AC Voltage leak, help me understand what's going on - Page 1
Forum section: electronics / beginners

author: JHAnd
author_rank: Contributor
timestamp: September 10, 2023, 08:10:20 pm
content: Hi!I work at an educational facility and was told that a student had gotten an electrical shock from a microphone connected to a mixer and an active sub driving two speakers.This is in europe so we have 230V on mains.This is I have found:1. The mixer and sub were not connected to ground. Someone has put a branch outlet with no ground between the equipment and the grounded wall outlet.2. I measure 100VAC between the line input connectors on the back of the sub and mains ground when the sub is not connected to ground.3. I measure 40VAC between the line output of the mixer and mains ground when the mixer is not connected to ground.4. When replacing the non-grounded branch outlet with a grounded one both the 100VAC and the 40VAC disappear to ground and the reading is then 0VWould I be right in calling the voltages above "voltage leaks" or is there a more correct name for it?What would the most probable cause be? Line filter caps?We have another sub of the same make and model and it also leaks... 90VAC... which has me thinking that this is a design flaw perhapsAny thoughts?

author: Benta
author_rank: Super Contributor
timestamp: September 10, 2023, 08:32:10 pm
content: No design flaw.It's simply stray capacitances you are seeing, perhaps aided by a couple of Y-capacitors in the power supplies.Modern DMMs have a high input impedance, making them sensitive to this.If you measured with an old-fashioned analog meter with typically 10...20 kohms/V input, my bet is you won't see anything.The flaw is more on your side by not providing protective earth (PE) to equipment that's designed for it. Isn't there someone at your "educational facility" that knows what he/she is doing? Otherwise hire a pro roadie for setting your stuff up (they can be hired by the hour).

author: TimFox
author_rank: Super Contributor
timestamp: September 10, 2023, 09:03:31 pm
content: Yes, most modern equipment with a three-prong plug appropriate to the country has two capacitors, one from L and one from N to the metal chassis, in the line filter that should be connected to PE through the third prong.Without a proper ground at the third prong, the voltage on the chassis with respect to grounded metal (e.g., plumbing) will be 1/2 the line voltage (1/2 x 230 V in your case) in series with a few nF of capacitance.1 nF at 50 Hz is about 3 M\$\Omega\$ reactive, or 50 uA of current. Multiply that by the actual capacitance and number of ungrounded line filters.That should not be lethal (unless you are standing on a ladder and fall off) but certainly will be felt by the victim who contacts both the metal chassis of the ungrounded equipment and a grounded piece of metal.Before I retired, we often encountered improperly installed outlets in foreign countries, and discovered the "Dan effect": "It's hard to shock Dan, but..."Improper grounding is very dangerous:https://media.defense.gov/2009/Jul/24/2001712264/-1/-1/1/ROI-Part%20%20II%20Final%20(24%20Jul%2009)_web.pdfis a US military report about electrocution deaths during the American war in Iraq: several of them occurred while the serviceman was taking a shower.

author: JHAnd
author_rank: Contributor
timestamp: September 10, 2023, 09:13:11 pm
content: Quote from: Benta on September 10, 2023, 08:32:10 pmEven if it is just stray capacitance, isn't it a bit high with 100VAC? I mean that could potentially kill or shock someone, or am I missing something?And why does the fuse not blow when the capacitance is connected to ground? My thinking is that 100V connected to ground would instantly blow the fuse...I have a low impedance testing function on my Fluke T150 perhaps I could try that and see if the voltage disappears. Still would you care to explain to me why the high impedance makes for a wrong reading then?And if it is not a design flaw... but shouldn't a design be able to be connected to a non-grounded supply. I mean what if a consumer buys this product but has an old house without grounded outlets. That's pretty common here.QuoteClearly someone has missed that this was not a grounded branch outlet.

author: Benta
author_rank: Super Contributor
timestamp: September 10, 2023, 09:54:41 pm
content: Quote from: JHAnd on September 10, 2023, 09:13:11 pmForget the voltage, it's about power. A modern DMM pulls negligible power from the measured object (= very low current, aka high input impedance). This makes it able to show these "ghost" voltages.An old-fashioned passive meter pulls power from the measured object to move the pointer. But there's so little here that it won't move.No, the fuse won't blow. If it does, you are already roast.Whatwillblow is the GFI (ground fault interrupter), which triggers at max. 30 mA, newer versions at 10 mA. Unpleasant, but harmless unless you carry a heart pacemaker. As it didn't trigger, the current your subject experienced is much lower.QuoteYes, this is an issue back from the days when every country had their own plug/socket systems. It's still an issue, but getting better. I have no knowledge of the Swedish system.But what can I say, except that when equipment is designed for PE earthing, then make sure it's connected that way. It's not rocket science. If that's too difficult, get an electrician or the mentioned pro roadie.Quote"Someone" did not miss that the wall outlet had earth. What you call "branch outlet" I interpreted as an extension cord without earth (a pro roadie would never do that). Otherwise your OP made no sense.

