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A Challenge Testing a YHDC SCT-013-000 Current Transformer

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Tags: #<Tag:0x00007f880e622148>
(Margaret K. Johnson) #1

I am attempting to read the current from this CT using my DMM. The setup connects a desktop heater to a extension cord I made that exposes the live wire. I use my DMM to take a measurement clamps one of the probes on the sleeve (-) and the other probe on the tip (+).

Our voltage is 120V.
My expectation is to get a reading in the mA. Something like 6 mA. What I am getting is 6uA. 6uA is way too low. Am I not measuring correctly?

Any advice greatly appreciated. Thank you.

(Robert Wall) #2

Your supply voltage is irrelevant.

You should see 50 mA, this is of course alternating current, for 100 A of primary current, that’s current in the wire that the c.t. is on.

So if you are trying to measure a 2.5 kW kettle, at 120 V that’s 20.83A, so the c.t. output should be 10.42 mA.

I haven’t looked at that 3rd party site, if it’s relevant, please upload your photo here.

(Margaret K. Johnson) #3

Hmmm… That’s not what I thought (so I don’t understand). I have my “extension cord” plugged into my 120V socket. plugged into that is a heater which is about 1200w. I put the CT on the black (live) wire. I put the DMM probes on the sleeve and tip of the 3.5 TRS. Given that V = 120V and P = 1200w, P = IV, I = P/V, I = 1200/120 = 10A. The CT has 1800 coils. So I expect 10/1800 = 5.6mA?

(Robert Wall) #4

That’s meaningless. (Because the manufacturer will tweak the number of secondary turns to give the best accuracy overall.) The ratio of a c.t. is ALWAYS specified as a ratio of two currents, the rated primary current to the corresponding secondary current. So your SCT-013-000 is 100 A : 50 mA. And that also tells me that the maximum current you can measure is 100 A.

Therefore reworking your maths:

Primary (load) current is 1200 W Ă· 120 V = 10 A (correct)
Secondary (meter) current is 10 A Ă— 50 mA Ă· 100 A = 5 mA.

So you were close - but far enough away to get confused by the discrepancy.

If you’re not measuring that (within a few percent), there’s either a problem with your c.t., your connections (but you’re measuring at the right place - tip and sleeve), or your meter (or the range you’re using - those pictures are blocked to me, so please upload them here if they’re meaningful).

All I can say is, whenever I’ve measured a c.t. that way, it gives the right answer for me.

(Margaret K. Johnson) #5

THANK YOU. There is much to learn from your reply. Ah! So practically, the number of coils quoted is most likely not accurate. This is great to know.

re:

All I can say is, whenever I’ve measured a c.t. that way, it gives the right answer for me.

yah - but you know what you’re doing. :slightly_smiling_face:

I will try yet again (isn’t that what us crazy folks do? Repeat and…repeat…)

(Robert Wall) #6

You are using the a.c. range on your meter?

(Margaret K. Johnson) #7

Here’s a picture of the meter w/ settings. Here’s a picture of “the microwave power”. Here’s a picture of the “extension cord” I made and use to measure current.

I just tried on a different CT (same model). Got the same results (.67mA).

(Robert Wall) #8

Can you upload those here? I cannot see them where they are, the image is blocked.

(Margaret K. Johnson) #9

of course.

(Robert Wall) #10

Can you try the same setup, but use something like an electric kettle, rather than a microwave oven?

(Margaret K. Johnson) #11

similar results. makes me wonder if the “buggaboo” is in the DMM? I don’t understand what could be wrong with the DMM. I get readings, so I assume the fuse is ok.

(Robert Wall) #12

You need to check that. When something doesn’t make sense, assumptions are the first thing to check.

(Margaret K. Johnson) #13

So right you are. Blown fuse. Working …reading = 5.5mA. I very much appreciate your kind help. Thank you.

1 Like
(Robert Wall) #14

I think I ought to add:

Meter fuses can be a source of great danger. You must NEVER rely on a no-voltage indication by a meter unless you have proved that the meter is indicating correctly both before and after the zero reading on the conductor under investigation. This is so that you have evidence that the fuse has not blown.

(Bill Thomson) #15

Sounds like that’s one of the reasons why Fluke doesn’t fuse the voltage inputs of their meters
and uses separate jacks for making current measurements. (which are fused)

Ref: https://www.fluke.com/en-us/learn/blog/digital-multimeters/choosing-the-correct-fuse-for-your-tester

(Robert Wall) #16

My DMM likewise has separate inputs fused only on the current ranges, but I wouldn’t like to guarantee that every meter does the same. And even so, relying on a no-indication, without proving the instrument, is fraught with danger.

(Bill Thomson) #17

Yep. Quite likely the cheapo meter manufacturers don’t do as Fluke does.
Not something I’d want to bet my life on either. :wink: