I realised there was one more test I could do in support for or against your specimen theory, even without the calibrator. Using the grid, heaters and multiple wire wraps like everyone else does, I knocked up a quick-n-easy test of my two SCT-006s sourced directly from China.
I used one of my trusty 20A/333mV CTs as a convenient reference. These go up to 70A before they start to distort but their low voltage output doesn’t make the suitable for OEM kit. Two CTs on the one piece of wire is a great way to make sure everyone is measuring the same signal at the same time.
I used the exact same 24R shunt I used in the earlier experiment on the shop supplied SCT-006 … not just the same value shunt but the same actual shunt.
First a relatively low current to test how well the SCT-006 agreed with the 333mV CT:
Green is the 333mV CT and it measures it at 182.53/333x20 = 10.96A
Yellow is the SCT-006 and it measures it at 328.46/600x20 = 10.95A
A pretty impressive match given it’s all just based on nominal values.
Then I amped it up:
Green is the 333mV CT and it measures it at 314.27/333x20 = 18.88A
Yellow is the SCT-006 and it measures it at 505.29/600x20 = 16.84A
Then repeated the last one with my other Chinese sourced SCT-006 (grid voltage had varied since the earlier reading):
Green is the 333mV CT and it measures it at 315.24/333x20 = 18.93A
Yellow is the second SCT-006 and it measures it at 496.53/600x20 = 16.55A
So all three of mine display similar distortion (and Robert’s seem to too). And I guess we can safely say that 19A isn’t far enough back from the 20A edge for 50Hz users.
It doesn’t rule out your theory though, just a couple more data points. It could be a batch thing. Does anyone know how to decode the YHDC serial numbers? The first 4 digits look like they could be a yymm code? Mine are:
1706 - both Chinese sourced
1710 - Wales sourced
Finally, while I didn’t get the vice out, I used as much force as I could muster with my thumb to try and close it more firmly while watching the scope, but nothing changed a bit.