Electronic – Reading a CT with burden resistor and voltmeter, why am I getting this reading

current measurement

So I have hooked up a 50:5A CT, with only the hot wire going through its center. I put a 1 Ohm burden resister between the legs of the CT and then measured the voltage on each side of that resistor.

The primary is 120V @ 60Hz.

Since I used a 1 Ohm resistor, the voltage reading should be roughly equal the the current across the CT. I read 183mV, so I would expect ~180mA, which equates to about 1.8A on the primary wire going through the CT.

Two separate clamp-on amp probes both read 3.3A on the primary. I'm confused as to why my voltage reading is not more like 330mV. What am I doing wrong here?

CT

Note: My meter says that the actual resistance value of the resistor is 1.1 Ohm.

Best Answer

I suspect that you are exceeding the VA rating. With your photo now added we can see the rating is 1.0 VA. That will be the maximum power you can draw from the CT with the rated secondary current, 5 A in this case.

From \$ P = I^2R \$ we can calculate the maximum burden resistance as follows:

$$ R = \frac {P}{I^2} = \frac {1}{5^2} = 0.04 \; \Omega $$

Since you have a burden of 1 Ω you may be presenting too high a burden on the secondary and the core of the CT may be saturating - although this would be surprising at < 10% primary current.

If you have another 1 Ω resistor add it in parallel and see if the accuracy improves (remembering to double the meter reading).

Notes:

  • At rated current and a 0.04 Ω burden you will get \$ V = IR = 5 \times 0.04 = 0.2 \; V_{RMS} \$ across your burden resistor.
  • You may be able to make a 0.04 Ω resistor from a length of solid-copper network or telephone cable if you know the wire gauge. You'll find a cable resistance calculator online fairly easily.
  • When your burden is right you can increase the primary ampere turns by looping your primary cable through several times. e.g., 3.3 A with 10 turns should read the same as 33 A, single turn.
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