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Discussion Starter · #1 ·

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Unfortunately it doesn't say it tolerates DC, only that it tolerates a DC pulse, so that makes it equivalent to a Type A RCD in function, not the Type B, F or EV required for steady DC tolerant earth leakage protection. There are several sensors around that work just fine when it comes to being DC earth leakage tolerant, though.
 

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Hi,
How to use a standard torroid for CT ...for use as RCD and DC fault current sense?
Ie, just like a standard CT like the ZCT409.

In a 32Arms 50Hz wall box.


ZCT409
So this post is just ( another) advert for some Chinese made tat, that has to perform a safety function, and is made to undefined QA and probably unreliable if any certification, right?
Sound ideal for a safety function...NOT
 

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Discussion Starter · #4 ·
Hi
Thanks, the thing is, as you see, the ZCT409 has two coils...one is some 150mH, and the other is approx 330uH. As you know, Line and neutral go through the aperture. The 330uH coil can be used like a fluxgate coil to pulse into it and so reduce the field in the core, as part of the feedback mechanism in "closed loop fluxgate sensor" style. At the same time, one puts a square wave of some few khz into the 150mH coil.......so that the core goes into and out of saturation in either direction.....the thing is, the symetry of the current in either direction depends on the imbalance current between line and neutral.........so you see, what the ZCT409 gives you is a fluxgate sensor.!!!.....highly accurate, able to detect imbalance current of between 15 to 30mA in the phase/neutral. It is also able to detect DC fault current. It can detect DC currents accurately, at levels from 5.4mA to 200mA.
So its a fluxgate, and very cheap, why arent more people doing this?
 

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Hi
Thanks, the thing is, as you see, the ZCT409 has two coils...one is some 150mH, and the other is approx 330uH. As you know, Line and neutral go through the aperture. The 330uH coil can be used like a fluxgate coil to pulse into it and so reduce the field in the core, as part of the feedback mechanism in "closed loop fluxgate sensor" style. At the same time, one puts a square wave of some few khz into the 150mH coil.......so that the core goes into and out of saturation in either direction.....the thing is, the symetry of the current in either direction depends on the imbalance current between line and neutral.........so you see, what the ZCT409 gives you is a fluxgate sensor.!!!.....highly accurate, able to detect imbalance current of between 15 to 30mA in the phase/neutral. It is also able to detect DC fault current. It can detect DC currents accurately, at levels from 5.4mA to 200mA.
So its a fluxgate, and very cheap, why arent more people doing this?

Yes, I do understand, but it seems you may not. To work as a DC tolerant residual current sensor that complies with the requirements means that it must still meet the requirement to detect an imbalance, with no change in sense capability, in the presence of 6mA of DC current superimposed on the AC current. In other words, the coil must not be affected by the steady magnetic field from this DC current. There is nothing in the specification for that sensor that suggests that it meets this requirement, it seems to be a standard type of AC sensor that has a pulsating DC tolerance, but not a steady DC tolerance. Without that tolerance to DC then that sensor is of no use for helping to protect a charge point.

I'm getting the strong impression that you are not really producing a new charge point design here in the UK at all. I also get the impression that you have very little understanding of the requirements and regulations that apply here. I have sugegsted that you just read up on the applicable requirements in another thread, yet it seems you may not have understood them, and what is needed in order to ensure compliance.
 

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Discussion Starter · #6 ·
that complies with the requirements means that it must still meet the requirement to detect an imbalance, with no change in sense capability, in the presence of 6mA of DC current superimposed on the AC current. In other words, the coil must not be affected by the steady magnetic field from this DC current. There is nothing in the specification for that sensor that suggests that it meets this requirement, it seems to be a standard type of AC sensor that has a pulsating DC tolerance, but not a steady DC tolerance. Without that tolerance to DC then that sensor is of no use for helping to protect a charge point.
Thanks, but how do you know it doesnt have the capability to do proper imbalance testing in the presence of the said DC fault current?.....surely there is no way you can tell this?

This CT doesnt have to be used as a normal CT, it can be used "fluxgate style"...and thats where the DC capabilty with RCD capability is opened up.....its easy.....why nobody doing it?
 

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Why not just include this in your design, then, do the required compliance testing and then show us the completed and fully compliant charge point?

The DC tolerance requirement isn't to do with using fluxgate sensing, it's simply ensuring that the magnetic flux within the sensing system caused by any DC current up to 6 mA does not alter the sensing characteristic of the sensor. The most common issue is saturation from the DC induced magnetic core flux. This shifts the working point of the sensor and may change the sensitivity and that is not acceptable for a device that is required to trip at a defined imbalance current.

