Transformer - Mains Oscilloscope Probing - Page 1
Forum section: electronics / beginners

author: 2X
author_rank: Frequent Contributor
timestamp: May 05, 2024, 09:31:30 pm
content: Hello,I have some doubt’s/queries about the oscilloscope probing on a transformer.All the below that I am writing is what I have understand from personal reading and searching and must not be taken as 100% right.For safety reasons the negative clips of abench oscilloscopeare connected between (there is the exception of a portable oscilloscope with theoptionof isolated channels) them and all together to the PE Ground pin of his power socket. This probe’s PE Ground connection causing some “problems” when it comes the probing of the oscilloscope to a device/PCB.At figure 1 because Neutral wire is connected through the oscilloscope’s negative clip to the PE ground this will lead to the imbalance currents between Line and Neutral conductors on the RCD breaker and the result will be the RDC triggering.At figure 2 there is a short circuit between Line and Earth-Neutral (if electrical system has Neutral Grounding).Figures 3 & 4 are OK and will not cause any short circuit because of the transformer’s galvanic isolation;from the other hand on high frequencies because of the parasitic capacitances between transformer’s core & windings and in addition of the oscilloscope probe’s characteristics like attenuation and capacitancethe “OK” in respect of the short circuit and safety it depends.Now this that I can’t understand and be sure is why/if the probing of the oscilloscope on the figures 5 and 6 (7 & 8 - secondary output 24Vrms) are wrong. This probing I have only triedwith portable oscilloscope with isolated channelsand with bench oscilloscopebut with differential probe at the transformer’s primary winding.The oscilloscope or the circuit at the figures 5 and 6 (7 & 8 ) will blow out or just the measurement will be wrong?I believe I missing something between potentials of PE Ground in respect to transformer’s secondary “virtual” Ground (I don’t know if there is an official term for this Ground), so for this I can’t understand the whole concept. I remember that I read on the internet that only the PE Ground has truly zero potential and the “electronics” Ground (officially Common Gound) is not zero but just a reference point where is considered zero; this is what I can’t understand. If you have any book or link or your opinion to suggest me all are welcomed.

author: Silenos
author_rank: Regular Contributor
timestamp: May 06, 2024, 01:18:58 pm
content: If you have a TN-S or TN-C-S installation, touching neutral with PE will trigger RCD, why do you connect it on a primary side? With TN-C installation it won't but you don't have a PE there.If I understand your measurement correctly, fig 3,4 on 2nd pic are ok, and if you need primary side on the another channel, do that with differential probe.And fig 5 and 6, apart from the triggered rcd, will blow up your scope unless you have channels rated for 230 V rms.And if you want to measure high frequencies and precise stuff in mains, use the instrument transformers, they either come with output conditioning or you do it yourself.

author: 2X
author_rank: Frequent Contributor
timestamp: May 06, 2024, 02:56:26 pm
content: Quote from: Silenos on May 06, 2024, 01:18:58 pmAt figures 1,5,6 If I had TN-C or TT installation ortheoreticallyif I removed the RCD (then the RCDs of the neighboring houses will be triggered) or the ground wire from the oscilloscope power socket (this must never done in practice for safety reason) and I had a probe with attenuation x10 or higher then:1. The measurements will be the right values?2. The PCB or oscilloscope or both will not blow out?Thanks for your reply.

author: Silenos
author_rank: Regular Contributor
timestamp: May 07, 2024, 10:44:00 am
content: Quote from: 2X on May 06, 2024, 02:56:26 pmEwww how on earth you come up with such an idea? That's why everyone has their ows sets of RCD and fuses...Ok, let me put it straight. You never have to hack the installation in osciliscope cases, ever. And the type does not matter, either. I was just explaining you what will happen when you connect neutral with PE, and I still have no idea why you insist on doing that except the evervague "safety reasons" - it is a mistake in any case, that's all.At least in my part of Europe neutral wire is considered "high voltage" (230/400 V) wire in the safety standards, and it is supposed to be treated like this. And in a real installation there is always some N-PE voltage floating due to return currents, hence the RCD action when connecting these wires.Disclaimer: you would be surprised what can the PE in wall socket be connected to, especially in older households, sites, with TN-C 2-wired installations. Though no idea of such "traditions" in your area.Ok, cut this subject, just don't mess with it.So now some board is appearing. No idea what exactly are you going to measure. What do you mean "right values"? Measuring anything with the osciliscope will always show "false values", as you are biasing the DUT with a probe. The transformer is also a machine collapsible to iirc simple RLC/magnetic flux circuit.

