Measuring difference current between live and neutral?

[nightfire]

I think the point was: please take into consideration the original poster's location.

I thought RCDs have to trip on DC, as well as AC nowadays, because there's a lot of equipment which has a rectifier and it's important to detect any leakage on the DC side.

Depending on your location and application scenario, not necessarily.
Here in germany you have typically a RCD Type A für most outlets and circuits.
In applications where DC currents are to be expected (or you really cannot be sure what type of device gets plugged in in the future) you are required to use a RCD type B.
This applies due to work safety regulations to distribution boxes on construction sites, where lots of contractors will use their equipment that they bring with them.

Or you take appropriate measures by choosing another appropriate type of RDC, maybe the Type A-EV for charging stations in EVSE/wallboxes.

[james_s]

Obviously 230V isn't an issue, but 110VAC is sometimes used here on building sites and in some industrial situations. It's supplied by a centre tapped (single phase), or a star connected (three phase) transformer/generator, so both conductors are 55V, or 63.5V, with respect to earth. I haven't seen what wire colours are used in 110V fixed installations, but appliances use ordinary flexible mains cable, with brown, blue and green/yellow conductors. I would hope 110V wiring is marked and kept separate from 230V/400V, irrespective of the colour code used.

I've seen those building site transformers and always thought it seemed like a good idea, at least prior to the development of GFCI/RCD protection. I have no idea what they use for color coding but here there is no difference in the coloring for the different voltages. Live/hot/line can be any color except for white (reserved for neutral) or green (reserved for ground). The most common is black, and the next most common is red, typically seen alongside black in a 240V circuit. 3 phase usually uses the same plus blue but again there is no legal requirement, it's just convention.

[Monkeh]

Obviously 230V isn't an issue, but 110VAC is sometimes used here on building sites and in some industrial situations. It's supplied by a centre tapped (single phase), or a star connected (three phase) transformer/generator, so both conductors are 55V, or 63.5V, with respect to earth. I haven't seen what wire colours are used in 110V fixed installations, but appliances use ordinary flexible mains cable, with brown, blue and green/yellow conductors. I would hope 110V wiring is marked and kept separate from 230V/400V, irrespective of the colour code used.

I've seen those building site transformers and always thought it seemed like a good idea, at least prior to the development of GFCI/RCD protection. I have no idea what they use for color coding

Even with RCDs they're a great safety advantage. RCDs are meant to provide protection in the unlikely even of a shock - reduced low voltage supplies are used where physical damage and unfavourable conditions (wet ones.) are significantly more likely than normal.

For colour coding, technically you would use two browns (remembering it's single phase so there's no polarity, and there's no neutral), but realistically they use exactly the same materials as everything else (kind of the point, really), so it's all brown and blue.

[Zero999]

Obviously 230V isn't an issue, but 110VAC is sometimes used here on building sites and in some industrial situations. It's supplied by a centre tapped (single phase), or a star connected (three phase) transformer/generator, so both conductors are 55V, or 63.5V, with respect to earth. I haven't seen what wire colours are used in 110V fixed installations, but appliances use ordinary flexible mains cable, with brown, blue and green/yellow conductors. I would hope 110V wiring is marked and kept separate from 230V/400V, irrespective of the colour code used.

I've seen those building site transformers and always thought it seemed like a good idea, at least prior to the development of GFCI/RCD protection. I have no idea what they use for color coding

Even with RCDs they're a great safety advantage. RCDs are meant to provide protection in the unlikely even of a shock - reduced low voltage supplies are used where physical damage and unfavourable conditions (wet ones.) are significantly more likely than normal.

For colour coding, technically you would use two browns (remembering it's single phase so there's no polarity, and there's no neutral), but realistically they use exactly the same materials as everything else (kind of the point, really), so it's all brown and blue.

I didn't realise they also have them in the US.

I'm not a fan. They were good back in the days before RCDs, but nowdays the manual handling hazard of lifting heavy transformers and the fact that 55VAC, or 63.5VAC can still deliver a considerable shock, make plain old 230V + an RCD a safer option.

I agree that if you add an RCD, 110V is safer than 230V, but I've not seen that before. If a place wants reduced voltage, they should add fixed wiring and a centralised transformer, or generator and RCDs, rather than those back breaking transformers.

