Earth leakage current measurement - Page 1
Forum section: electronics / projects

author: adam4521
author_rank: Regular Contributor
timestamp: November 06, 2021, 11:07:27 pm
content: I'm designing a power quality measurement/display project -- isolated ADC plus microcontroller plus Raspberry Pi/display -- the idea is you can plug a mains powered device in and it will show real time voltage/current waveforms, harmonic currents, RMS measurements, power factor etc. The acquisition ADC I'm favouring is MCP3919 which is economical and designed for this kind of application. I'm planning to use resistor dividers and shunts for voltage and current measurement pretty much as per the application notes for that ADC series.A feature I'm thinking about including is leakage current measurement. This needs to detect differential current between live and neutral and be quite sensitive, most electronic equipment will have EMC filters that leak less than 0.5mA to earth/ground.My current thoughts:1. Hall effect devices are not sensitive enough.2. I will not be able to measure live and neutral currents though separate shunts and then subtract them after ADC. The difference current is a tiny fraction of the total current, likely much smaller than ADC resolution.3. I'm not keen on using series shunt in the CPC and measure voltage drop because of compromise of electrical protection (but I'm interested in members thoughts on this).4. It seems that I could use a sensitive current transformer on the L and N conductors and pickup the difference current from that. Some handheld leakage current testers such as UT251A do advertise a resolution down to 1µA.From what I have researched so far about 'zero sequence current transformers' is that they have a high permeability magnetic core for sensitivity, and presumably a low hysteresis loss. Not ferrite, but permalloy tape (or similar). Maybe I could wind the live and neutral conductors through it a few times to increase the gain/SNR.Am I thinking on the right lines or is there a better way of doing this?

author: pqass
author_rank: Super Contributor
timestamp: November 07, 2021, 12:20:41 am
content: OpenEVSE has a GFCI circuit built-in. See:https://www.barbouri.com/2017/03/10/diy-open-evse-v4-23/#attachment_587Just drop the final comparator and feed your ADC from the cathode of D3; but you'll likely need to scale it and/or redesign around 3V3 operation or your ADC Vref. It uses thishttps://store.openevse.com/products/gfci-current-transformertransformer. By wrapping both N and L through it (once or several times) any leakage will be seen as a non-zero AC wave. The transformer also has another 5-turn coil to test that the GFCI operation is working. This may be interesting to you for calibration purposes.Also, for determining power factor, this may be interesting:https://mousa-simple-projects.blogspot.com/2017/12/power-factor-measurment-using-arduino_18.html

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 07, 2021, 04:38:04 am
content: Quote from: adam4521 on November 06, 2021, 11:07:27 pmUse this "ground loop breaker" circuit as the shunt:https://sound-au.com/earthing.htm#s9

author: Terry Bites
author_rank: Super Contributor
timestamp: November 07, 2021, 06:24:54 pm
content: seehttps://www.ebme.co.uk/articles/electrical-safety/electrical-safety-tests

author: geggi1
author_rank: Frequent Contributor
timestamp: November 07, 2021, 07:55:28 pm
content: You could make a trifilar current transformer.Fiurst you make the primary side of two windings of a higher cross-section capable of handling the load current. By running phase on one winding and neutral on the other winding you got a differential transformer.The secondary side is of a thinner cross-section and should have some kind of resistor loading to limit the maximum voltage. Run the DAC directly of the secondary side but do some testing prior to connection to ensure tat you dont pass the maximum rating.The differential transformer will only give out the difference between L and N. The ratio between primarys and secondary side will give an output current that is converted to voltage by a shunt resistor (loading)

author: adam4521
author_rank: Regular Contributor
timestamp: November 08, 2021, 03:03:04 pm
content: Thanks everyone -- really interesting references that I hadn't seen elsewhere. It's interesting that a number of the formal test approaches do actually insert a test device in the protective conductor. I suppose if you put the big diodes in parallel across the shunt, the protection should be ok! I've ordered sample differential current transformers so I'll find the sensitivity/transfer function of those first to see if they are good enough, and consider the shunt resistor + diodes as 'Plan B'.

author: Zero999
author_rank: Super Contributor
timestamp: November 08, 2021, 03:13:56 pm
content: A current transformer is only any good for detecting AC leakage. If the device has a rectifier and there's leakage after that, it won't be detected by a transformer. What's worse is DC leakage can blind a transformer to AC leakage, by causing core saturation.There are Hall effect sensors specifically designed to measure leakage current, but they're not cheap. What's your budget?https://fastron.com.au/products/ct-04-p

author: fcb
author_rank: Super Contributor
timestamp: November 08, 2021, 03:24:23 pm
content: A balanced transformer will only measure AC difference.Measuring the current though a resistive shunt in the earth line would be the 'gold standard' method for an R&D/test lab. The shunt should be low-Z enough and high-current to meet any legal requirements, but that isn't as onerous as you think.

