# Difference between CTs

Hello everyone,

I’ve my iotaWatt working perfectly but i’ve some problems with my CTs. I’ve 2 models of CTs like in the next pictures: CT1 and CT2

So what’s the main difference between both CTs? I can say that 100A : 50mA refers to the CT’s current in the primary and secondary but the 30A / 1V is making me confused.

Thanks!

The “30 A : 1 V” c.t. has a built-in burden resistor, so it produces a 1 V output for a 30 A primary current. It isn’t really a current transformer in the strictest sense, rather a transducer.

If you have further questions specifically about interfacing that device to Ioatwatt, I suggest The IoTaWatt forum is a better place to ask. That is managed by Bob Lemaire, its creator, and he and the other users there are more likely to have the specific knowledge to help you.

Thank you for the answer. To be honest, i thought that the CT had a internal resistor but I wasn’t sure.

Thanks!

If you read the data sheet, it does say that there’s a “sampling” resistor inside. I guess that’s the translation into Chinese translated back into English.

Right. And if the output is in current, it has a protective diode instead of a resistor.

Now I’m confused with the " : " and the " / ". In the 100A : 50mA it means that 100A in the primary are converted to 50mA in the secondary. In the 30A / 1V it means that for a 30A input i’ve 1V in the output, right?

Thanks again!

Conventionally, in England, a true current transformer is always specified by the ratio of the currents (not, as many do, by the turns ratio - so a 2000 : 1 c.t is a totally different thing to a 100 A : 50 mA c.t, even though they have nominally the same number of turns on the secondary winding).

In cases like this ‘:’ and ‘/’ mean the same thing.

That’s correct. A current transformer wants to drive a current in the secondary circuit. It will generate whatever voltage it needs and is able to, to make that current flow. If there’s nothing there for the current to flow in, a large c.t. will “flash over” - generate an electric arc - either across the surface, between the terminals, or internally and damage itself. The diode - actually a bi-directional surge suppressor - provides a current path when the voltage exceeds about 16 V (22 V in early versions).
There’s a picture here: https://learn.openenergymonitor.org/electricity-monitoring/ct-sensors/files/YhdcCTReportIss6.pdf

Ty Mr. Rober.Wall. Got enlightened!

It may be more correct to say:
most, but not all, manufacturers include a protective device in their current-output CTs.

e.g. I have a 30 Amp CT made by CR Magnetics that has no burden and no protection built in.

And, as Robert always advises, even if the CT does have protective diodes, it’s best to never
depend on them, i.e. treat the CT as if they weren’t there.

Indeed.

It is ALWAYS safe to short-circuit a current transformer. You should never rely on there being a protective device inside a current transformer. If you need to disconnect it from the burden, always isolate the primary circuit and short-circuit the c.t’s secondary winding. Then you may re-energise the primary circuit.

Hello again guys. First of all, thanks for all the explanations. I’m learning so much!

Now I’m testing and learning about Rogowski coils and found that they give an Voltage output related to the current that flows through his condutors. Are Rogowski coils somewhat like CTs? Since their output is the derivative of the current I need an integrator to convert the sinal output. Am I right?

Thanks!

Here’s some info about RCs compared to CTs, and how thery work:

BTW, this is a good example of info that can be retreived with a simple Google search.

I cannot speak about the Ioatwatt, but I would expect problems trying to use a Rogowski coil with the emonTx or emonPi, simply because our input circuitry is designed for a floating c.t., and the output from a Rogowski coil’s integrator-amplifier is unlikely to be biased to the correct voltage.