[capt bullshot]

Hi,
Our contractor has selected the following imbalance CT for our 32Arms mains distribution box, its a ZCT409.Do you know why this was chosen?
The idea is to make an RCD, so it needs to detect an imbalance of 30mA between line and neutral.
This CT gives an imbalance current of 500mA rms if the imbalance is 225Arms (!!!)
So if the imbalance is just 30mA, then the output current is just 66uArms.
Do you know why such a CT was selected for this job? The ZCT409 doesnt saturate if the imbalance grows to 225A......which seems good....but when thinking about it.....the fuse/circuit breaker would blow if well under 225A flowed. So why do it like this? We also have a CT on the line conductor, so we could detect overcurrent like that, and have no need of the 225A rating of the ZCT409? What am i missing?
Surely we need an imbalance CT which allows a higher output current when imbalance is 30mA? Surely all we need is to select a CT which doesnt saturate as long as the imbalance is less than 32Arms?...make it 40Amrs for some margin?
Also, as attached, we are using a 130R burden resistor. So even if there was an imbalance of 225A, then the voltage across the burden resistor would grow to 65V, which would blow up the opamp that amplifies the burden resistor voltage (its supplied by +/-12V). Do you know why this has been done?
Please advise on What have i overlooked in my dim-wittedness concerning this?

ZCT409 datasheet:
https://www.ctzentar.com/zero-phase-current-transformer-for-leakage-current-detection-zct409_p28.html

As far as I can see from the "datasheet", this CT gives you 125...150mV into 150R at 500mA difference current.

Rated current means: one conductor can carry 225A, and the other one 225.5A giving you 500mA difference. The core detects the difference only, pretty much independent of the rated current, or actual current flowing through the conductors. This doesn't mean the CT will translate 225A difference into 65V at 130R. The core most probably will saturate somewhat above the rated 500mA difference current.
Anyway, the CT might have been chosen because it's got a large opening, so the wires are easy to pull through, not for the 225A rating. All the others are rated for less than 30mA difference current, so this one most probably is the correct choice for your requirement to detect 30mA difference. Lower rated cores (25mA or 22.5mA) might saturate before the output reaches the 30mA threshold. On the first glance, it looks oversized though.

[Monkeh]

If a place wants reduced voltage, they should add fixed wiring and a centralised transformer, or generator and RCDs, rather than those back breaking transformers.

Major sites do. The transformers are to use the tools where the infrastructure isn't present. There's not much point in using an RLV supply if you just trail huge 240V extensions around the place, is there?

[Zero999]

If a place wants reduced voltage, they should add fixed wiring and a centralised transformer, or generator and RCDs, rather than those back breaking transformers.

Major sites do. The transformers are to use the tools where the infrastructure isn't present. There's not much point in using an RLV supply if you just trail huge 240V extensions around the place, is there?

I agree, although I've seen those yellow transformers connected to long extension leads, on many occasions. It's common practise where I work now and has been at many other companies, I've worked at. I'd much rather use 230V tools, with an RCD, which should be mandatory for all appliances run of >50VAC, if they're used in places where cables can easilly be damaged, or water ingress is an issue.

[Terry Bites]

Dont play with electricity. You're too young to die.

[james_s]

I didn't realise they also have them in the US.

I'm not a fan. They were good back in the days before RCDs, but nowdays the manual handling hazard of lifting heavy transformers and the fact that 55VAC, or 63.5VAC can still deliver a considerable shock, make plain old 230V + an RCD a safer option.

I agree that if you add an RCD, 110V is safer than 230V, but I've not seen that before. If a place wants reduced voltage, they should add fixed wiring and a centralised transformer, or generator and RCDs, rather than those back breaking transformers.

They don't have them here, I saw them in England though. My friend had one that he was using to power American lamps, it was something someone had given him and he explained what it was supposed to be used for. I've never seen one being used in its native environment on a building site though, I assumed it was something they would set up by the power point and leave it there then run extension cords to any tools that needed it. These days cordless tools have gotten so good that there is less and less need for any kind of direct mains powered stuff on jobsites other than large stationary tools like table and radial arm saws.

[Monkeh]

I've never seen one being used in its native environment on a building site though, I assumed it was something they would set up by the power point and leave it there then run extension cords to any tools that needed it.

Indeed. On major sites a very large transformer is provided and the 110V centre-tapped supply is spread around. The transformers are so the tools can be used outside of these environments, rather than duplicating tools.

Cordless tools have indeed killed off a lot of this stuff. You do still see big concrete breakers and the like, and site lighting of course, plus manly drills (core drills).

