Hi all,
Is it ok to ask a general question pertaining to electrical engineering ?
Specifically I want to measure a current of 500A rms (50Hz). I have a CT which is 500A/5A . I am also able to source 5A/5mA PCB mount CTs. Is it possible to combine both to measure the 500A with about 2% accuracy ? I was thinking of looping the output of the 500A/5A through the 5A/5mA with a burden resistor of 1k on the 5mA side. Is this a doable solution ?
I was browsing through the site and found a lot of amazing work, so I hope I dont get barred or something for posting an irrelevant question.
Cheers
Yes, of course - provided it’s somewhat relevant.
Yes it is.
No it isn’t and you won’t be.
I have in fact suggested this before - but I’m not at all surprised that you didn’t find it. Your 5A : 5 mA c.t. used in this way is called an interposing c.t..
But your choice of burden resistor seems wrong. A 1 kΩ burden carrying 5 mA will give you 5 V rms which, if you have a nice sine wave, has a peak-peak voltage of 14.2 V nearly.
If you are using an Arduino with an input range of 0 – 5 V, then your burden should be 330 Ω.
If you are using an emonTx with an input range of 0 – 3.3 V, then your burden should be 220 Ω.
For safety, I recommend you earth the 5 A current loop at one point. It will have no effect on the operation of the system.
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Thanks much , Robert.Wall. Yes, I had overlooked the RMS to Vpp conversion.
I am assuming you mean the 500A/5A CT ? Since it is going to be dead short loop passing through the interposing c.t. , will I be connecting the shorting link to protective earth ?
Thanks again.
Yes, the secondary of the 500 A : 5 A c.t. and the primary of the 5 A : 5 mA c.t. - it’s just to protect the 5 mA side in case of an insulation breakdown. There should never be any danger from not doing so, but if there’s a handy earth nearby, then I would.