Inductance of a 1 metre length of mains cable

[ocset]

Hello
We tried to measure the inductance of a 1 metre length of three core mains cable (L,N,E).

We did this by soldering a 2.2uF film capacitor to live and neutral at one end. –Also attaching a scope probe to this capacitor. Then we touched live and neutral at the other end across a 10V power supply. 

We determined the inductance from the frequency of the subsequent ringing..
The frequency of the ringing was 111kHz.
This means an inductance of 933nH.     
We are amazed because this seems so high. Can this be right? 
 

[ejeffrey]

The parallel wire inductance formula gives about 520 nH for the geometry you specified.  So 933 seems a bit high.  There will be a small ESL for the capacitor (10 nH?  total guess). Possibly the rest is a parasitic inductance of your test power supply,

What kind of power supply and how was it connected?  Obviously any cable from the supply to the terminals will increase the inductance, but many PSUs just have high output inductance.

An easy way to test is to repeat the experiment with 2m of cable or with 0m of cable.  That should help you calibrate the system parasitic inductance and separate it from the cable.

[ejeffrey]

One more thing: I assumed the geometry factor d/2a = 2.  That is, the wires are spaced by twice their diameter.  I would say this is reasonable for something like a 15 A jacketed IEC power cable where each wire only has one layer of insulation inside the bundle.   If d/2a was 5, then 933 would be reasonable.   A low amperage lamp cord where the conductors are thinner and have more insulation between them, you might get about that high.

[Zero999]

The figures you've measured, seem acceptable.

Just one question: why do you want to know the inductance of a mains cable? It's negligible at mains frequencies.

[T3sl4co1l]

The figures you've measured, seem acceptable.

Just one question: why do you want to know the inductance of a mains cable? It's negligible at mains frequencies.

Mr. treez has been faffing about with EMC problems since, well, ever.  It would be reasonable to expect that's the immediate concern.

A reminder that the mains cable has characteristic impedance, i.e., it simply transmits whatever energy is sent down it, into the mains LISN, to be read out as conducted EMI.  That impedance isn't terribly different from the 50 ohms (per line) of the LISN, in fact the differential (L-N) impedance is then 100 ohms, very close to the cable indeed.

If you have concerns about conducted EMI, fix it inside the EUT.  You can't fix it outside.

Tim

[Zero999]

The figures you've measured, seem acceptable.

Just one question: why do you want to know the inductance of a mains cable? It's negligible at mains frequencies.

Mr. treez has been faffing about with EMC problems since, well, ever.  It would be reasonable to expect that's the immediate concern.

A reminder that the mains cable has characteristic impedance, i.e., it simply transmits whatever energy is sent down it, into the mains LISN, to be read out as conducted EMI.  That impedance isn't terribly different from the 50 ohms (per line) of the LISN, in fact the differential (L-N) impedance is then 100 ohms, very close to the cable indeed.

If you have concerns about conducted EMI, fix it inside the EUT.  You can't fix it outside.

Tim

I agree: the cable impedance is much more important than either the inductance or capacitance and any EMC problems need to be fixed, inside the device. Adding a ferrite bead to the cable may help, if it's borderline  but I doubt it will fix something which completely fails.

[mikerj]

The parallel wire inductance formula gives about 520 nH for the geometry you specified.  So 933 seems a bit high.  There will be a small ESL for the capacitor (10 nH?  total guess). Possibly the rest is a parasitic inductance of your test power supply,

What kind of power supply and how was it connected?  Obviously any cable from the supply to the terminals will increase the inductance, but many PSUs just have high output inductance.

An easy way to test is to repeat the experiment with 2m of cable or with 0m of cable.  That should help you calibrate the system parasitic inductance and separate it from the cable.

Another way to remove the influence of the supply would be to short the conductors at one end of the cable, then connect a charged cap to the other end of the cable.

[Neilm]

I have seen the inductance of leads be significant in certain fault conditions - when the 13A fuse blew the inductance caused a voltage spike that then arced over to the "safe" area. Never had EMC issues from just the mains lead although all the EMC relevent specifications specify the layout of the cord during the testing

[Zero999]

I have seen the inductance of leads be significant in certain fault conditions - when the 13A fuse blew the inductance caused a voltage spike that then arced over to the "safe" area. Never had EMC issues from just the mains lead although all the EMC relevent specifications specify the layout of the cord during the testing

I'd be surprised if it was the inductance which caused the voltage spike. Could it have been the a voltage spike from the mains which caused too higher current to be drawn, the fuse blowing and arcing over the "safe" area?

[floobydust]

1mH/km is typical cable inductance, or 0.2uH/ft. Used as a typical value in some regulatory standards.
Capacitance line-line is another number to include. 30nF/km or 33pF/ft is same.