Fixture to safely measure mains AC amperage - Page 1
Forum section: electronics / testgear

author: fazalmajid
author_rank: Newbie
timestamp: June 05, 2022, 01:25:31 pm
content: This may be a silly question, but I would like to safely measure current on an AC mains circuit to calibrate power-measuring switches, by putting my multimeter in A mode in series with the mains connection. I'd rather use a fixture designed specifically for this rather than rig one up myself, but I have no idea to look out for. So ideally a box that has both a male and female BS1363 plug/socket, and test leads that go into the multimeter, as that seems safer than having exposed points to plug multimeter probes into. I haven't found anything like this, and have no idea what search terms to look for.Thanks in advance!

author: AVGresponding
author_rank: Super Contributor
timestamp: June 05, 2022, 01:39:40 pm
content: How much current are we talking about? And what kind of accuracy and precision? Most handheld DMMs are limited to 10A, and at that you can only measure for a short time and need to let the shunt cool off before measuring something else.Usually for something like this you'd use an inline power meter (for domestic use, like one ofthese), or in a commercial/industrial setting you'd use a good quality clamp meter or a much more expensive power analyser like one ofthese.

author: fazalmajid
author_rank: Newbie
timestamp: June 05, 2022, 01:47:15 pm
content: Thanks. I'm looking at 5% accuracy, 13A current max (in practice, closer to 1/10 of that).An inline power meter is probably what I need to get, I am just dubious on the accuracy of the £16 made-in-China ones you get on Amazon and would like something actually calibrated.

author: BeBuLamar
author_rank: Super Contributor
timestamp: June 05, 2022, 01:54:39 pm
content: Use a current clamp meter. Accuracy is slightly better than 5% in most case and can measure to very high current. Significantly higher accuracy would cost a lot of money.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 05, 2022, 01:58:13 pm
content: 5% is easily within the capability of a current clamp, and it would not be difficult to make an inline breakout box that gave you a double-insulated loop of the neutral conductor to use the clamp on. The breakout box would not need to be calibrated.Something like this would be perfectly adequate:https://www.rapidonline.com/voltcraft-vc-310-digital-clamp-meter-64-33282.5% basic accuracy, 2, 20, 200A ranges, and only £20. No need to spend more.Or a Uni-T 210 off Amazon etc

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2022, 04:01:56 pm
content: A cable with a current transformer would be an option too for higher currents. As the core is closed the current transformer can be more accurate than a clamp meter with the extra mechanics.If the current is smaller (e.g. 100 mA range) one could use a few primary turns on the current transformer.

author: HKJ
author_rank: Super Contributor
timestamp: June 05, 2022, 04:22:12 pm
content: In US you can get this adapter for clamp meters:But I have not seen it in Europe.I have made my own test cable (For my power meter):This is for both voltage and current, a current only version would be safer.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2022, 05:16:30 pm
content: There is also the option to get one of those cheap (e.g. $20 range) power meters to plug in the receptable. Many also work OK to measure the mains voltage and current in addition to the power.

author: bdunham7
author_rank: Super Contributor
timestamp: June 05, 2022, 05:24:55 pm
content: https://www.newtons4th.com/wp-content/uploads/2020/06/Break-out-Box.pdf

author: Gregg
author_rank: Super Contributor
timestamp: June 05, 2022, 05:36:51 pm
content: Big Clive uses a Hopi inline meter that shows amps, watts, power factor and Hzhttps://www.aliexpress.com/item/2251832189644565.htmlBut I see lately he has replaced it with another similar or perhaps a clone.

author: nightfire
author_rank: Frequent Contributor
timestamp: June 06, 2022, 02:01:09 am
content: For spontaneous measurements of mains current a standard current clamp is the best way to go.Especially considerung that the area you are probably working at is CAT III or even CAT IV, a current clamp also protects you due to the galvanic isolation.For permanent installation, some power converters exist, that are clippable around the hot wire(s). Depending on the type, on the secondary you can get whether current or voltage.Mostly those are used to interface to stationary meters, but there are also add-ons for multimeters, like the Fluke i410- here the current gets measured and presented to the DMM as voltage.

