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Maybe you could avoid disconnecting the CT when a large current is flowing?  Providing you have fitted some protection diodes to your input circuit, I think you would be unlucky if anything were to blow up when your CT is re-connected.

I was more thinkng on the danger of having the CT with a current through it without a load. Maybe it gets hot and burns.

Imagine you need to do a 1 day maintenance to the arduino, it would be nice to have the CT installed and not have to remove them for this task.

Any idea on how to create this dummy load?

If it has an internal burden resistor like the 030, it should only have a volt or so across it, if it has no burden like the 000 it will normally have a pair of zener diodes (5V I think) that will limit the voltage, The problem comes if it has neither, as it will generate a high voltage when open circuit. The problem isn't the CT blowing, but blowing the Arduino when it's plugged back in if it has no protection.

The Eco Eye monitor comes with a test plug with an LED, but I don't know what it has in the CT, you could experiment with an LED as that will give an indication that the CT is "live".

So the processor inputs are damaged when plugging in a CT where a current is flowing? Then I think I'll need 1 or 2 new ATmegas because they always give a measuring about 65400....

Maybe we should think about some kind of overvoltage protection => 1k resistor to Atmega...

I have the 000 and it makes a buzzing noise. isnt this dangerous?

If the Ct is outputing AC, no easy way to just put a resistor and a led as dummy load, right?

I only hear a buzzing noise when there is no load at the CT, so the internal transient-voltage suppressor of the CT conduct all the current. The resulting voltage is too high for a Atmega input.

If you heat the noise while the CT is connected to the EmonTx you should check if the 18Ohm resistor is placed correct and if the solder points are ok.

The output is AC. You might use 2 antiparallel LEDs and a series resistor as dummi load. The problem is that the CT can output up to about 50mA (at 100A primary current), that might be too much for a led. On the other hand the solution (if the maximum current is about 50A or lower might be 2 normal diodes in series and antiparallel, so the maximum voltage of the CT will be 1,4V. If connected to the system at 1/2 Vcc the maximum voltage at the analog Atmega inputs would be 3,3V/2+1,4V=3,05V => everything ok.

But: The diodes don't start to conduct at exactly 0,7V, they start at lower voltages, so a little bit of current would be lost at higher currents. If the input current is far below the maximum of 100A this should't affect the measurement. At 50A, there will be about 25mA maximum at 18Ohms => about 0,45V. At 25A is is only 0,225V. And 25A means allreday 5750W at 230V or 17250W when the load is on all 3 phases.

Maybe I should test this solution by soldering 4 diodes (1N4148 or 1N400x) into the jack of the CTs because the maximum power consumtion of my house should be below 15kW.

Simple.

If you need to disconnect the c.t. whilst it is under load, why not move the 18 Ohm burden resistor into the plug, and replace it on the circuit board with a (say) 10 kOhm resistor just to give the input a mid-rail bias?

I use a 33ohms resistor. So first thing I tried was soldering a same 33ohms resistor to a female 3.5mm jack. I disconnected from the arduino and connected the (female jack + 33ohms) but the CT kept making the buzz noise. So I guess something is missing right?

As for the 10kOhms resistor, I dont understand what you mean, for what is for?

thanks Robert!

Ok mounting the 18Ohm Resistor into the plug is even a better and easier idea, so the voltage on the CT will never be to high again. I'll do this modification to all CT sensors when I get them.

As for the 10kOhms resistor, I dont understand what you mean, for what is for?

If you were to move the burden resistor into your plug, your Anduino's input would then be floating if you removed the plug.  By putting a high-valued resistor on the board in place of the burden, this would mean that the Arduino's input would always be connected to the mid-rail voltage.  I think that's the point Robert was making.

If your CT buzzes when connected to a standard 33R resistor, then I would expect it to buzz when connected to any similar burden.  If it doesn't, then something is not how you understand it to be.  I suppose it could be the protection diodes in your input circuit that's normally keeping it quiet.  Have you got them going the correct power rails, OV and +5V?

Sergegsx

The 33 Ohms is your burden resistor? When you unplug the c.t. you disconnect from that, yes? The problem is: without a burden resistor, the c.t, which is a current source, will attempt to generate infinite voltage in order to drive its current into an open circuit. But is quite OK to short-circuit a c.t.

Move the 33 Ohms into the male PLUG on the end of the cable that comes out of the c.t.  (Solder it to the same place as the red and white wires, or check which connect to the c.t. secondary with an Ohmmeter (it should be around 100 Ohms).  With the burden resistor there, you will never disconnect it.

The 10 KOhms: Look at the emonTx input circuit. The bias for the ADC input is connected to it through the burden resistor. When you move that resistor into the plug, you have an open circuit and no bias supply. The 10 K is big enough not to affect the calibration significantly but will ensure the bias remains correct. It will also help to protect the input from damage. Any high value ( >= 10 K) is good enough.

thanks everyone for your information.

I didnt want to move the burden resistor to the CT male plug because, correct me if I am wrong, it is better to have it near the arduino in order to have current along the wires up to the arduino instead of voltage. voltage will drop but current will not. therefore when CT and arduino are at a considerable distance having the burden in the arduino will prevent .

I moved the resistor to the male plug just to test, but the CT was still making noise when disconnected from the arduino, so there is something missing. by only installing the burden in the CT does not solve the buzzing noise. 

any part missing?

Clear on the 10kOhms resistor thanks !

About the buzzing, I dont know all thats happening, but when I connect the CT to the arduino it does not buzz.

When I disconnect the CT from the arduino it buzz

When I put a burden resistor in the CT plug (without arduino) it buzz.

So I guess there is something missing.

when I connect the CT to the arduino it does not buzz.

OK, so there must be something in your "Arduino + input circuit" that stops the buzzing. 

What happens if you separate your input circuit (including its protection diodes) from the Arduino? 

I must ask the stupid questions: There are 3 connections in your plug and socket (called "Tip", "Ring" and "Sleeve"). The c.t. uses only two of these, normally tip and sleeve.  Are you sure you are connecting the burden resistor to the correct two? Are you sure you have a 33 Ohm resistor, have you checked it with a multimeter?

I do not think moving the burden resistor a few cm from the circuit board into the plug will make any difference provided you have a good quality plug and socket. You will not see a voltage drop between the plug and the Arduino because the current flowing into the input pin is almost zero.