Y Capacitor, current leakage & Electromagnetic Interference (EMI) problem. - Page 1
Forum section: electronics / beginners

author: davidwyl
author_rank: Contributor
timestamp: December 28, 2016, 12:27:52 pm
content: over the years i purchased many phone charger from china. Most of the charger will give me shock "tingly" sensation when touching the phone while charging.definately there is some current leakage and can give me unpleasant feeling.Since im not using all these charger, so i decided to investigate and tear it apart. in order to reduce to "tingly" feeling when touching i need to use a really low value for y capacitor or remove it completely. After i removed the capacitor, the "tingly" sensation is gone. From my knowledge remove y capacitor will increase EMI interference.my question is:Is it safe to use the charger without y capacitor??how bad is EMI Interference to my phone/raspberry pi/arduino??why manufacture put that capacitor in the first place?? to reduce the EMI but give people a shock every time touching the phone?

author: Simon
author_rank: Unknown
timestamp: December 28, 2016, 12:55:15 pm
content: I think the problem is the chargers are not earthed but they have connected the connector body up as though it were earth. these things should be outlawed, we have an earth for a reason but we don't do quality control anymore and safety laws are not a concern for the chinese.

author: orolo
author_rank: Frequent Contributor
timestamp: December 28, 2016, 01:00:37 pm
content: I saw something related just yesterday:In this case the fault was in the transformer, apparently. Be careful with those cheap wonders.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2016, 01:10:14 pm
content: The Y capacitor provides a return path for the capacitively coupled energy from the primary to the secondary transformer inside the switched mode power supply. Without it, the secondary will float at an AC voltage equal to the SMPS's switching frequency, with respect to the mains voltage or earth.Yes, the chargers aren't earthed. One problem is some countries have two pin sockets for many devices, which aren't earthed and the power supply circuit is designed so that it can be used the world over. The tiny shock is not harmful and it's perfectly safe, so long as the Y capacitor meets all the relevant safety standards.The are other potential problems with Y capacitors though. One is the tiny current can damaged sensitive devices which don't have ESD protection, although this can be solved by proper design. Another problem is, if many unearthed appliances with Y capacitors are connected together, the leakage currents will add up and can then exceed the safe levels.Removing the Y capacitor is generally a bad idea. One alternative is to connect the Y capacitor to earth or, if the mains connector is polarised, neutral. That way all the EMI will be diverted to earth and there will be no leakage current.Another possibility is adding a screen between the primary and secondary of the transformer and connecting that to the mains rectifier, via a Y capacitor, but you'd need to rewind the transformer to do that. It's something which could be done by the manufacture but it would cost more.

author: davidwyl
author_rank: Contributor
timestamp: December 28, 2016, 01:19:14 pm
content: I believe these power supply is not damage . For example this Orico brand, I bought at least 6chanrger from them all give me the shock feeling . Initially I thought just device faulty but recently I bought another 2 from them, sadly it giving me same shock feel.I believe this is something to do with the manufacturer try to control the emi under the requirements and totally forget about the current leakage.This Orico sold thousands of charger under their company name I'm wondering why I'm the only one feel the shock and no one else. I didn't see any complaint whatsoever.Is this something to do with the country I'm in is humid? Main socket voltage? Earth? I dunno, maybe if I take these charger and use in China the electric shock will gone. I seriously dunno.

author: davidwyl
author_rank: Contributor
timestamp: December 28, 2016, 02:39:52 pm
content: Since removing the y capacitor is a bad idea , Is there a safe way to modify these charger to eliminate the shock feeling?

author: Simon
author_rank: Unknown
timestamp: December 28, 2016, 03:01:59 pm
content: yea, give it an earth.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2016, 03:05:20 pm
content: Quote from: davidwyl on December 28, 2016, 02:39:52 pmQuote from: Hero999 on December 28, 2016, 01:10:14 pm

author: davidwyl
author_rank: Contributor
timestamp: December 28, 2016, 04:54:50 pm
content: Thanks for the information...All These charger i owned are a 2pin non-polarised type so my option is to connect the y capacitor to earth?How to Connect it?Primary (AC side) of the transformer ---> Ycapacitor--->earth??So I disconnect the Ycapacitor on the secondary side and connect it to earth pin?

