Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 6 replies
  • Subscribers 32 subscribers
  • Views 1651 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Capacitive reactance in twin and earth

Former Community Member
Former Community Member
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
Parents
  • 0
    This is slightly more complex than your post. Single wires have an inherent inductance, effectively in series with the conductor. In a cable (and let's say just 2 wires at the moment) there is a capacitance between the wires perhaps 150pF/metre, but varies with the size of the conductors and distance apart. The inductance might be 20 nH/metre. If you sketch this out you will find you have low-pass filter configuration, but there is a difference, which is that because both capacitance and inductance are continuous along the length (that is there is a value for each however long one measures, say 1mm) the low pass effect is not seen and we have a "transmission line" instead. At 50Hz or so we do not see transmission line effects on short cables (100s of metres) but we do on long lines on pylons for example, because it is related to the transmission wavelength (about 60km at 50Hz).


    A loop "impedance" test is concerned with the cable and Earth return path resistance plus the series inductance reactance at 50 HZ. The inductive reactance is very tiny compared to the resistance and is largely ignored except on long lengths of the largest cables (the kind which it is difficult to lift a couple of metres). It is not affected by the cable capacitance because this is a parallel effect and very tiny, and does not affect the fault current.


    David CEng etc.
Reply
  • 0
    This is slightly more complex than your post. Single wires have an inherent inductance, effectively in series with the conductor. In a cable (and let's say just 2 wires at the moment) there is a capacitance between the wires perhaps 150pF/metre, but varies with the size of the conductors and distance apart. The inductance might be 20 nH/metre. If you sketch this out you will find you have low-pass filter configuration, but there is a difference, which is that because both capacitance and inductance are continuous along the length (that is there is a value for each however long one measures, say 1mm) the low pass effect is not seen and we have a "transmission line" instead. At 50Hz or so we do not see transmission line effects on short cables (100s of metres) but we do on long lines on pylons for example, because it is related to the transmission wavelength (about 60km at 50Hz).


    A loop "impedance" test is concerned with the cable and Earth return path resistance plus the series inductance reactance at 50 HZ. The inductive reactance is very tiny compared to the resistance and is largely ignored except on long lengths of the largest cables (the kind which it is difficult to lift a couple of metres). It is not affected by the cable capacitance because this is a parallel effect and very tiny, and does not affect the fault current.


    David CEng etc.
Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines