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Thread: Elevated voltage on supply neutral measurement

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    Elevated voltage on supply neutral measurement


    SANS 10142: 8.7.6: With the main switch off, measure the voltage between the supply neutral and any earth external to the installation. Notify the supplier if the reading exceeds 25 V.


    Is it required that you disconnect the supply neutral from earth (in a TN-C-S installation) in order to perform the test? I hope so otherwise it doesn't make much sense to me.

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    Diamond Member AndyD's Avatar
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    No, you wouldn't disconnect the supply N-E link which is on the council/Eskom supply side, the test is just made with the installation isolated. Many times you might not have access to this link, it could be in a locked cabinet or supply/metering panel. Also the whole idea is to confirm that the link is still in tact especially with PME supplies where an open circuit PEN conductor can be dangerous.
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    andy , are you sure about that? regarding domestic supplies with the n-e link in the outside enclosure,its quite easy to seperate it and test the voltage. if its linked why would you bother testing ,it can only be zero. testing factories is another story.

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    Diamond Member AndyD's Avatar
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    Lol, I was pretty sure right up to the point where you asked me if I'm sure

    I'll check up on this when I've got some time but thinking about it logically disconnecting the N-E link on the suppliers side isn't something I'd do lightly even on a small domestic supply, I'd certainly be wary of current flowing through that link even with the DB isolated, there's fault conditions that could be occurring locally that could result in a backfed current.
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    I think what they want you to do is make a temporary earth point (new peg in the ground) "any earth external to the installation". This obviously because of the fact that many installations will have the N & E linked on-site, so testing the potential between them is a pointless exercise (unless you either disconnect them from one another).

    But to me, testing if the PEN conductor is intact is much easier done by an on load voltage measurement. If the PEN conductor is high impedance, a volt drop will occur over it and thus a lower voltage across your load. If this then fails, additional testing can be performed to ascertain if the fault is on the supply or consumer side?

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    it is a risky test for sure. i think thats one of the reasons using a big line tap is not allowed anymore. an earth neutral bar is compulsory. the main reason is for testing the earth spike.
    its mainly your load side neutral that will give you a nasty kick if you forget to switch the main switch off and you get feedback. the prepay meter without a circuit breaker also causes feedback. if you have an earth neutral bar then you dont disturb the load neutral.

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    Full Member MullerR's Avatar
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    When looking at the Regs:

    "8.7.6 Elevated voltage on supply neutral"
    With the main switch off, measure the voltage between the supply neutral
    and any earth external to the installation. Notify the supplier if the reading
    exceeds 25 V.
    Disconnect the installation and notify the supplier (see annex K) if the
    reading exceeds 50 V.

    This test is to actually test if the "link" between N-E is still in tact. Think about it logically, Neutral is earthed at the Transformer, thus if that neutral and Earth link is anywhere broken along the line, you will have a voltage between neutral and earth when you test it according to 8.7.6..

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    The only way that I can make sense of this is by considering that there are other loads on the phase (as per below drawing). You are testing "Installation 2". "Installation 1" is the guy next door or down the road. R4 is the impedance from your "earth external to the installation" to the earth at the transformer. R2 represents a high impedance on the PEN conductor. If, as an example, R1 and R2 are of equal value then you will drop half the supply voltage over each and thus measure 115Vac between neutral and the external earth.

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    This is how I understand it, but if correct, it does mean that if the other loads on the phase you are testing varies you will get varying values of the N-E voltage.

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    Diamond Member AndyD's Avatar
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    Because the TN C S installation is derived from a TNC supply on the council/Eskom side there are inherent dangers that would occur if the supply neutral is high resistance or open circuit. If the supply is TN C S without a local earth rod (not PME) then the touch voltage in the equipotential zone wrt ground would rise. If there is a local earth rod then that rod (along with parallel paths via bonding) would become the return path for the full load current and, depending on its Ra reading, the touch voltage in the equipotential zone would again rise. Also it's particularly dangerous if the premises has inadequate bonding or there's a final circuit where the earth has been exported to another zone, then a touch voltage is occurring between anything connected to the supply and the other services such as water pipes and gas installation, building elements etc. RCD's in the DB would not protect against shocks from a TN C S supply broken/high resistance neutral so the supply authority is quite rightly very concerned that they never occur.

    The test outlined is one of the ways you could detect this type of fault and because they're insisting it's done with respect to a known good earth that's outside the equipotential zone of the installation I would see no reason to separate the CNE link.

    One thing I would personally have an issue with is the failure threshold of 25v for notifying the supplier and 50v for disconnection. If the installation is disconnected at the time of the test then these figures are very high, I'd be concerned if I was to see more than a single figure voltage between the PEN and ground and have in the past requested remedial work from the council on supplies with an elevated neutral voltage as low as 6 or 8 volts on a PME'd TN C S supply.
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