author: Gyro
author_rank: Super Contributor
timestamp: December 28, 2016, 06:38:14 pm
content: Unfortunately Y capacitors are there to eliminate (decouple) noise at RF frequencies. This means that they need to be very close to the noise source (the switcher). You can't just wire them externally to earth on lengths of wire, that would most likely create a radiating loop antenna, making the radiated interference worse.As long as the Y caps are genuine, then they can't pass enough current to be anywhere near dangerous. Y caps are highly specified parts with very high breakdown voltage. The far higher risk from no-name adaptors is the quality of the insulation in the transformer, safety clearances on the PCB, inadequate overcurrent protection (squiggly line thin PCB traces) on the primary side.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2016, 07:38:26 pm
content: Quote from: Gyro on December 28, 2016, 06:38:14 pmIt's not ideal but I still think it's better than removing the Y capacitor altogether. Don't use a long piece of separate cable. Use three core mains cable. The close proximity of the earth conductor to the live and neutral should minimise the interference.Ideally there should be an X capacitor and another Y capacitor from the primary side to earth.QuoteTrue but that doesn't stop it from being a nuisance or a problem if the 0V of many devices is connected together, as in the case of audio and visual equipment.

author: davidwyl
author_rank: Contributor
timestamp: December 29, 2016, 02:19:17 am
content: Thanks hero999 for the diagram.What value of x y capacitor should I use?I have bunch of 2nf capacitor around, will that ok to use in this?

author: davidwyl
author_rank: Contributor
timestamp: December 29, 2016, 02:23:33 am
content: Quote from: Gyro on December 28, 2016, 06:38:14 pmI'm fully aware the power supply is not dangerous but it always give me unpleasant feeling. If I sit on my bed it is fine, once I step on the floor I will get very unpleasant tickling feel. Worst is when I'm using metal earphones, I can feel the tickle feeling on my ear too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2016, 03:35:32 am
content: Add a ground pin and connect the negative side of the power supply to GND.This safely shorts the ground leakage current, and provides a return path for the RF noise besides. (Some more Y caps from AC line/neutral to GND may also be necessary for reduced radio interference.)Tim

author: davidwyl
author_rank: Contributor
timestamp: December 29, 2016, 05:03:25 am
content: If I just connect the Negative on the DC side to the Earth pin, is this safe to connect this way?Sent from my ASUS_Z00AD using Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2016, 05:48:34 am
content: Even better, find where the USB shields go. Should be the same -- check it with a meter.Tim

author: davidwyl
author_rank: Contributor
timestamp: December 29, 2016, 08:53:37 am
content: Quote from: T3sl4co1l on December 29, 2016, 05:48:34 amThat is connected to the usb shell. Tested the connectivity.Sent from my ASUS_Z00AD using Tapatalk

author: Zero999
author_rank: Super Contributor
timestamp: December 29, 2016, 09:37:24 am
content: Quote from: T3sl4co1l on December 29, 2016, 03:35:32 amThat will work too. The only thing you now need to be wary of is the negative is now referenced to earth, which won't be a problem in this case.Quote from: davidwyl on December 29, 2016, 05:03:25 amThe connections are fine but please use proper three core mains cable, with an outer sheath, rather than individual conductors like that for the real thing.

author: davidwyl
author_rank: Contributor
timestamp: December 29, 2016, 10:09:08 am
content: Quote from: Hero999 on December 29, 2016, 09:37:24 amIs there any situation where DC negative referenced to Earth might be dangerous / causing any issue?Sent from my ASUS_Z00AD using Tapatalk

author: Zero999
author_rank: Super Contributor
timestamp: December 29, 2016, 04:14:48 pm
content: Quote from: davidwyl on December 29, 2016, 10:09:08 amIt could be a problem if the circuit you're powering has a rail splitter and is connected to another device who's negative line is connected to earth.Look at the attachment. Device 1 has a virtual earth circuit which splits the 12V from power pack 1 to + & - 6V. The 0V rail is connected to another device running from a power pack 2 which has its negative rail connected it earth. The circuit will work fine, until power pack 1's negative rail is connected to 0V.This won't be an issue for your circuit, as the 0V goes to some USB connectors which are quite often earthed anyway.

