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EICR TT installation coded C2 by electrician

Gomez Addams

Hi,


Anybody got any thoughts on this situation?

I have had an EICR done on a property. There were no issues reported apart from earthing. 

The electrician measured the rod resistance at 534 ohms. He insists it has to be less than 200, but his preference is less than a 100.
He said the 30ma RCD wouldn't trip at the measured value, so didn't bother to test it. Bizarrely, he said he pressed the test button which, of course, tripped, but he coded it C2 "unsatisfactory".

I had already tested it with my meter. I got 400ohms, a worst case trip time of 9ms, best 6ms and 28ma on the ramp test.

I pointed out the 200ohms is a recommendation not a requirement and asked him to justify his C2, he refused and stated he stands by his findings.

I haven't checked yet if there is an obvious reason for the rod to be high, but it seems to me the requirements of the regulations have been met.


  • 0 in reply to WiredScience

    The touch voltage won't exceed 50 at 534Ohms @ 30ma. One of the tests I did measured the touch voltage at 13v @ 395 Ohms.

    Megger CM500. All of the RCD tests, 1/2, I 0/180/, 5I 0/180 and ramp.

  • 0 in reply to Chris Pearson

    Ze measured at 0.27. On the report the spark added the rod resistance to the Ze.

  • 0 in reply to Gomez Addams

    The comparison with the neighbouring properties is the killer information actually.

    - while others "400-500Ω is around 5x to 10x higher than I'd get for a simple 4' rod sunk into half decent soil around here "

    I have worked in a (dry sandy) place where we sunk two sets of 8 foot rods in parallel about one rod length apart, and still only  just got down to about 200 ohms, varying a bit with method of measurement (I think 340-370 or so per single rod). Equally, a few years ago now I did some electricals at the Essex Scout Jamboree, and in the ever damp Essex clay we saw 80-100 ohms off half length rods (2ft) adequate results from container gensets dropped on their skids even with no electrodes, and  single figure ohms from a metal framed marquee.  So what is OK really varies with location.

    Mike

  • 0
    Gomez Addams said:
    It trips on the button

    That doesn't depend on an earth connection in most, if not all, RCDs.

    Gomez Addams said:
    No justification was given for the C2. When asked he would not discuss it. He just stated it has to be below 200 ohms. 

    I don't think there's a point in arguing this. In TT systems, Regulation 643.7.1 b) is used to verify effectiveness of automatic disconnection of supply (for protection against electric shock).

    This first requires measurement of earth electrode resistance as per 643.7.1 b) (1). BS 7671 (Note to Table 41.5) and the relevant IET Guidance Notes all say that an earth electrode measurement exceeding 200 Ω indicates the earth electrode resistance is possibly not stable.

    The next step of testing ADS in TT systems relying on RCDs, according to Regulation 643.7.1 b)(2), is to test the RCD. The guidance in IET Guidance Note 3 for RCD Test Method 1 (test between live conductor and cpc downstream of RCD) implies it may not be safe to conduct the test if the EFLI exceeds the maximum limits (in the case of earth electrode, that would imply check the earth electrode is stable before testing).

    Therefore, it may be the case that the person carrying out the test considered it not safe to proceed with RCD tests. It would be a potential breach of health & safety legislation for them to proceed with the tests if they believed it was not safe to do so.

    Perhaps there are two ways to proceed from here:

    1. Carry out some work to reduce the earth electrode resistance (e.g. repair, replace, or supplement with a second electrode ... doesn't have to be a 'rod' but installing in accordance with BS 7430, at a suitable depth, is strongly recommended).
    2. Get a second opinion.
  • 0 in reply to WiredScience

    I fully intend to check after the weekend if there is an issue with the connections to the rod, and if not pop a second rod in. I have four houses on TT in a radius of less then 100m. 3 are fine this ones not. I really don't think the ground conditions are going to be significantly different in such a small area.

    I'm not disputing that the value is high and lower would be better. I am disputing the C2 and the "potentially dangerous" that comes with it.

    As an aside. My own house is one of the 4 on TT. When my kids were little they said they could feel a tingle when the were washing their hands. We are on a plastic water supply. I felt nothing, but I measured everything and found nothing untoward. Eventually, I felt it too when I had a cut finger. It wasn't off the taps or the steel drainer, it was in the running water. I suspect they could feel it because they didn't have hands like leather. I got rid of it with 20m of copper under my front garden. The touch voltage was about 30V to start with, It was about 3V when I was done. Insulation measurements were all good. I was never sure where leakage came from. I suspect it was any or all of fluorescent tube fittings, fridge freezers or switching power supplies.

  • 0 in reply to Gomez Addams

    The instructions for your meter state that the temporary electrode should be 20 - 30m away, so that may account for some of the discrepancy, plus of course you're not measuring the supply side, maybe another 20 ohms? 

    Personally I always loop test if the supply is connected as it's far less faffing about.

    It does sound like he's inexperienced when it comes to TT, or perhaps EICRs in general.  I would have coded as C3, with no expectation that any improvement would ever be sanctioned by the client. 

    I know of at least one DNO that insist on less than 200 ohms for new installations and they can, and have, issued demands for improvement within a 3 month time frame.

    You sound like you are competent, so why not put in another rod (or 2) and move on knowing that your paying tenants are safe?  

  • 0

    How to fix the situation ...

    (1) Measure the rod resistance properly and replace if excessive.

    (2) Just bang in another rod. If you cannot measure the resistance properly, ensure that Ze is satisfactory.

  • 0 in reply to AJJewsbury

    Sorry, my statement before was a bit brief.


    The meter has a jack socket to plug the probe into. The 2 wire test lead is used to do this as per Megger's instructions. The equivalent circuit in the user manual shows how the resistance is measured. So one lead is on the rod, the other on live and the long lead is on the probe. Current is injected into the rod and the probe in parallel. The meter the current injected and the developed voltage are used to calculate the resistance. It is a loop measurement, but with a "true" ground reference as the manual puts it. I did try the probe in more than one spot and the results were comparable.

    The local substation here is about 250M away. 

  • 0

    I used a a long wire and a ground probe.

    Just one wire? How did you account for the resistance around your test electrode? Was that on a d.c. Ohms range? The salts in the soil and metals of the electrodes can often make little batteries, generating a small voltage that can easily influence the result of a simple d.c. test (you might well see differences simply by reversing the meter connections). The resistance around a temporary electrode is usually considerable - a 4-wire (or sometime 3-wire) test method using separate test electrodes for voltage and current and an a.c. test current is usually needed for dead testing of electrodes to get sensible results. Loop testing (from the mains L) is usually simpler - although it does include the resistance of the source electrode (e.g. at the substation & for PME additional electrodes in the distribution system) - which can yield higher figures than your electrode alone, the difference is normally small.

       - Andy.

  • 0 in reply to Chris Pearson

    Good question. He wouldn't engage with me at all when I questioned his report. I did it with a Megger CM500, a long wire and a ground probe. Measured a least 15M from the rod in more than one place.

    Today I called by to speak to my tenants about something else. I asked about the electrician, who I have never met, he was referred to me by a letting agent. My tenant told me he said he could only half the checks because "he didn't have the right equipment with him". She didn't know what he meant, which is fair enough, why should she? She asked him if the house was safe and he told her yes. I am wondering if he had any idea of what he was doing at all.

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