CT Connection Best Practice?

I’ve been setting up CTs in my panel and have been cautious/paranoid about damaging the IoTaWatt. I’m still trying various CT configurations (on which breakers) and moving things around a lot.

I’m using a mix of ECS25200, ECS1050, and ECOL09 CTs.

Do the TVS diodes protect the IoTaWatt from damage un/plugging a “live” CT while it’s powered up? Is there a chance I could fry my IoTaWatt by making changes while it’s running?

Is the best practice to unplug the IoTaWatt, make all my CT connections/changes, then power up the IoTaWatt? Is pulling the 5V power enough, or should I also unplug the 9V reference?

Can the CT be clamped over wiring while un/plugging into the (turned off) IoTaWatt?

Is there a procedure to properly shutdown the IoTaWatt (or just unplug it)?


The TVS diodes actually protect the CT. Without them, the CT would try to produce a voltage against an infinite resistance and produce very high internal voltage that can (and does) damage the CT. So it’s considered generally safe to unplug a TVS protected CT while working on it. If you are going to leave a CT unplugged for more than a few hours, I’d recommend removing it from the primary, or shorting the 3.5mm jack (won’t hurt the CT).

No. The IoTaWatt has it’s own TVS diodes inside, primarily to guard against static discharges. They are “rail-to-rail” diodes so they protect all of the internal components. As part of the safety and emissions testing, IoTaWatt has been tested against repeated discharges of thousands of volts.

No, best practice would be to just plug and unplug TVS protected CTs without powering down the IoTaWatt. There are no ordinary configuration procedures that require powering it down or restarting it. For the few configuration changes that require restart, like changing the external name, it’s done automatically.

Pulling the power plug should only be necessary if you need to open the IoTaWatt to remove the SDcard or replace the battery, or in the rare instance it is advised as part of a diagnostic or recovery procedure. Disconnecting the 9V reference should never be necessary.

It does not matter if the CT is connected to the IoTaWatt when you clamp or unclamp a TVS protected CT. Unprotected CTs should be connected to the IoTaWatt before clamping to protect the CT.

If you want to shut down, then yes, just disconnect the USB power.

Thanks for those questions. I now realize that these things need to be explained in the documentation, which I have neglected lately. I have a short list of new things to add, and I’ve put these on it.

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Thanks for the detailed explanation. I’m barely above the level of “hack” when it comes to this stuff. Good to know the IoTaWatt protects itself from newbies like me : )

I agree a little more explanation in the docs would be helpful.

On a manufacturing note, the only other nicety that would have helped is to have the wifi mac address printed on the case. Probably not necessary for 90% of people, but it was something I was looking for when adding it to my LAN.

The IoTaWatt is still very cool and painless to setup. I’m excited at all the possibilities to “see” what’s happening with my power usage. Thanks again for making it.

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