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Thread: Earth loop impedance testing

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    Earth loop impedance testing

    A question for all the bright sparks, It you are busy doing an inspection report and find the earth loop impedance reading higher than it should be, what do you do?

    I was told by the technical division of the Durban electricity department 2 very interesting points:

    1/ That i am suppose to do an earth loop impedance test at each and every plug socket or power outlet, interesting because according to page 276...8.7.5 Earht loop impedance test should be done at the the main switch.

    2/ If the reading is higher than it is suppose to be, for example 10 ohms at a 60 amp breaker, it is my responsibility to isolate the power imediately, report it to the electricity department and they will sort out the issue within the same day, funny they should say that because if i have switched off the power in the most recent test (15/11/2012) the power would still be off, i still havent had a reply from them todate. If i dont switch off the power i will be held responsible for any injuries caused by bad earth, surely the responsibilty would become theirs, if they dont attend to the problem within a 24 hour period?

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    Diamond Member AndyD's Avatar
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    What earthing arrangement did the premises have? TNCS?

    We perform an external earth fault loop impedance test (Ze) on the incoming supply with the earth disconnected and the installation isolated plus we do earth fault loop impedance testing (Zs) from the farthest end of any circuits we install to ensure MCB protection times are met.
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    Durban has a TN-S earthing system.

    I use a fluke 1653, so i can and do tests at every single point,

    voltage : L-N, L-E, N-E and include the phases if it is a 3 phase system.
    polarity.
    E/L (RCD as Andy refers to them) trip time which should be done in both in 0 and 180 degree (not a requirement in South africa) you can just use a normal plug in e/l tester.
    E/L tripping current.
    loop/line impedance and at the same time the prospective earth fault current.

    It gets a little more complicated when we test points in other applications like hospital theatres for example.

    The tests above are normally only done if the power is already on, for example in exsisiting installation inspection reports when properties are sold.


    What does the law(SANS) say about it, at what point should the loop impedance be carried out.

    What would be the ruling in court if someone was injured if you didnt switch off the installation?

    Just like elevated voltage rule, which happened a couple of weeks ago, i was called out on a Sunday morning to find the that the voltage was way about recommended value, so i didnt even think twice, I isolated the entire installation and boy did i collect for that. Customers where complaining threatning me win court cases etc, i eventually found the neutral bussbar between the transformer and the main frame had been stolen, i only reset the breakers at 4 pm. Lucky for me non of shop owners went through with the threats to sue me.
    Last edited by ians; 16-Feb-13 at 10:39 AM.

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    SANS 0142 section 8.7.5.2 tells you to install an ELU
    Eric

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    Gold Member Sparks's Avatar
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    Looking at the COC as a whole you will see it is split into sections. The Loop,PSC, Supply Voltage,Elevated voltage,erath resistance are all tests done at the DB with the installation isolated. This is to determine whether the supplier is providing withing spec. Once these specs have been met the installation can be connected provided the installation complies. There are specs regarding bonding conductors, earth continuity which are tested for at each point of consumption will determine if the installation conforms to minimum requirements.

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    Did you ever resolve this with the municipality. I've just done an inspection and test on a very old property and it has an Earth loop impedance (at the main switch) of 16 Ohms! Way too high to allow 2 times the current to trip the 60A mains breaker. Most houses I've tested in the suburb have Earth loop impedances of less than 0.2 Ohms. All are TN-C-S by the way. One interesting thing is that the house has a Zellweger Uster ZE-22 ripple meter installed. I've tried to find a contact at Tswane Metro to see if they are still using these and if it could be introducing extra impedance into the circuit but to no avail. Also can I disconnect the thing? I'm probably going to be doing a complete rewire of the property as it dates to 1937 but the Earth loop problem is really a supplier problem. Early days though!

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    Gold Member Sparks's Avatar
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    I see my previous response has the spelling of a new SA graduate, my apologies. I however reiterate what I said. Do the test at the main switch with it off. The ripple relay will not influence the reading at all. What must be tested at all the points of consumption for the COC is the earth continuity to the DB. This must comply with the regs according to the CB rating of the particular circuit. The local supplier is obligated to rectify a non-compliant loop impedance reading immediately. If you cannot get hold of a local supply employee willing to lift his backside you can resort to threats of claims for damages and the possible charges of culpable homicide being laid against said authority as well as public denounciation in the interest of consumer safety as well as fraudulent abuse of monopoly.

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