Basic CT Questions


Sorry for the basic questions, I've bought an emonTXv3 and a load of CT sensors but I'm brand new to this so need a few pointers. I've read all the docs I could find but I'm not 100% sure so wanted to verify some things.

1) If I want to use a CT sensor to monitor the light circuit coming out of the consumer unit, then do I need to connect it around only around the live wire coming out?

2) Similar to 1), if I want to monitor a specific appliance using a CT sensor, should I make an extension lead to go inbetween the plug socket and the appliance and connect the CT around the live wire only? Does it matter that the CT won't be a tight fit and will be loose around the wire?

3) Or is there a better device that I should buy to do 2) ?

4) I've read that I shouldn't connect a CT while the power is on. Does this apply to the ones in the shop - ?
I was wondering if it was OK to connect them live as they don't touch the wire like some other CT sensors?

Many thanks,

Robert Wall's picture

Re: Basic CT Questions

1. Yes.
2. Yes, and no, it doesn't matter that the CT is loose on a wire. It makes no detectable difference whether it's straight through the middle, at one side or slanted.
3. The Shop CT with the emonTx V3 is not really suitable for single appliances, as the CT is good to 100 A and the 3 principal inputs are scaled for that. (Input 4 is scaled for a bit under 20 A maximum, so much better for a single appliance, or your lighting circuit.) If you're able to accept that accuracy (percentage-wise) falls dramatically when the current gets below about 1% (125 W / 250 W depending on what your supply is, then you're OK, otherwise you should consider modifying the emonTx V3 to increase the sensitivity of inputs 1 - 3 by changing the burden resistor.
4. You shouldn't dis-connect a CT while its primary circuit is energised. This is because a CT is a current source, so it will generate whatever voltage is necessary to drive the current it wants to into its load. Therefore, quite high voltages can be generated. The YHDC SCT-013-000 has protective zener diodes inside that limit the voltage to 22 V, so while there's probably not enough current available to damage the V3, for the sake of switching off the load (or unclipping the CT from the cable) it's better not to risk it.

niallel's picture

Re: Basic CT Questions

Thanks so much for the quick reply, makes it very clear.

With regards 3. would you say that if I want to monitor each circuit coming out of the consumer unit then the CT would be a good fit, especially for the lights if the majority is switched off - the Wattage will be less than 125W.

I don't really want to modify the emonTx, I'm liable to break the whole thing.

For monitoring individual appliances, what would you recommend to work with emonhub/emoncms?

thanks again,

Robert Wall's picture

Re: Basic CT Questions

The emonTx was designed for whole-house monitoring. I don't know of a device that is more suitable for individual appliances, unfortunately.

Modifying the emonTx should be relatively simple if you have some experience in electronics - for each channel that you want to modify, you need to remove one surface-mount resistor and replace it with a wire-ended one of higher value.

But if this is too daunting, I'd suggest you try the emonTx as it is and see if the performance is acceptable. You should get reasonable accuracy down to about 200 mA ( ~45 W with a 240 V supply) using input 4. So with more than about 4 medium-sized CFLs, you should be OK for accuracy, though it will read something to much lower currents. There's a Building Blocks article that explains the theory behind what happens to the ADC with very small inputs.

thiebolt's picture

Re: Basic CT Questions


I have had the same issue: 100A CT with all lights turned on in the corridor leading to ... less than 5A (remember that according to the manufacturer, 0-5A is not considered a range to use the CT). So i decided to switch to 30A CT (same manufacturer): to do so, bear in mind that 30A CT already features a burden resistor, this means that you have to remove 3 x 22R resistors

Hope this helps

Robert Wall's picture

Re: Basic CT Questions

As far as I'm aware, the 30 A CT is identical to the 100 A version except that it has the built-in burden resistor. It is therefore a less flexible solution.

thiebolt's picture

Re: Basic CT Questions

Thanks for pointing out that, i was not aware of the fact that all of the SCT-013 series are in fact the same CT either without a bruden resistor (100A) or with an integrated one (all of the others).

Anyway, 30A is still far above my requirements (most of the time, light circuit is either protected with a 10a or 16a and most of the time real power consumption is far below this level), so i bought both 5 an 10a CT (non -split core version) and i removed emonTxV3's burden resistors: this allows me to directly solder my burden resistor at the output pins of those CT.



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