[Faringdon]

Rated current means: one conductor can carry 225A, and the other one 225.5A giving you 500mA difference. The core detects the difference only, pretty much independent of the rated current, or actual current flowing through the conductors. This doesn't mean the CT will translate 225A difference into 65V at 130R

Thanks for raising this issue...the 225A conductors are not part of this CT, so  i dont know why they bother to specify it?...and  as you   rightly point out....its the difference current that this CT is for, not the absolute values of the currents in the  individual conductors......?...or is it kind of just in case of a problem (fault), and one conductor carrys 225A and the other carries near nothing?

[capt bullshot]

Rated current means: one conductor can carry 225A, and the other one 225.5A giving you 500mA difference. The core detects the difference only, pretty much independent of the rated current, or actual current flowing through the conductors. This doesn't mean the CT will translate 225A difference into 65V at 130R

or is it kind of just in case of a problem (fault), and one conductor carrys 225A and the other carries near nothing?

That'd be a permanent wiring fault, combined with a fault of your device - because the attached circuitry is supposed to break the circuit very quickly under such condition. I don't think this is the reason for the 225A rating, but I don't know for sure.
Otherwise, as the core will saturate at 225A primary current flow, it'll heat up. Maybe the 225A is some kind of limit for "it just doesn't start to burn" under such condition.
Maybe it's just the intention of the manufacturer: this core is large enough to pull wires rated for 225A through?

[geggi1]

The simplest way to measure the difference of current in two conductors is in a differentiation transformer. The simplest differentiation transformer is a torriodal transformer that you already got and just run phase and neutral through the transformer. Because you are measuring AC the vector sum of current in phase and neutral will be zero when the currents are equal. When there is a difference between current i phase and neutral you will get the vector sum on the secondary side. If you are measuring the secondary side with a high impedance MCU or voltmeter you will probably have to make some kind of loading on the secondary side.

[connectTek]

In Australia it's "Active and Neutral"

[Faringdon]

Hi,
Is the aforementioned ZCT409 CT actually a fluxgate sensor?

ZCT409:
https://www.ctzentar.com/zero-phase-current-transformer-for-leakage-current-detection-zct409_p28.html

One of the wire pairs is about 140mH so is obviosuly the secondary, but the other is some 330uH...so could be a "field coil" as used in some types of fluxgate transducer?

[EPAIII]

Well, silly me! I always considered no wires to be safe to touch until I have tested them. WITH A NEON STYLE TESTER THAT WAS TESTED FIRST ON A KNOWN LIVE CIRCUIT.

Silly me!

With over 50 years of experience, I have been bit, but not recently.

Actually "Live" is normally often used here in the UK...

E.g.

Yeah, you'll find a lot of such examples.

Because something is a common mistake doesn't make it right; quite the opposite, it's even stronger reason to spend effort trying to set the record straight. This is safety critical terminology mistake. And terminology is important because that connects to our thinking, which affects our assumptions.

It's a common misassumption that neutral is "safe" because it's not "live", and this only originates from people accidentally labeling the other wire "live". This false assumption kills people, because line and neutral are often swapped, either completely legally in nonpolarized plugs, or due to fairly usual installation mistakes. So both wires need to be considered live regardless of color or label; if you don't, you are taking a serious risk up to 50% in case of nonpolarized plugs.

That's also the reason why PE is made so different; color is nowadays a special two-color mix which makes us pay special attention; the prong(s) in the plug is/are located in such way they stand out and swapping PE with anything else is visually obvious. Yet even with these precautions, swapping PE with either of the live wires does happen and people die as a result. It's just that swapping line and neutral is like 100 or 1000 times more common.

It's line and neutral.

surely you mean phase and neutral

Depending on country, yes. "Phase and neutral" or "Line and neutral". Line is a crappy word because it's too easy to mix with live, I'd prefer "phase" everywhere. In three-phase systems, L1, L2, L3 refer to three lines. On the other hand, phases are often called U,V,W. Same thing, different names.

It's not an error. Phase, or line, was always called live in older standards and documentation. The terms have just changed over time, with the intention of making it easier to understand.

It's actually quite unusual for live and neutral to be swapped in the UK, because installations are always checked before being signed off. In older installations, without RCDs, it's also possible for the neutral and earth to be swapped and it will carry on working without any problem. The only time the neutral is hazardous, is if it''s broken.

I do agree one shouldn't assume the neutral conductor is safe to touch, but there's nothing wrong with using the term "live" to denote the phase conductor. It may be a little archaic, but it's still correct.

Anyway, it's trivial to bias a bipolar signal around 1.5V or 2.5V, so it can be read by a microcontroller's ADC input.

