When testing large MVA power transformers, like 66kV to 16kV, the winding resistance in DC ohms is milliohms.

It takes a high current supply to do this, then the V is measured across the winding to determine the resistance using Ohm's Law. If the resistance is less than 1 ohm, why does a 9V digital multimeter show infinite ohms?

I am guessing/assuming that there is a physics explanation. My guess is that the electric field created in the conductor (winding) from a 9V battery is not enough to polarize the mass of electrons in the primary winding.

It also intrigues/confuses me that I can use a 9V DMM to measure other primary windings on smaller transformers with no problems. How long is the coil of wire in a primary winding (approximately)?

So why does a 9V DMM yield infinity when the real primary winding resistance is in milliohms?

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    \$\begingroup\$ Copper is a very linear material. Getting electrons to flow through it isn't like sliding a piece of furniture. If you've actually got the probes on the right wires, the meter should read milli-ohms (or zero, if the resistance in question is too low for the meter to register). Fix your technique? \$\endgroup\$
    – TimWescott
    Nov 10, 2020 at 16:22
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    \$\begingroup\$ I understand the theory okay, but this situation is known/observed among several Electrical Power Test technicians, but no one seems to have a good explanation of why this is. \$\endgroup\$ Nov 10, 2020 at 16:23
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    \$\begingroup\$ What happens if you hold the meter in place for a good long time? You didn't give the KVA rating so my estimate is probably wildly off -- but if the input impedance of the transformer is 10H, it could take a Good Long While before the circuit stabilizes. \$\endgroup\$
    – TimWescott
    Nov 10, 2020 at 16:26
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    \$\begingroup\$ wild guess: bad contact between the probes and the transformer caused by corrosion? \$\endgroup\$
    – user253751
    Nov 10, 2020 at 17:10
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    \$\begingroup\$ Inductance is a good guess, because sampling DMM's may never get a constant current flowing in the coil. Multi-turn coils also can generate significant voltage from local magnetic fields - enough to saturate the amplifier in the DMM. Try measuring the AC voltage on a disconnected transformer. \$\endgroup\$ Nov 10, 2020 at 19:36

1 Answer 1


According to what I heard here in the laboratory of the company where I work, this effect is related to the magnetization of the core. It seems to me to make sense, since the core of the large transformer is actually much larger than in distribution transformers.


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