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Live - Neutral reverse from plug in tester

Former Community Member
Former Community Member

Hi, being retired for 2 years i was in trade as domestic electrcian for 10 years then commercail for next 40 years. So have high level od experience in field, yet never experienced the below unrational fault.

Really baffling me now (and an  independant competant electrican contractor) is a recent problem where i found using Martindale plug in tester (also using  independant contractors KEW plug in tester) that on a ring circuit somewhare in its middle 2 outlets read L-N reverse.

Yet taking off the outlets, the cables are connected correctly and live tested live in its correct position so Plug top fuse was correctly protecting appliances. This is on a recently extended ring circuit for room extension. I have read other blogs where they say the earth could be live or at least have a voltage presence, or that the earth CPC in fact could be floating so having volts induced to it.  Both scary thoughts.   

Any one had same issue please and what was found to then correct fault. 

I have yet to do a local earth test of the cpc at the two outlets and check the consumer unit earth connection to the main incoming supply.  Also all other outlets in house test correctly so assume main earth connection is not a problem and the RCD as main isolator passes its tests,

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    yes, as a first bash, for typical 2,5mm PVC cable, expect about 100pF of capacitance core to core per metre length (so you'd need hundreds or maybe thousands of km of it to make a motor start capacitor- it is really a very small effect !!) The core to core capacitance  does vary a bit between makers, not being a well controlled quantity, more of an accident of core geometry and insulation thickness, but the microamps that flow at 50Hz are enough to register on a modern digital meter (10 or 20 megaohm input loading typical ) and also to cause some LED fittings and some CFL lamps to pulse in a ghostly way when supposedly off. Confusing if you are not aware, especially on 2 way lighting, with different adjacent cores live or connected to the lamps in the two ‘off’ states. 

    A resistor or a capacitor of perhaps 0.1uF or so is sometimes useful to provide a load to collapse these picked up voltages, on short lengths sometimes the pick up can also be modulated a  bit with contact to conductive fingers, but it is a brave soul who does that, as the degree of voltage collapse is not guaranteed.

    Mike.

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  • 0

    yes, as a first bash, for typical 2,5mm PVC cable, expect about 100pF of capacitance core to core per metre length (so you'd need hundreds or maybe thousands of km of it to make a motor start capacitor- it is really a very small effect !!) The core to core capacitance  does vary a bit between makers, not being a well controlled quantity, more of an accident of core geometry and insulation thickness, but the microamps that flow at 50Hz are enough to register on a modern digital meter (10 or 20 megaohm input loading typical ) and also to cause some LED fittings and some CFL lamps to pulse in a ghostly way when supposedly off. Confusing if you are not aware, especially on 2 way lighting, with different adjacent cores live or connected to the lamps in the two ‘off’ states. 

    A resistor or a capacitor of perhaps 0.1uF or so is sometimes useful to provide a load to collapse these picked up voltages, on short lengths sometimes the pick up can also be modulated a  bit with contact to conductive fingers, but it is a brave soul who does that, as the degree of voltage collapse is not guaranteed.

    Mike.

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