author: davidwyl
author_rank: Contributor
timestamp: December 29, 2016, 04:41:50 pm
content: Quote from: Hero999 on December 28, 2016, 01:10:14 pmIf I make the socket polarised (live always connect to pin1 , neutral always connect to pin2 & not reverseable), then I can connect the DC negative straight to the Neutral? Is what what u mean?Simply diagram will be much appreciated

author: Zero999
author_rank: Super Contributor
timestamp: December 29, 2016, 05:08:00 pm
content: Quote from: davidwyl on December 29, 2016, 04:41:50 pmOh no, never, connect the 0V directly to neutral.I meant, change the Y cap so it goes to neutral, rather than the rectifier.

author: davidwyl
author_rank: Contributor
timestamp: December 30, 2016, 02:27:11 am
content: Quote from: Hero999 on December 29, 2016, 05:08:00 pmIs the x capacitor necessary? What range of value normally? 1nf - 2nf good enough?I have bunch of 1nf capacitor can I use that for x and y?Sent from my ASUS_Z00AD using Tapatalk

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2016, 09:00:01 am
content: Quote from: davidwyl on December 30, 2016, 02:27:11 amThe X capacitor should already be built-in to the power supply.

author: Giaime
author_rank: Regular Contributor
timestamp: December 30, 2016, 11:40:15 am
content: Quote from: Hero999 on December 29, 2016, 04:14:48 pmDirect connection between ground and earth (PE - protective earth) constitutes a PELV-type system: usually it's the most common practice for class I equipment (that is, with protective earth), for example IT stuff, audio/video equipment, etc. Just to give you an example, in lighting appliances (at least in Europe) it's not allowed, and only SELV systems are allowed (>1MOhm insulation resistance @ 500V between ground and PE).One of the potential dangers of PELV systems is that if you have an earth fault, with huge currents flowing into the PE building conductors, it is allowed for the PE terminals to "shift" from the "real" earth up to 50V. This additional potential can "shift up" all potentials attached to it: if you have a 24V PELV system for example, it is safe to touch since it's less than 60V. But if you have an earth fault, some points in the 24V PELV system can reach to 74V which by some safety standards is not safe anymore to touch!

author: Gyro
author_rank: Super Contributor
timestamp: December 31, 2016, 05:57:35 pm
content: Quote from: davidwyl on December 29, 2016, 02:19:17 amQuote from: davidwyl on December 30, 2016, 02:27:11 amOk, I think I detect a misunderstanding here...The X and Y capacitors are not simply component positions, they have specific and certified characteristics. In the case of X caps, a safe self-healing behaviour so they don't create a fire risk. For the Y capacitors, they have very high insulation voltage (far above rated voltage) to prevent risk of electrocution. The appropriate parts will be printed with X1, Y etc and will be covered in certification symbols, UL, CSA, VDE etc.You can't just use any capacitors you have lying around, that would be extremely unsafe. You need the correct X and Y certified parts.

author: orin
author_rank: Frequent Contributor
timestamp: December 31, 2016, 06:18:12 pm
content: Quote from: Hero999 on December 28, 2016, 01:10:14 pmDon't ever assume neutral is wired correctly. Just assume live and neutral are swapped and then what happens?The worst I've seen here is live jumpered to ground in a kitchen outlet! I don't know how that didn't kill someone since the microwave was plugged into it and was pretty much next to the sink.

author: Zero999
author_rank: Super Contributor
timestamp: December 31, 2016, 06:49:36 pm
content: Quote from: orin on December 31, 2016, 06:18:12 pmNothing really bad. If live and neutral are swapped, then the leakage current will be double what it would be without the modification but it should still be within the legal limit.

