Hey guys quick test for you.I bolloxed my apprentice today, he was doing zs test and the tester came up with the above reading L 186V N 40 odd he had no idea what the fault could be (has passed all test mods at colledge).Do i expect too much of him?.Prize for 1st right answers!!.
This occurs when the N-E impedance becomes significant causing a voltage drop. The actual voltage drop becomes supply load dependant, which can lead to variable test results and seemingly unpredictable behavior at the installation e.g. apparently random tripping of RCDs.
On a TN-S or TT system a small potential above earth is virtually inevitable due to all the consumer loads sharing that supply N.
On a TN-S system a significant potential may occur if the outer sheath or armouring of the supply cable or their connections become eroded or corroded.
On a TT system lack of a local earth or poor earth electrode or connections to it.
On a TN-C-S system the most likely cause of this fault is a voltage drop along the PEN conductor caused by faulty connections to earth electrode(s) or eroded earth electrode(s) on the DNOs PME network.
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