Current sensing transformer datasheet meanings?

[cupoftea]

Hi,
We are using the PCT236 current transformer to sense the current in the live line of our 32Arms mains distribution box. (this doesnt relate to other posts, which concern 'imbalance' torroids).
The datasheet is very sparse (as shown below)
What is the number of secondary turns?
What does "10A(30)/10mA" and "20A(40)/8mA" mean in the datasheet?
What does accuracy class "0.5,1" mean in the datasheet?
How can we find out the saturation current?
What material is this torroid made of?
What does "RL(Ohms)" mean in the datasheet?....is it the recomended burden resistor?
This torroid has an internal diameter of 9mm, so why would you choose it when your line conductor is only 4mm in diameter?

PCT236 datasheet:

High Precision DC Immunity Current Transformer 100A Manufacturer | Ctzentar.com

Xiamen ZTC Technology focus on precise transformer since 1993,offer High Precision DC Immunity current transformer 100A,precise transformer wholesale.Factory price.

www.ctzentar.com

 

[Easy peasy]

This is a standard CT, the brackets are the max short term currents thru the hole,

10A / 10mA is the CT ratio, 1000:1 in this case

Class 0.5 = 0.5% accuracy, and so on

RL is the maximum burden resistor for the stated accuracy.

 

[Easy peasy]

The formula for calc B in a CT is the same as for any other Tx

dB / dt = V / ( N. Ae ), so for a swing of +/- 100mT, with a period of 10mS each way ( 50Hz ) and we assume a ferrite core of 5mm x 5mm cross sectional area, = 25mm^2, then we get a Vave / N = 500 E-6

Therefore, for 100mV of output, the N must be 2000T, NB the 100mV must include the volt drop due to the IR of the wire as well as that across the burden R.

Fro 20mV of o/p the N will be 400T, for the given max flux swing, and so on ...

[ N.B for a sine wave of current, Vsine o/p rms = Vave x 1.11, so for 20mV ave, we would have 22.2 mV rms for sine, for the same flux in core ]

 

[cupoftea]

10A / 10mA is the CT ratio, 1000:1 in this case

Thankyou very much indeed....also , presumbaly the "(30)" is the maximum current permitted?

The formula for calc B in a CT is the same as for any other Tx

dB / dt = V / ( N. Ae ), so for a swing of +/- 100mT, with a period of 10mS each way ( 50Hz ) and we assume a ferrite core of 5mm x 5mm cross sectional area, = 25mm^2, then we get a Vave / N = 500 E-6

...thanks this is fantastic...and i assume it applies only to ferrite?...as iron and powdered iron have a permeability which changes as B increases.

The problem i am finding with ferrite for CT's at 50Hz, is that they are either inaccurate...or, if you put up with a low vsec...then they are accurate...but you get very little voltage (signal) and so your resolution is very poor.?

 

[Easy peasy]

why the assumption only for ferrite ?

 

[cupoftea]

Thaks, because as i thought, ferrite has a constant permeability as B increases.

Also, any more news on the old technique of bettering CT output reading by the method of reading the voltage across the secondary coil resistance?...just like they read the coil resistance of a buck inductor by the resistor-less sensing method?

 

[Easy peasy]

there is no "more news" because there was no "old news" a buck inductor is not a CT, quite some way from it in fact.

p.s. in the formula above for dB/dt, there is no dependence on Ur, or Ui.

 

[cupoftea]

there is no "more news" because there was no "old news" a buck inductor is not a CT, quite some way from it in fact.

Thanks, was referring to a way of reading the output (secondary) of a 50Hz ferrite CT that ive seen on this forum a few times before.....it involves application of a somewhat high-ish frequency.......i'll see if i can dig it out, unless you may know of it? Maybe those who know of it won't divulge for entirely understandable and honourable reasons..

--- Updated Aug 25, 2021 ---

why the assumption only for ferrite ?

Thanks...as you know, with iron CTs, (unlike ferrite CTs) , its very easy to get a very high secondary inductance, and thus get a very low secondary magnetising inductance...and thus less distortion to the signal coming out of the CT secondary.

 

[Easy peasy]

there is no better way of reading mains freq current from a CT than using it as a plain CT.

 

[cupoftea]

there is no better way of reading mains freq current from a CT than using it as a plain CT.

Thanks...so in the way that you can "see" the voltage across the buck inductor's ESR, in spite of the high frequency square wave that exists across this inductor (for the purpose of Buck inductor current sensing)......you dont think there is some kind of equivalent thing that can be done with a CT secondary?

..so you are saying that to read the output of a 50Hz mains CT (secondary) , we are stuck with simply amplifying it......thats the only way?

..or if nothing else, there is some way of reducing the common mode noise in the measurement?...and this "unknown" method to reduce common mode noise being quite out of the ordinary?

 

[cupoftea]

OK thanks, i can't tell you any more, but if you know about RCDs , and other fault "situations" that need to be detected on power systems, then you'll probably know what i am referring to in relation to a standard CT in an RCD being used in a "different" way. I have read about it on here , but can’t now find.

I wonder how accurate/repeatable it is in comparison to a “straight” CT setup being used with an burden resistor and simply amplified output?

 

[cupoftea]

Thanks,
I have seen it explained many times on this forum....and it is the best way to use a 50Hz "imbalance" current transformer...(ie, for an RCD function).....the most accurate way by far.......you simply set it up with the line/neutral conductors going though its aperture....any imbalance will start to result in the flow of secondary current.......you then simply already have an (comparator based) oscillator (a few khz) to switch into the secondary coil.....the secondary coil starts to go into saturation as a result...but you have a comparator (of the oscillator) which then switchs the current the other way...ad infinitum.....so you go into deep saturation each way, but dont fully saturate...because as you start to deeply saturate, the secondary current gets switched the other way.....the amount the oscillation goes in either direction depends on the flux level that is in the CT at that instant.....so in other words, you can tell by reading the output of the CT what is the level of current imbalance in the line/neutral conductors....because you get a non-symetry of the oscillating waveform, depending on the flux level and flux "polarity" or "direction" that was already in the CT at that instant......ive seen this explained many many times on this forum and other forums. I am sure you would agree that this is the most precise way to run a 50Hz mains "imbalance" CT? (ie, a CT for an RCD).

...ie, so that you can measure exactly what is the "imbalance" current between live and neutral

https://prorelay.tamu.edu/wp-content/uploads/sites/3/2018/03/BeyondTheKneePoint_6811_20180222.pdf

 

[Easy peasy]

I'm not sure any CT based RCD uses this approach due to cost, and it is no more accurate, usually used for DC and low freq AC sensing

the paper you listed makes no mention of that technique what-so-ever.

 

[cupoftea]

usually used for DC and low freq AC sensing

Thanks, in this case, mains 50Hz...is that what you meant by the "low frequency"?...or lower?

Also, if you need to do both 6mA DC testing, and the AC testing for 30mA imbalance aswell...then doesnt it make sense to use the "fluxgate" type approach, to kill two birds with one stone?

Can you use a "non fluxgate CT" for Fluxgate type sensing?.....if so, is there an associated inaccuracy?....i mean, arent the secondaries very low inductance with fluxgate sensor CTs?