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: June 06, 2022, 03:20:04 am
content: In the US, I use the orange splitter HKJ suggested, along with a Fluke 375 FC for 2% accuracy +/- 5 digits, Since you requested “safely” in your question, it is by far the safest way to do the measurement, not to mention the ability to log and monitor with Fluke Connect remotely for utmost safety. With the flexible loop, it can handle up to 2500A, it is higher than anything I would want to get close to.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 06, 2022, 01:05:39 pm
content: Are you wanting to take visual ad hoc measurements, or continuous high rate, or auto logged, or ....

author: fazalmajid
author_rank: Newbie
timestamp: June 06, 2022, 05:49:00 pm
content: I am trying to calibrate IoT power plugs with power metering function (2Nice/Gosund UP111) running the Tasmota open-source firmware, using a calibrated multimeter (or inline power meter) as a reference. So I would put a purely resistive load like heat lamps (since incandescent bulbs are hard to get now) on a dimmer, change the dimmer until I get 10W, 20W, 30W etc on the calibrated meter, then plug it into the UP111, see its readings and tabulate them. Of course inductive loads and power factor correction introduce another wrinkle.

author: fazalmajid
author_rank: Newbie
timestamp: June 06, 2022, 05:50:41 pm
content: Quote from: HKJ on June 05, 2022, 04:22:12 pmThanks, HKJ! I am a big fan of your battery and charger reviews.

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2022, 06:03:48 pm
content: Quote from: fazalmajid on June 06, 2022, 05:49:00 pmIf you are wanting a PF near 1.0, you need to use a variac and not a dimmer. A phase-control (dimmer) circuit is likely going to result in some pretty poor results from current clamps and probably your IOT power plugs. A P3 KillAWatt or the (direct) amp measurement of a good DMM with TRMS will be much better.

author: nightfire
author_rank: Frequent Contributor
timestamp: June 06, 2022, 06:20:31 pm
content: Some other version sold in germany via Conrad etc.:https://ht-instruments.de/produkte/optionales-zubehor/line-splitter/But: With those comparatively low currents ordinary current clamps have lots of tolerance- with a light bulb of 10 Watts or maybe 100 Watts, we are talking at 230 VAC something around 40mA to maybe 150 mA - thats not much for those big clamps that go easily up to 400 A.And leakage current clamps usually work well below 20 mA.

author: fazalmajid
author_rank: Newbie
timestamp: June 06, 2022, 06:39:05 pm
content: So there seems to be a UK BS1363 version of the AC line splitters, theT.I.S. 454. Unfortunately, it also seems to be discontinued...It does seem they have a newer version, theT.I.S. 455, but I can't find any resellers yet. It seems to be just a rebadged version of theHT Splitterthat nightfire mentioned.

author: factory
author_rank: Super Contributor
timestamp: June 06, 2022, 06:50:52 pm
content: Did you check stock at your nearest store, my local one claims to have two in stock.David

author: themadhippy
author_rank: Super Contributor
timestamp: June 06, 2022, 07:22:56 pm
content: If you can live without the x10 just wire a plug and socket up in singles

author: alm
author_rank: Super Contributor
timestamp: June 06, 2022, 09:14:56 pm
content: Quote from: bdunham7 on June 06, 2022, 06:03:48 pmWhat is the advantage of a TRMS DMM over a TRMS current clamp here that makes the DMM so much better? Crest factor? Bandwidth? Comparing the earlier mentioned Fluke 375 to Fluke 175 doesn't show much difference: 500 Hz bandwidth, crest factor of up to 3 for the clamp, compared to 1 kHz bandwidth for ACI, crest factor of up to 3 for the DMM.Obviously measuring current is not a good indicator of power as soon as there's any phase shift between voltage and current, like with a capacitive or inductive load. A DMM is no better than a clamp meter there. For this you either need to do digital or analog multiplication of instantaneous voltage and current, like decent plug-in power meters do.

