIC have been advising that Zs testing is unnecessary, on final circuits that have RCD protection. The guidance given is to carry out an RCD test, and enter N/A in the column for Zs readings on the schedule of test results.

This has been the catalyst to quite a bit of reaction and discussion, which is great! I firmly believe that electricians should be encouraged at all times to challenge, discuss and consider all electrical issues, whether it be changes or amendments to BS7671, or advice given from a non-regulatory body, as in this case.

So, the article I am referring to came from the March 2019 issue of 'Professional Electrician & Installer', which is a free magazine available generally from Electrical Wholesalers. The article is entitled ' Verification of ADS', and is presented by the NICEIC generally, without any individual named as author, and finishes with an advert for their 'Guide to Inspection, Testing and Certification'.

There is a reasonable explanation of ADS and how Zs testing is used to verify everything is hunky dory. The article continues with the alternative method of calculating of Zs instead of testing.

Then comes the tricky bit - 'Verification of ADS by verifying the effectiveness of the RCD'.

"Where an RCD is installed upstream of the overcurrent protective device ...." (i,e in a split load RCD domestic consumer unit) .. " Regulation 643.7.1 permits a different approach to be used to verify ADS ..... where continuity of the protective conductors is verified as required by Regulation 643.2.1, ADS for the final circuits connected downstream of an RCD can be confirmed by verifying the effectiveness of the RCD".

The article then continues by stating that the RCD test should be conducted "at the most convenient place downstream of the device, such as a socket outlet", and the measured reading for each RCD recorded.

"For the column entitled 'Maximum measured earth fault loop impedance' not applicable 'N/A' should be recorded".

It concludes by stating that under the conditions outlined above "undertaking live testing to determine the earth fault loop impedance of the circuit is unnecessary".

So, lets pick this apart a bit and see what is being suggested. What exactly does Regulation 643.7.1 state, and, perhaps more importantly, how should it be interpreted?

643.7.1 - General

The verification of the effectiveness of the measures for fault protection by automatic disconnection of the supply is effected as follows:

a]. TN system Compliance with Regulation 411.4 shall be verified by;

1/. measurement of the earth fault loop impedance (see Regulation 643.7.3)

2/. verification of the characteristics and/or the effectiveness of the associated protective device. This verification shall be made;

- for overcurrent protective devices, by visual inspection or other appropriate methods ...

- for RCDs, by visual inspection and testing.

Then just to confuse the issue the following paragraph states .... "the effectiveness of ADS by RCD shall be verified by using suitable test equipment..."

Interpretation ! Well, it is clear that for fuses and mcb's, to comply with Reg. 411.4 , both conditions 1 and 2 need to be satisfied, namely measure the Zs then check the values against the tables, having adjusted for temperature. But, what about RCD's? Initially it seems to be saying that we should 1/. carry out a Zs test first, then 2/. do the inspection and RCD test. But then the Regs seems to contradict itself by stating that for RCDs the effectiveness of ADS can be verified by using suitable test equipment, an RCD tester and referring to Table 3A in Appendix 3, without any mention of a Zs test? Any chance of clarity please?

The next paragraph then also states "Where the effectiveness of the protective measure has been confirmed at a point located downstream of an RCD, the protection of the installation downstream from this point may be proved by confirmation of the continuity of the protective conductors".

So hence the viewpoint of the NICEIC which initiated this article.

Common Sense

Now, there are a number of issues that I am going to discuss in relation to this conundrum.

1/. RCD's have to be maintained to keep them working effectively - allegedly we are testing RCD's every six months, using the 'trip button' test facility. As we know, this maintains the RCD, keeping all its little moving bits still moving, and without this the RCD would possibly seize up and be as much use as a chocolate teapot. Now lets be honest, how many of you actually do carry this out, even in your own home for your own families? How many householders are aware of the importance of this, and religiously test all their RCD's? Remember, if the RCD does not operate, we are now reliant on the fuse or mcb for ADS, so perhaps we should have carried out that Zs test after all?

I can state categorically that the functional testing DOES make a difference, as an example, on a caravan park I tested some years ago 50 out of 110 plots failed the RCD tests because no regular functional testing was being done. I encouraged the site owner to regularly test the RCD's (after I had replaced the 50 sticky ones), and two years later only 9 RCD's failed, a great improvement (but still not 100%).

2/. There are a number of different types of RCD (Types AC, A, F and B), which have different characteristics, and should be used for different types of load. If the wrong type of RCD is used, it may not operate effectively under certain types of equipment fault, hence will not disconnect, again we are now relying on the mcb or fuse, and hence the Zs value is important.

3/. I like the Zs test, it gives me a cosy feeling of reassurance. My Zs value indicates that connections are ok, polarity is correct, I have a good earth, I have a neutral, my circuit is healthy, If I just did a R2 continuity test (that's all that is needed under 643.2.1) and then an RCD test at ONE socket outlet on a radial circuit, I feel I will not be getting sufficient information about the circuit.

4/. I also like to carry out PFC tests at the end of circuits to check overcurrent protection under short circuit conditions. So while I am doing that, why wouldn't I do a Zs test? Also, as the RCD test is a live test anyway, why the fuss over doing a Zs test 'live testing'?

5/. Just because the Regs. and other articles say we can get away with 'just doing this' because its easier, does not mean it is right. As electricians we should not be looking for short cuts and the easy way out, we should be using our own judgement as to what is right, and what is ethical. Can we honestly say we have done all we can to ensure the safety of the installation now, and for continued use? If there are tests we can do to verify safety systems are going to work, then do them!

6/. I do not completely trust RCD's, they do fail from time to time as they are a fairly complex unit.

This also seems to be part of the continuing 'dumbing down' of the industry. Make it easier for the 10 day and 20 day wonders to do testing, just whack an RCD everywhere and check it trips. We have had the introduction of metal consumer units in domestic properties to offset the increase in fires due to poor installation techniques, cheap materials and non-competent installers. We have had the recent recommendation of AFDD's to offset the increase in fires due to poor installation techniques, cheap materials and non-competent installers. Seems to be a pattern developing!

We look to bodies like the NICEIC, ECA and NAPIT to help us, guide us, inform us and protect our industry from all sorts of nonsense. The IET need to give clarity on these issues, for instance perhaps re-write Reg 643.7.1 so it does not appear to contradict itself, but clearly states what is required without ambiguity.

My message to the NICEIC over this article is that whilst I can see where you are coming from, and technically it can be argued that you are correct in the essence of what you have said, do you REALLY think that this is the best way forward, for the industry, and for those using electrical installations? Sometimes we have to look at things from a different perspective considering the human element, not just reading from the rule-book. But....., I thank you for raising the issue as it has provided quite a good discussion point, getting lots of girls and guys debating, which I think is great for the industry!

Stay safe people!


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