author: Zero999
author_rank: Super Contributor
timestamp: November 08, 2021, 05:57:52 pm
content: Quote from: fcb on November 08, 2021, 03:24:23 pmIt's not that simple. For example, if the device needs to be connected to other equipment, the leakage current might flow through other protective earth conductors.

author: fcb
author_rank: Super Contributor
timestamp: November 09, 2021, 07:50:25 am
content: Quote from: Zero999 on November 08, 2021, 05:57:52 pmIndeed. However, if the OP is building a device for temporary analysis then you can control for other leakage paths. The problem is that measuring the imbalance (down to DC) between L&N in the presence of considerably bigger signals would be tough.An example might be an EV - if you wanted to measure the earth leakage (say to 0.1mA resolution) whilst charging at 32A; it would be much easier to measure 0.1mA across a shunt than accurately measuring (L) 32.0001 and (N) 32.0000.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 10, 2021, 03:48:34 pm
content: Maybe you can get it working with a current transformer.But maybe go back to the idea of a shunt resistor.Use some beefy (30A +) diodes so the diodes can handle the full mains rated current and connect them anti-parallel to the resistor. Then you can dimension your shunt resistor to have up to a few hundred mV before the diodes start conducting.The easiest way to get some high current diodes is to use the body diodes of MOSfets.

author: EPAIII
author_rank: Super Contributor
timestamp: November 11, 2021, 01:49:42 am
content: Just curious here, what kind of losses can you expect when using the trifilar transformer? Just asking for a ballpark answer.Quote from: geggi1 on November 07, 2021, 07:55:28 pm

author: Smokey
author_rank: Super Contributor
timestamp: April 02, 2023, 06:53:15 am
content: Interesting discussion. What was the actual solution?

author: geggi1
author_rank: Frequent Contributor
timestamp: April 02, 2023, 09:51:45 am
content: If it is AC leakage you are after you can use a differential transformer.All the live wires are running through a toroide core but not the PE.The secondary side is a multi turn winding.Because all currents are suppose to be the same the current on the secondary in zero when there are no earth leakage.If you have a earth leakage there will be a current.

author: jonpaul
author_rank: Super Contributor
timestamp: April 02, 2023, 12:22:59 pm
content: In 1980s..1990s, We designed and consulted on power supplies and medical electronicsMeasurements and leakage requirements ( eg UL, VDE) dépend on your location, goals and types of equipment being tested, eg double insulation appliances, tools, earthed metal housed electronics, medical equipment, lighting.....There exist many professional power Analyzers and Hypot testers, like Rod L, Yokogawa, Fluke, etc.Great examples.Common mode mains leakage standards and regulations cover a wide range of RMS AC current limits, eg5 ma max to be dangerous to life100 uA medical equipment10 uA patient connected medical deviceHypot leakage voltage levels can be 2500V.Consult the standards or regulations in your location for appropriate leakage current levels.I doubt if any of the current sense techniques mentioned, cover this wide range,of common mode leakage current.Bon courageJon

author: Zero999
author_rank: Super Contributor
timestamp: April 02, 2023, 01:19:02 pm
content: Another thing to note about Hall effect sensors is the sensitivity can be boosted by adding multiple turns, so a device designed to sense up to 400mA, could be scaled down to 40mA.

author: DavidKo
author_rank: Frequent Contributor
timestamp: April 05, 2023, 08:01:18 am
content: If you want to measure the ground leakage current. The device MUST HANDLE THE SHORT CIRCUIT current. The ground is used for safety reasons and should not be disconnected in case of the failure and should carry all current before the breaker disconnects the device (for example the phase can get in contact with metal part, the peak current will be higher than breaker/fuse current). So any device you will use for residual current measurement should handle the short circuit current and also the voltage drop should be safe for the environment, where it will be used. Worst case is that the ground wire will take over all working current instead of neutral in case that the ground residual-current breaker is not used.

author: jbb
author_rank: Super Contributor
timestamp: April 05, 2023, 07:36:00 pm
content: It might be a bit expensive, but LEM have a part for that…https://www.lem.com/en/ctsrIf you’re only making a couple of units it could be worth it to save your time.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 05, 2023, 07:51:35 pm
content: Have you looked into Energy Metering ICs (AD, Ti and others) they have all the current and voltage sensing interfaces built into the smart ADC - they're precise and low cost too.Some have a high impedance current sense amp. That lets you switch in a high value sense resistor for leakage measurements.

author: Zero999
author_rank: Super Contributor
timestamp: April 05, 2023, 08:50:48 pm
content: Quote from: jbb on April 05, 2023, 07:36:00 pmI've used an LEM device before and it was very good. I don't consider it to be expensive, for what it is. As I said, the sensitivity can be increased by adding more turns.https://uk.farnell.com/lem/ctsr-0-3-p/current-sensor-0-3a-voltage-o/dp/2664154