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2022, 10:21:36 pm
content: Quote from: alm on June 06, 2022, 09:14:56 pmYes, mostly bandwidth...and I wasn't thinking of the 175 as a 'good' DMM in this context. Nothing against it, but as you point out, it isn't any better than a decent clamp for this. Many DMMs have more suitable ACI BW of 10kHz and more.QuoteYes, power is different, but the EEVBlog 121GW and the P3 KillAWatt, among others, can manage at a reasonable price. And there's obviously more than phase shift to worry about as even linear PSUs with transformers can get very spiky. But the OP asked about current.

author: BeBuLamar
author_rank: Super Contributor
timestamp: June 06, 2022, 10:23:42 pm
content: The 175 has slightly higher accuracy 1.5% vs 2% and 2.5% for the 375. Also the 375 has resolution to 0.1A while the 175 has the resolution of 0.01A. Also the 175 is less expensive than the 375. But since you said "Safely" I would much rather use the 375 or other clamp meter.

author: alm
author_rank: Super Contributor
timestamp: June 07, 2022, 08:54:51 pm
content: Quote from: bdunham7 on June 06, 2022, 10:21:36 pmThat's true, but a TRMS meter (clamp or otherwise) with sufficient bandwidth and crest factor should be able to manage a spiky current waveform. But no amount of fancy measuring of the current will help if you try to measure power and the voltage and current waveforms are substantially different (either in phase or shape). And since the goal of this is apparently to calibrate a power-measuring device, I think it's good to make the point that just measuring the current may not be sufficient for non-resistive loads, like a small SMPS without power factor correction.

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2022, 09:14:43 pm
content: Quote from: alm on June 07, 2022, 08:54:51 pmRereading the first post, I think you may be right that the OP has not considered this properly.

author: fazalmajid
author_rank: Newbie
timestamp: June 07, 2022, 09:35:35 pm
content: I was planning on using a fan next, but you're right, I have no idea how to extrapolate for non-resistive loads. Those devices do measure and report on the power factor, though, so I am hopeful.

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2022, 09:41:11 pm
content: Quote from: fazalmajid on June 07, 2022, 09:35:35 pmThen you might want to consider an actual power meter. I don't know the BW of a KillAWatt, but apparently the 121GW multimeter is ~1kHz. Perhaps that is good enough after all, at least at the accuracies you are probably after. Recombining separate current and voltage readings to obtain power is harder that it looks as you have to account for both BW and time skew.

author: fazalmajid
author_rank: Newbie
timestamp: June 07, 2022, 10:16:37 pm
content: I've ordered anOlson PMBoxpower meter. I'd still like to verify on a resistive load.

author: bicycleguy
author_rank: Frequent Contributor
timestamp: June 08, 2022, 03:46:20 am
content: Quote from: bdunham7 on June 06, 2022, 10:21:36 pmThe 121GW can only handle 50V AC or DC max in VA mode. Many complaints during the kickstarter. The fix was a very ambiguous note in the specifications, note 11, 500VA, with 50V and 10A.

author: bdunham7
author_rank: Super Contributor
timestamp: June 08, 2022, 05:04:21 am
content: Quote from: bicycleguy on June 08, 2022, 03:46:20 amOops! I'm glad nobody took my advice, or at least AFAIK...I wonder why?

author: 5U4GB
author_rank: Super Contributor
timestamp: September 25, 2024, 09:20:19 am
content: The downside of the plastic line splitters is that they're fairly bulky and won't work with smaller current clamps, which can't close around them. There's various DIY alternatives where people wire separate lines between rewirable plugs and sockets, leading to concerns about mechanical reliability at the plug/socket, lack of earth wire in some of them, and use of the same-coloured wire for active and neutral in others. What about coming in from the other direction and getting a short extension cord with moulded-on plug/socket, stripping a short length of the outer insulation from it, and clamping around the inner insulated conductors? It's losing one layer of insulation, but avoids all of the problems with the DIY rewirable-plug setup.