author: davidwyl
author_rank: Contributor
timestamp: December 31, 2016, 10:52:27 pm
content: Quote from: Gyro on December 31, 2016, 05:57:35 pmI do have proper x1 y1 rates capacitor around I just not sure what is the proper value to use.I tried adding a 2.2nf capacitor from DC Negative to Neutral , the leakage current does reduce and no more tickling feel.Sent from my ASUS_Z00AD using Tapatalk

author: davidwyl
author_rank: Contributor
timestamp: December 31, 2016, 10:57:18 pm
content: Quote from: Hero999 on December 31, 2016, 06:49:36 pmI plan to modify the connector on the power supply to a polarised connector so it can only go in one way. So Live always connect to live and Neural always Neutral.So what is a better & more recommended way to do this?can I presume connect DC negative to Earth is safer than connect it to Neural? ?Sent from my ASUS_Z00AD using Tapatalk

author: tggzzz
author_rank: Super Contributor
timestamp: January 01, 2017, 01:49:57 am
content: Quote from: davidwyl on December 31, 2016, 10:57:18 pm... always assuming that everything between the socket and the power station has been wired correctly

author: Gyro
author_rank: Super Contributor
timestamp: January 01, 2017, 01:01:47 pm
content: Quote from: davidwyl on December 31, 2016, 10:52:27 pmAh good, that's a relief.Y caps are typically 2.2nF. X caps are normally in the 100nF to 220nF range, occasionally 470nF or higher for large SMPSs.

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2017, 01:40:25 pm
content: Quote from: tggzzz on January 01, 2017, 01:49:57 amWhich is actually a reasonable assumption to make. If it wasn't, then nothing can be safe. Suppose live and earth are transposed in the socket? Only the RCD will protect you. That's assuming there is an RCD.

author: tggzzz
author_rank: Super Contributor
timestamp: January 01, 2017, 02:32:39 pm
content: Quote from: Hero999 on January 01, 2017, 01:40:25 pmWho saidanythingis safe?It is always best to state assumptions."Trust but verify" - that principle has saved me on more than one occasion, e.g. when switching off a domestic socketdidn'tremove the power.

author: orin
author_rank: Frequent Contributor
timestamp: January 01, 2017, 11:39:00 pm
content: Quote from: tggzzz on January 01, 2017, 02:32:39 pmIndeed. It's a good idea to check all the outlets when you move in somewhere. There's a reason they sell these here:http://www.homedepot.com/p/Commercial-Electric-Tools-Outlet-Tester-Green-MS112H/206029154

author: orin
author_rank: Frequent Contributor
timestamp: January 03, 2017, 02:20:45 am
content: Quote from: orin on January 01, 2017, 11:39:00 pmAnd following his own advice... I picked up the slightly better version at a local Home Depot today. Results so far:1 missing ground - see attached1 reversed live/neutralAt least they were easy to fix - or would have been if they weren't both behind the bed with about 9" of room to work in.With them being about 6' apart and both incorrectly wired, you'd think they'd both be on the same circuit and I'm sure that once we'd turned the breaker off I double checked with a Tequipment AC Sensor. Wrong. I found out the hard way when I grabbed both sides of the ungrounded one to tighten its new ground wire.

author: tggzzz
author_rank: Super Contributor
timestamp: January 03, 2017, 09:18:54 am
content: Quote from: orin on January 03, 2017, 02:20:45 amExcellent!I hope Hero999 now understands thatthe world and textbooks are almost orthogonal to each otherprofessional engineershave to think about whatmightgowrong, whereas amateur hackers can get away with only thinking about what can go rightbest practices and caution may seem unnecessary and pedantic, but they are based on experiences hard-won over the decades."Let's makenewmistakes"

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2017, 11:44:34 am
content: Quote from: tggzzz on January 03, 2017, 09:18:54 amI understood all of that before this thread was even posted. My point is that the end user isn't an engineer or electrician.All appliances will be designed with the assumption that the socket is correctly wired. That's all the engineers who designed the device can do. They can't be held liable for external wiring faults. For example, if it was the live and earth which were reversed or there was an earth fault, with no earth, in the building's wiring, and the end user was electrocuted, when they plugged a device in, the designer/manufacture/seller of the device is not responsible for their death/injury. The wiring of the fixed installation is outside the engineers' who designed the device control.This is why before selling or letting a house in the UK, it's mandatory to get the wiring tested by a competent electrician and periodic inspection is recommend.