author: TimFox
author_rank: Super Contributor
timestamp: September 10, 2023, 10:32:28 pm
content: "And if it is not a design flaw... but shouldn't a design be able to be connected to a non-grounded supply. I mean what if a consumer buys this product but has an old house without grounded outlets. That's pretty common here."There are many commercial products that do not need a PE connection: they are called "double-insulated", meaning that two levels of insulation must fail before there is a connection between the mains input and any exposed metal.Products that have three-prong plugs (including computers, test equipment, some power tools, and some electronic sound equipment), on the other hand, require that the exposed metal (including microphone shells) be grounded for human safety.

author: themadhippy
author_rank: Super Contributor
timestamp: September 11, 2023, 12:04:36 am
content: QuoteBit of a generalisation,what about country's were its 3 pins or nothing? i can think of sevral bits of double insulated equipment with exposed metal parts but no earth connection in the 3 pin plug.Quoteminimum call around here is 4 hours.

author: Benta
author_rank: Super Contributor
timestamp: September 11, 2023, 12:12:04 am
content: @TimFox, you forgot the disclaimer "APART from the UK, "...Please take care next time.

author: themadhippy
author_rank: Super Contributor
timestamp: September 11, 2023, 12:15:40 am
content: Quotenot just the uk, anywhere a bit of double insulated kit is wired to a 3 pin plug, the post could lead to confusion with people assuming they need to put something in the earth pin.

author: ArdWar
author_rank: Frequent Contributor
timestamp: September 11, 2023, 12:41:51 am
content: Quote from: Benta on September 10, 2023, 09:54:41 pmNot really, absent any proper PE connection it's exactly half mains voltage with fairly high impedance. Itshouldbe no more than a few couple mA maximum, with the voltage drooping rapidly the moment you touch it with something reasonably conductive. That amount of currentshouldbe still safe for direct touch.Heck, here it still can get to half mains where customer premise wiring are usually poorly done TN-C system.

author: Benta
author_rank: Super Contributor
timestamp: September 11, 2023, 12:44:07 am
content: Quote from: themadhippy on September 11, 2023, 12:15:40 amPlease see here:https://www.worldstandards.eu/electricity/plugs-and-sockets/Only Type G (UK and a few others) and Type M (South Africa) do not have a 2-prong option. That'syourissue.All others are compatible with Type C or Type A (the most common plugs on all consumer equipment).And there's no confusion. If the equipment has a three-prong plug, it should go into a three-hole receptacle with earth.If it's a two-prong plug, it can go anywhere (like bad girls).

author: Benta
author_rank: Super Contributor
timestamp: September 11, 2023, 12:50:14 am
content: Quote from: ArdWar on September 11, 2023, 12:41:51 amYep, with you 100%. The most you'll likely feel is a faint buzzing on your skin.But the OP mentioned a PA-like setup, and a micropkone without earth connection can be unpleasant on the lips: thin skin, moist, lots of nerves. That probably caused the complaints from people.

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2023, 03:39:58 am
content: Quote from: Benta on September 11, 2023, 12:12:04 amParse my statement carefully next time.There exists double-insulated equipment (with two-prong plugs) in some places.

author: jonpaul
author_rank: Super Contributor
timestamp: September 11, 2023, 03:51:06 am
content: EMI filter not compatible to doub insul equip.The CM noise VDE Y caps must return to earth.Either you have a double ins appliacne or tool OR you have a CM filter with Y caos L1><Earth><L2.j

author: Barry Hardhouse
author_rank: Contributor
timestamp: September 12, 2023, 12:46:42 pm
content: Just to answer one of the OPs original questions:"Would I be right in calling the voltages above "voltage leaks" or is there a more correct name for it?"In the UK. we use the term "touch current", and I believe this is the phrase in other English speaking countries, too. As the name implies, because the source impedance is high, we generally think of this in terms of the current available, rather than the open-circuit voltage.Hope this helps.