Thanks yes, and we coudl amplify it up with INAMPS, and then precision rectify it...the thing is , we need to detect the imbalance current as accurately as possible first.
We wonder whether we should use an imbalance CT of iron, iron laminate, powdered iron, or ferrite, and if ferrite, NiZn, or Mn Zn?

The links I gave are to Hall effect sensors. Both live and neutral wires go through the same hole. Under normal operation, the magnetic fields produced by the currents cancel, but if there's a leakage, it's not cancelled and measured by the sensor.

There is no need for any instrumentation amplifiers and rectifiers. A single op-amp, with some resistors can perform level shifting and amplification and the rectification done in software.

Example:

Output voltage from the transducer =  +/-1.2V
Voltage required for MCU ADC input: 0V to 5V
Reference voltage: 5V

I've just realised, the sensor I linked to gives +/-1.2V out, relative to a reference terminal, so no level shifting is required. Presumably the reference is around 2.5V, so you'll get 1.3V to 3.7V out. A non-inverting amplifier, with a gain of 2 could be used to give 0.1V to 4.9V out, but it's probably not worth it.

http://www.multimic.com/e/products/detail/1791

[EPAIII]

The simple fact that the opening is larger may very well be why that CT was chosen. You guys may be greatly over-thinking this.

...<snip>...

Please advise on What have i overlooked in my dim-wittedness concerning this?

...<snip>...

...<snip>...

Anyway, the CT might have been chosen because it's got a large opening, so the wires are easy to pull through, not for the 225A rating.

 ...<snip>...

[capt bullshot]

Hi,
Is the aforementioned ZCT409 CT actually a fluxgate sensor?

ZCT409:
https://www.ctzentar.com/zero-phase-current-transformer-for-leakage-current-detection-zct409_p28.html

One of the wire pairs is about 140mH so is obviosuly the secondary, but the other is some 330uH...so could be a "field coil" as used in some types of fluxgate transducer?

I don't think so, because a flux gate sensor would require somewhat more complex electronics than just the resistor that is specified. It might be a "test" coil, to inject some differential current to test the system.

[Gyro]

Well, silly me! I always considered no wires to be safe to touch until I have tested them. WITH A NEON STYLE TESTER THAT WAS TESTED FIRST ON A KNOWN LIVE CIRCUIT.

Silly me!

With over 50 years of experience, I have been bit, but not recently.

I see no relevance to the discussion. 

[Faringdon]

I don't think so, because a flux gate sensor would require somewhat more complex electronics than just the resistor that is specified.

Thanks, is there some "hackers" way of making an accurate fluxgate sensor out of a standard 50Hz mains CT torroid? (ie, out of a torroid thats meant for use as a normal NP/NS = IS/IP function, no switching as in fluxgate).

Also, to turn a "Normal" CT torroid into a fluxgate type sensor, does it really require that much electronics extra?.....isnt it just an oscillator to switch voltage across the secondary till it nearly saturates in each direction.....?....then amplify what you get out of the secondary after doing that switching....and read it with a microcontroller ADC which interprets it and finds the "imbalance current" , aswell as any DC current level?

[uer166]

Also, to turn a "Normal" CT torroid into a fluxgate type sensor, does it really require that much electronics extra?.....isnt it just an oscillator to switch voltage across the secondary till it nearly saturates in each direction.....?....then amplify what you get out of the secondary after doing that switching....and read it with a microcontroller ADC which interprets it and finds the "imbalance current" , aswell as any DC current level?

fkn LOOOOOL, I'm sorry this entertainment is what keeps me from ending it all some days.

[james_s]

Well, silly me! I always considered no wires to be safe to touch until I have tested them. WITH A NEON STYLE TESTER THAT WAS TESTED FIRST ON A KNOWN LIVE CIRCUIT.

Silly me!

With over 50 years of experience, I have been bit, but not recently.

I see no relevance to the discussion. 

The relevance is that a wire should always be considered potentially energized until proven otherwise and treated accordingly. This is regardless of what color it is or whether the name we used to refer it implies that it is dangerous or not.

[Faringdon]

fkn LOOOOOL, I'm sorry this entertainment is what keeps me from ending it all some da

Thanks, i take it youre saying its not possibel to convert a normal CT torroid for imbalance current detection, eg ZCT409, into a fluxgate sensor?...and measure up to 200mA DC and an AC imbalance of 30mA on a 32Arms connection.?

zct409
https://www.ctzentar.com/zero-phase-current-transformer-for-leakage-current-detection-zct409_p28.html