author: wraper
author_rank: Unknown
timestamp: January 03, 2017, 12:03:26 pm
content: Quote from: Hero999 on January 03, 2017, 11:44:34 am, the only thing engineers can assume it's not correctly wired, because, very likely, it is not. Live and earth reversed a is completely different level of wiring fault compared to the live/neutral reversal. Here in Europe we usually don't have socket polarity at all, exceptions I can recall are UK, France and Denmark.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2017, 01:25:25 pm
content: Quote from: Hero999 on January 03, 2017, 11:44:34 amNot true! IEC/UL 60950-1 requires devices, intended for such wiring schemes, be equipped with two fuses. (In a suitable location, the fused neutral should be labeled, too.)Such wiring schemes are apparently common in commercial settings, like server farms and hospitals, not to mention older facilities (where schemes as bad as high-leg delta were installed, at least here in the wild wild U.S.).Likewise, the ground leakage current, with any one of the two mains conductors cut, must be within limits. Thus you can't cheat by filtering to neutral.Tim

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2017, 04:26:06 pm
content: Quote from: wraper on January 03, 2017, 12:03:26 pmIf the appliance has a non-polarised connector (as in your case) then it isn't a wiring fault. The appliance will be designed with the assumption that either conductor is live. In the case of a polarised connector, only the live is fused and switched, unless it's a double pole switch of course. For example, I doubt 3 phase equipment in Europe is designed for phase-neutral reversal, because it has a polarised connector.Quote from: T3sl4co1l on January 03, 2017, 01:25:25 pmAre you sure?I don't think I've encountered an appliance with both live and neutral fused and would consider it suspect if I did.I believe double fusing is only permissible when the connector is not polarised. I did some searching and found this:http://incompliancemag.com/article/double-fusing-or-fusing-both-sides-of-the-line/In the UK, polarised connectors are used and it's not permitted to have a fuse in the neutral.http://mechatrofice.blogspot.co.uk/2014/08/why-fuse-is-not-used-in-neutral.htmlQuoteI agree with you about it remaining safe with any one of the mains conductors cut. Incidentally, domestic appliances should also be safe, even with the earth conductor cut, since, under normal conditions, the earth leakage shouldn't be high enough to cause a dangerous shock.However, there's no reason why the neutral can't be used for filtering, so long as the leakage current wouldn't exceed the safe limit, when the neutral is broken.

author: tggzzz
author_rank: Super Contributor
timestamp: January 03, 2017, 10:27:32 pm
content: Quote from: Hero999 on January 03, 2017, 11:44:34 amSolid reference forselling, please. I've never seen one.Letting may well be different.

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2017, 11:41:51 pm
content: Quote from: tggzzz on January 03, 2017, 10:27:32 pmhttp://www.electricalsafetyfirst.org.uk/find-an-electrician/periodic-inspection-explained/EDIT:Wait a minute, this says it's not mandatory, just recommended:https://www.reallymoving.com/property-professors/surveying/do-sellers-provide-the-gas-and-electricity-safetyIn any case, even if it's not mandatory, it's a good idea to get both the gas and wiring inspected before buying a house.

author: tggzzz
author_rank: Super Contributor
timestamp: January 04, 2017, 12:52:10 am
content: Quote from: Hero999 on January 03, 2017, 11:41:51 pmActuallyneithersay it is required for selling; that's the difference between "should" and "must".A decent inspection would be very expensive, requiring all joints to be inspected and cables to be traced. A cheap inspection might only reveal simple errors.

author: orin
author_rank: Frequent Contributor
timestamp: January 04, 2017, 05:39:47 am
content: This is good stuff, but getting way off topic in this thread. So I started a new thread:https://www.eevblog.com/forum/chat/home-electrical-safety/

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2017, 08:42:03 am
content: Quote from: tggzzz on January 04, 2017, 12:52:10 amI stand corrected.I've just bought a new house myself. A brand new house, which is why it came with a certificate to say it had been inspected by an electrician and gas safe technician. The inspection must be a requirement for new buildings.Quote from: orin on January 04, 2017, 05:39:47 amBe warned: it's likely to get into a long winded and possibly heated discussion because:a) Legislation varies from one jurisdiction to another.b) And everyone has differing views about what level of risk is acceptable.