As before, please just read the requirements. All the information you need is in the documents I referenced in the other thread. If you can show compliance with those requirements in your completed design then that's fine, but trying to design a new product via a debate on an internet forum is neither professional or likely to result in an optimised design.
 

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Discussion Starter · #8 ·
The DC tolerance requirement isn't to do with using fluxgate sensing, it's simply ensuring that the magnetic flux within the sensing system caused by any DC current up to 6 mA does not alter the sensing characteristic of the sensor. The most common issue is saturation from the DC induced magnetic core flux. This shifts the working point of the sensor and may change the sensitivity and that is not acceptable for a device that is required to trip at a defined imbalance current.
Fluxgate sensors actually run the core up to saturation anyway, so they are not affected by saturation......fluxgates are the well known solution for sensing AC and DC, with the best accuracy. You just need a micro to read the output and interpret it.
 

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Fluxgate sensors actually run the core up to saturation anyway, so they are not affected by saturation......fluxgates are the well known solution for sensing AC and DC, with the best accuracy. You just need a micro to read the output and interpret it.

I have used fluxgate sensors 30 odd years ago as compass sensors, like this one from 1992, so am pretty familiar with the way they work, thanks (just pulled this gimballed flux gate sensor from a shelf by my desk, for example):

149135


Designing a charge point that is compliant with the requirements and regulations and has all the required protection isn't at very challenging. I have built several home made charge points over the years, and one of those had been in daily use for several years until I chose to replace it (nothing wrong with it, just wanted a tidier looking enclosure, so built a new one).

What I do not understand is whether you are really designing a new charge point for the UK market, or whether you are just playing some sort of game on here.
 

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Discussion Starter · #10 ·
Thanks, do you agree that "any" 30mA RCD CT for EVSE can also be used, with some extra circuitry, to allow it to detect the DC 6mA? Basically, it wont saturate because you have a comparator to switch the coil current the other way just as saturation starts to get deep......why is everyone spending a fortune on LEM RCDs for EVSE (30mA AC and 6mA DC), when you can just make one yourself, out of a cheap CT?

For a fluxgate CT, does it need a wide or narrow BH loop........ditto for an AC CT?
 

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Why don't you just complete the design of your charge point, then let us review it here?

Be far simpler and easier for all of us, rather than have these endless questions about very basic and fundamental requirements, and the ways to meet them.
 

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Discussion Starter · #12 ·
.......the reason we dont complete is because we are stuck on the 30mA/6mA current sensor.....either an very expensive LEM type, or a cheapo CT torroid with an oscillator to pulse into its secondary, and make it into a Fluxgate?

We want to use the cheap, but if it really works, then why are people paying a fortune for LEMs?

There must be something wrong with using a cheap ZCT409 torroid as a 30mA/6mA fluxgate, but what is it?

.................
Also..
Some fluxgate sensor questions?
1....Can an RCD fluxgate sensor be done with a standard CT with a single coil? eg a torroid through which the power conductor pair are meant to simply traverse the aperture.
2....What makes a fluxgate sensor accurate?.....is it the feedback loop?
3....If a batch of fluxgate cores have different saturation current levels, will they not give different current readings?
4....If a batch of fluxgate cores have varying BH loop widths, then wont they give different current readings?
5....If a batch fo fluxgate cores are pulsed with different frequencies, then wont they give different current readings?
 

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It seems to me that you want to use the members of this forum to give you a free design consultancy service, that you can then incorporate into a product to sell back to the very group that you keep asking to give you free product design advice.
 

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Discussion Starter · #14 · (Edited)
Thanks, but just to request can a 30mA/6mA Fluxgate sensor for EVSE be made with a torroid with a single coil on it? (the phase/neutral conductors going through the torroid aperture).

As you see below, an offtheshelf 30ma/6ma RCD is £114.....


why people pay this when they can make themselves?
 

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Discussion Starter · #16 ·
Hi,
We wish to make a RCD for an EVSE single phase “wall box”. We will use a simple CT for the 30mA imbalance detection. This may get “blinded” by any DC leaking in either line or neutral.

Therefore, we will also have a second CT the same as the first one, but will constantly also inject 30mA AC 50Hz through its aperture with a wire loop. As soon as this second CT does not detect the 30mA , then that means it has been “blinded” by a DC current flow in either line or neutral, so therefore we turn off the relay.

(We will inject the 30mA with constantly alternating polarity.)

Would this pass regulations? Why not? It surely guarantees safety, and is far cheaper than offtheshelf RCDs for 30mA/6mA which are £30 upwards.
 
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