author: 2X
author_rank: Frequent Contributor
timestamp: May 07, 2024, 12:20:00 pm
content: Quote from: Silenos on May 07, 2024, 10:44:00 amAs I know all switching power supplies have a tiny leakage and in TN-CS because the Neutral and Ground cables are common at the transformer and also at each household; all the leakage current from one house flows throught the earth where is common for all houses and there is apossibility(it depends from earth composition) some part of this leakage current go through the neutral wire of a neighboring house and will trigger the RCD (the difference current between the Line and Neutral must be higher than the limit of 30mA so to triggered the RCD) besides this house don't have a leakage itself. Also there is apossibilityif a house don't have a working RCD or RCD installed at all and there is a leakage can go all through the ground (not pass from neighbouring house neutral conductor so to trigger the RCD) and energise metal parts/devices of this house where can cause electric shock if someone touch them (touch voltage >= 50V can caush Electric Shock - the value of the touch voltage it depends from the amount of leakage current).In my question I wrote with bold letters "this must never done in practice for safety reason" and I ask all of these because I have never try something of them in practice because I am not sure. With "OK" measurement I mean if the actual voltage of secondary is 24Vrms and with probing (at figures 5,6,7,8) the oscilloscope will show a different value... higher or lower from 24Vrms.Also, I don't know from when something that can't be done in practice or if the assumption for this (even if its tottaly wrong) is forbidden to be asked and must be answerd with ironic way. Anyway, thank you for your time and for your replies.

author: themadhippy
author_rank: Super Contributor
timestamp: May 07, 2024, 12:30:01 pm
content: QuoteNot a strange assumption if your old enough to remember voltage operated devicesQuoteAny of your leakage current will appear on the supply before the neighbours rcd'd so they wont see any imbalance

author: 2X
author_rank: Frequent Contributor
timestamp: May 07, 2024, 12:45:55 pm
content: Quote from: themadhippy on May 07, 2024, 12:30:01 pmI forget to point that the RCD is not working properly or there is no RCD. Thanks for your reply.

author: Silenos
author_rank: Regular Contributor
timestamp: May 07, 2024, 04:08:34 pm
content: Quote from: 2X on May 07, 2024, 12:20:00 pmDid you see it? Have you caused it? Sounds highly unlikely as current just doesn't like to flow through earth, when it has the metallic return path to the transformer (N or PEN cable). If not, it will flow to the closest earthing and return to the metal, likely within the same site or the distribution grid earthing. Good household earthing has like few ohms of "static resistance", higher than ++25mm2 Al cable, and the strickest are transformer stations earthing coming down to iirc fraction of the ohm. Also, in the TN-C/S systems the last common point of PEN->PE and N is supposed to be before RCD, so there is no way to inject residual current after the RCD through the earth.And I am not ironic or whatever, still don't understand what are you going to do. The figs 5,6,7,8, will likely fail, as I mentioned before.If you measure the voltage of secondary side of the transformer with nominal 24 V, and unloaded, you are very likely to get the voltage significantly higher than 24 V, and the measurement method does not matter.Quote from: themadhippy on May 07, 2024, 12:30:01 pmElaborate, please?

author: themadhippy
author_rank: Super Contributor
timestamp: May 07, 2024, 04:31:41 pm
content: QuoteBefore current operated devices we had voltage operated devices,they worked by measuring the voltage to earth,basically a coil in series with the main earth conductor .If theres an alternative path to earth they dont work,and if that alternative path to earth is via a neighbors installation the voltage goes through there coil as well