author: tggzzz
author_rank: Super Contributor
timestamp: January 04, 2017, 09:13:48 am
content: Quote from: Hero999 on January 04, 2017, 08:42:03 amA previous co-worker bought a new house, complete with certificates. Unfortunately every house in the road had been built with only 25% of the cement required in the mortar. Pointing could be abraded with a thumb, and brick walls easily pushed over.The builder's preferred remedy was to repoint. After a year's litigation they agreed to repurchase the entire road. I don't know what happened afterwards. Apparently it wasn't the first time the firm had been caught with substandard mortar.

author: mmagin
author_rank: Frequent Contributor
timestamp: January 04, 2017, 07:54:01 pm
content: Quote from: orin on January 01, 2017, 11:39:00 pmAnyone who uses one of these really should do so understanding the circuit inside. One configuration that it will NOT warn you about is where hot (line) and neutral are reversed and ground (earth) is hot.I have seen this condition myself in a place I used to rent where a 3 prong outlet was incorrectly retrofitted in an old house and too much of the hot conductor was bare and contacted the (not bonded to ground) metal box.

author: davidwyl
author_rank: Contributor
timestamp: January 05, 2017, 11:07:42 am
content: Out of frustration I went and ordered an 2 Anker brand charger. Unfortunately it is still the same leakage problem. if I'm barefooted and using the phone while charging, I can feel something is poking my hand.... I'm in Malaysia which follow UK type of plug, I'm Wondering people in China / US won't feel this leaking current? No mild shocking feeling when using metal phone while charging?Anker is one of the best selling charger in China & Amazon US, not one person complaint about this tickle feeling when using the phone.Is this something to do with country power socket standard?Sent from my ASUS_Z00AD using Tapatalk

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2017, 05:49:06 pm
content: Quote from: davidwyl on January 05, 2017, 11:07:42 amI doesn't matter where you are in the world or what sort of plug socket you have. Virtually all DC power supplies, which don't have an earth connection will do this. Another way round this would be to power the charger from an isolation transformer but they're quite bulky.

author: Gyro
author_rank: Super Contributor
timestamp: January 05, 2017, 06:32:54 pm
content: Quote from: davidwyl on January 05, 2017, 11:07:42 amYou could always treat it as a measure of quality - The manufacturer was actually prepared to spend the assembly time and component cost to put in some EMI suppression.Maybe the time to worry is if youdon'tget a "tickle" from a non grounded charger, then you'll know they've gonereallycheap!

author: davidwyl
author_rank: Contributor
timestamp: January 06, 2017, 12:30:15 am
content: Quote from: Hero999 on January 05, 2017, 05:49:06 pmIm really still think that if I take this charger to china / US I might not have the tickle feeling  . That's why no complaints about Orico/Anker charger in those countries....Those original charger that come with the phone usually no issue..I tested Samsung phone charger , Asus charger even Nokia charger & xiaomi phone charger, all have no this tickle issue.And non of this charger I tested has an Earth.Sent from my ASUS_Z00AD using Tapatalk

author: davidwyl
author_rank: Contributor
timestamp: January 06, 2017, 12:35:46 am
content: Quote from: Gyro on January 05, 2017, 06:32:54 pmFrom the cheapest no brand charger to expensive Orico/Anker charger, all giving me tickle feeling .Where else those original Samsung /asus/xiaomi/Nokia phone charger have no issue at all.So I truly believe EMI suspension is so much easier & cheaper to do than current leakage (tickle feel) issue.Sent from my ASUS_Z00AD using Tapatalk

author: Zero999
author_rank: Super Contributor
timestamp: January 06, 2017, 09:20:13 pm
content: Quote from: Gyro on January 05, 2017, 06:32:54 pmIt should be possible to do EMI suppression, with minimal leakage. The transformer could have a screen connected to the rectifier, either directly or via a small capacitor. This should prevent any capacitively coupled EMI from being transferred to the secondary.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 07, 2017, 01:42:14 am
content: In some cases, the transformer can also be arranged to give minimal coupling (without adding internal shields). Mostly applicable to 1:1 flyback transformers, but it follows from transmission line transformer theory and any ratio can be constructed (in principle, with the higher ratios being less and less practical..).Medical grade supplies probably use shields, and balanced windings if applicable, to minimize EMI and isolation capacitance. They typically need only 100s of pF of Y1 caps, if that.Tim