author: EPAIII
author_rank: Super Contributor
timestamp: September 13, 2023, 07:06:54 am
content: I would tend to call it a leakage current.And it is a certain amount of current that creates danger from a shock, not the Voltage. You can walk across a carpet on a dry, winter day and accumulate a static charge of thousands or even tens of thousands of Volts on your person. But when you touch a ground the shock you experience is not really dangerous because the current that flows is very low. Children will run across a room and then touch someone to shock them - all for fun.The question about what was being missed in this was asked. What was missed was an understanding of the role of Voltage division. When a small capacitance (in the plug) is part of the current path, it presents a large impedance (resistance) at power line frequencies. So, if the path from there to ground is also a large impedance (resistance) then the total Voltage (230 VAC) is divided roughly in half and a fairly large Voltage is felt or measured. The human body or a modern, digital VOM would be such a large impedance. But the older, analog VOMsmaypresent a far smaller impedance to ground so the Voltage division is now a far smaller fraction of that total 230 VAC and little or no Voltage is at that point in the "circuit" to be measured. Draw the equivalent circuit, with impedance (resistance) values and do the math and you will see the differences.So they put capacitors in 230 VAC plugs from hot and neutral to ground as some kind of noise filter? Who thought of that? Wouldn't it be better to put them in the outlets where the presence of the ground is better assured? Oh, well, nothing's perfect.Quote from: Barry Hardhouse on September 12, 2023, 12:46:42 pm

author: wasedadoc
author_rank: Super Contributor
timestamp: September 13, 2023, 07:57:14 am
content: Quote from: EPAIII on September 13, 2023, 07:06:54 amI've never encountered or even heard of such capacitors in plugs. Any noise filters are inside the equipment.

author: tggzzz
author_rank: Super Contributor
timestamp: September 13, 2023, 08:54:44 am
content: Quote from: EPAIII on September 13, 2023, 07:06:54 amI very much doubt that.QuoteThat is a good example of "amateurs think about how things might work, professional engineers think about how things might fail".

author: JHAnd
author_rank: Contributor
timestamp: September 16, 2023, 09:41:10 pm
content: Thank you for your answers,So if I understand yo correctly... (see attached drawings at bottom of post)...the voltages with reference to ground would be like this:A1 230VB1 0VC1 0VA2 230VB2 115VC2 0V...The voltage on B2 is fed forward to the microphone through the mixer. So if someone touches the mic while holding a guitar connected to a grounded guitar amp the circuit would look like this:(see attached photo at bottom)R1 would then be the resistance from lips to fingers. R1 would be in series with one Y-capacitor.Since the Y capacitor is a very small value (between 0.001µF and 1µF) the reactance/impedance is high because the capacitor gets filled up quickly and then stops the current. The capacitor changes polarity for every half cycle of AC but for every half cycle only a small current gets through, the rest is stopped. A bigger capacitance value would let more current through for every half cycle and therefore have smaller less reactance/impedance.Is above statement correct?How would I calculate the current through R1?Lets say that R1 is 3 kOhms and each Y capacitor is 0.001µFReferences:typical Y-cap valueshttps://eepower.com/capacitor-guide/applications/filter-capacitor/#capacitive voltage dividerhttps://www.electronics-tutorials.ws/capacitor/capacitive-voltage-divider.html

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2023, 10:04:14 pm
content: Complex algebra: referring to your drawing with two capacitors and one resistor.First, there is a voltage of (1/2) x (230 V) = 115 V in series with (2) x (1 nF) = 2 nF (two 1 nF capacitors in parallel) driving the 3 k\$\Omega\$ resistor.At 50 Hz, the impedance of 2 nF is -j(1.6 M\$\Omega\$) (capacitive and quite high).The total (complex) impedance is therefore 3000 - j(1.6 M\$\Omega\$) (almost totally capacitive reactance).Ignoring the small real part from the resistor, the current through the resistor is therefore (115 V)/(1.6 M\$\Omega\$) = 72 uA.If you change the resistor or capacitors to a larger value, remember to add the real and imaginary parts of the impedance in quadrature (root sum of squared values); the smaller of the two components is not important until it becomes > (0.1) x (the larger component), where it changes the total by 0.5%. If it be (0.5) x (the larger component), that's a 12% change.Edit: corrected for my series/parallel error found by wasedadoc below.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 16, 2023, 10:19:12 pm
content: Quote from: TimFox on September 16, 2023, 10:04:14 pmThe two capacitors are in parallel, not series, as far as the impedance of that 115 Volts source is concerned. Impedance is therefore -j1.6 MΩ.

