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Capacitive reactance in twin and earth

Former Community Member
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I'm after some information regarding the cause of capacitive reactance which is part of the overall impedance of a circuit when carrying out loop impedance testing. 


I am not an electrician so apologies if this is standard knowledge to some of you. 


I understand how a capacitor works and what capacitance is. Since capacitive reactance is part of the three types of "resistance" of a circuit I am assuming that the reason this capacitive reactance is included is because twin and earth can to a lesser extent behave like a capacitor even when laid flat and not coiled? With AC power in the conductors I'm assuming that an weak electric field of attraction between the two conductors separated by the wire insulation forms. When the AC emf  passes the peak of the emf in the sine wave and current "flow" is now in the opposite direction from what it was moments beforehand then the induced emf created by the electric field between the two conductors results in a small reactive current flowing in opposing direction to the main current hence capacitive reactance?


 Thanks in advance
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    If you want some numbers to conjour with, typical 2.5mm twin and earth measures between 60 and 100 picofarads from L or N to E with the unused core floating  and rather less L to N with E floating.

    Capacitance is a measure of the energy stored in the electric fields around and between conductors at differing voltages - there does not have to be anything that actually  looks like plates - that is just a good  shape for max C in minimum volume. (a plump human on a step ladder has about 500 - 1000 pF to the rest of the planet, and may feel a small tingle from a live wire despite being insulated to DC.)


    Inductance per unit length is a measure of the energy stored in magnetic fields around moving charges (currents) and for that typical cable again is about 1 micro henry per metre if only one core is carrying current; but if the currents in the cable are balanced so current in  L = minus current in N the round loop inductance is quite a lot less than the sum of the inductances of the two cores, as the magnetic fields oppose and overlap and partly  cancel - the figures Dave quoted are for a line pair.


    As per above, all these effects are normally negligible at 50Hz unless you have very long lengths or are interested in effects that are a small fraction of  the main current and voltage.

    mike.

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  • 0
    If you want some numbers to conjour with, typical 2.5mm twin and earth measures between 60 and 100 picofarads from L or N to E with the unused core floating  and rather less L to N with E floating.

    Capacitance is a measure of the energy stored in the electric fields around and between conductors at differing voltages - there does not have to be anything that actually  looks like plates - that is just a good  shape for max C in minimum volume. (a plump human on a step ladder has about 500 - 1000 pF to the rest of the planet, and may feel a small tingle from a live wire despite being insulated to DC.)


    Inductance per unit length is a measure of the energy stored in magnetic fields around moving charges (currents) and for that typical cable again is about 1 micro henry per metre if only one core is carrying current; but if the currents in the cable are balanced so current in  L = minus current in N the round loop inductance is quite a lot less than the sum of the inductances of the two cores, as the magnetic fields oppose and overlap and partly  cancel - the figures Dave quoted are for a line pair.


    As per above, all these effects are normally negligible at 50Hz unless you have very long lengths or are interested in effects that are a small fraction of  the main current and voltage.

    mike.

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