# Electric grid power transformer winding: If it is milliohms, why does 9V DMM show infinite ohms?

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?

• 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? Nov 10, 2020 at 16:22
• 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. Nov 10, 2020 at 16:23
• 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. Nov 10, 2020 at 16:26
• wild guess: bad contact between the probes and the transformer caused by corrosion? Nov 10, 2020 at 17:10
• 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. Nov 10, 2020 at 19:36