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2023, 10:31:49 pm
content: Quote from: wasedadoc on September 16, 2023, 10:19:12 pmYes, my mistake; that is correct for the Thévenin equivalent of the capacitive voltage divider.I corrected the post for 2 nF capacitance.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 16, 2023, 11:59:47 pm
content: Quote from: JHAnd on September 10, 2023, 09:13:11 pmEquipment that is used on 230/240v supplies is often, by default, designed for use on the US 240v system, where both legs of the 240v supply are 120v "hot" w.r.t. Neutral, & hence Earth (ground).A capacitor from each leg to chassis is provided, so that, barring a capacitor failure, the chassis is at a "virtual earth" potential.All good in North America, but the same setup in countries with a 230/240v supplies becomes a capacitive voltage divider, elevating the chassis to half Mains potential w.r.t Neutral & hence earth.The current available through the capacitors is not really dangerous, but it does bite!But wait, there's more!If we "fix things" so that the chassis is earthed, there is a possibility that some older versions of these NA oriented designs may, because of their original envisaged use, use capacitors with voltage ratings which, although adequate for a NA "split-phase" supply, are not adequate in the new situation where one has (close to) the full 230/240v applied across it, & the other one has (close to) zero volts across it.

author: TimFox
author_rank: Super Contributor
timestamp: September 17, 2023, 04:47:03 am
content: I'm not aware of low-power equipment (< 1 kW) being used in North America on 240 V CT (120-0-120) power.The usual gear plugged into that 240 V connection are air conditioners, ranges/stoves, and driers.

author: EPAIII
author_rank: Super Contributor
timestamp: September 17, 2023, 06:05:44 am
content: In the plug or in the equipment, what's the difference? Seems like the same circuit.Quote from: wasedadoc on September 13, 2023, 07:57:14 am

author: JHAnd
author_rank: Contributor
timestamp: September 17, 2023, 07:35:07 pm
content: Not sure I understand why the two caps values should be added. Why would the lower capacitor in the drawing affect anything. Wouldn't the current travel through the resistor from the first cap only and then to ground and back to neutral in the transformer station.

author: TimFox
author_rank: Super Contributor
timestamp: September 17, 2023, 09:12:45 pm
content: This is the AC Thévenin equivalent of the capacitive voltage divider formed by the total voltage and the two capacitors.To calculate the output impedance for the Thévenin equivalent network, you short out all the voltage generators and measure the impedance at the two output terminals, which in this case is the two capacitors in parallel.For the usual resistor voltage divider, the output impedance is the two resistors in parallel.For the Thévenin equivalent network, the voltage driving that output impedance is the voltage you measure with the output open-circuited, in this case (1/2) x (230 V).The Thévenin equivalent for voltage sources and the Norton equivalent for current sources are elementary network analysis for linear circuits and worth studying.In your example circuit, with two 1 nF capacitors, each -j (3.2 M\$\Omega\$) at 50 Hz, and a very low load resistance of 3000\$\Omega\$, you could instead calculate the current through only one capacitor (1 nF driven by the full voltage (230 V), since the voltage across the resistor is much less than the full voltage. You will get the same answer, but the Thévenin approach is more flexible if you need to vary the resistance.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 17, 2023, 11:59:47 pm
content: Quote from: TimFox on September 17, 2023, 04:47:03 amOne particular "domestic" use of 240v supplies in the USA is for ham radio linear amplifiers.Being allowed 1500 Watts PEP does make a 240v supply attractive.Apart from that, the NA made stuff I am most familiar with is that used in TV Broadcasting, where much equipment shows it is designed to comply with the rules for 240v use in the USA.Things such as fuses on each leg of the Mains, for instance. (strictly speaking, that is forbidden for single phase equipment in Australia, but seems to have been given an exemption).If there is no express rule against the use of 240v small appliances in NA, logically, there must be rules for such devices, which, apart from the "capacitive voltage divider" thing, cover the requirements in multiple other countries, so it mainly makes sense to design to such rules.Japan historically, had both 100v & 200v single phase standards, depending on where you lived, but their mass market is the USA, so they made stuff to the prevailing NA standards.China is a 220/380v country, so I would expect equipment made there to adhere to similar standards to Australia, EU, UK, etc., but again, their mass market is the USA, so they probably design for the 240v split phase system in that country as a default, although the stuff is unlikely to be used in that situation.

author: TimFox
author_rank: Super Contributor
timestamp: September 18, 2023, 12:59:12 am
content: Of course: stuff pulling greater than, say 1 or 2 kW would do well on 240 VAC mains.However, guitar amplifiers, stereo systems, desk computers, computer peripherals, DSOs, etc. have line filters with Y capacitors and in North America they run from a 3-prong 120 V outlet.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 18, 2023, 03:40:54 am
content: Quote from: EPAIII on September 17, 2023, 06:05:44 amBecause the point of the capacitor is to be part of an EMI filter. If you put the filter in the outlet the power cord becomes a giant radiator. Plus you don't know the needed filter specs since that depends on the equipment.