Isolations Transformers remove ground - but what if circuit in question needs it - Page 1
Forum section: electronics / projects

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 20, 2021, 03:25:55 pm
content: I am working on a SMPS circuit that has a ground in two places on the PCB one at the mains point of entry and another on the opposite side of the PCB.When I use an Isolation Transformer, it has no Grounding just Line and Neutral.So how does the circuit operate without Ground ?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 20, 2021, 03:33:24 pm
content: Because the plug can be inserted either way in many countries, "line" and "neutral" make no difference.Thus, mains-powered systems works with two LIVE conductors, and potentially PROTECTIVE EARTH.So forget about line and neutral, but also forget about "ground", there is no such thing in mains.Live, live and PE. Live, live and PE.In order to electrons being able to flow and do work, you need closed circuit. The two live conductors do that.Protective earth can be connected in the metal chassis, or can be left unconnected if the device has so-called reinforced or double insulation. There is no major functional difference regarding the basic operation.PE can be connected to the output (usually -), or not. Makes no fundamental difference, but good to know in case you are going to put multiple supplies in series. The supply is isolated nevertheless because neither of the LIVE wires connect to the output.Protective earth is just something that is guaranteed by your house wiring to be at the same potential as your metal piping etc. So that if you touch your kitchen sink and the power supply simultaneously, you won't get a shock no matter what's happening inside the (possibly faulty) supply because the two are tied strongly together, to same potential, through the PROTECTIVE EARTH wiring.

author: TimFox
author_rank: Super Contributor
timestamp: July 20, 2021, 03:47:58 pm
content: The most important thing about the answer above is that any exposed metal should be connected to “PE” = “Ground” to protect the operator against shock from the power line.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 20, 2021, 03:54:16 pm
content: Quote from: TimFox on July 20, 2021, 03:47:58 pmYes, or, anything connected toeitherof the live wires properly double insulated as per regulations, in which case exposed metal is OK. Many two-prong plugged consumer devices do that because it's sometimes hard to completely avoid exposed metal (think about USB connector). The symbol for this is two squares inside each other. Practically, this means something like adding an extra plastic sheet so that if the wire insulation breaks down, the exposed wire still can't touch the metal, but for details OP needs to consult the relevant standards as required by the legislation.

author: TimFox
author_rank: Super Contributor
timestamp: July 20, 2021, 05:07:47 pm
content: An example of an accident waiting to happen with an isolation transformer:Before I retired, my US employer installed large equipment in foreign countries, many of which used 240 V single-phase power with a two-round-prong European-style plugs.One of our field engineers (who recognized how dangerous this object was and was careful when using it) bought a 240 to 120 V transformer overseas, without PE connection. That wasn't the bad part:Both sides of the transformer had "universal" two-post female outlets that could mate with a US or European male connector.It was supplied with a power cord fitted with a US male on one end, and a European male on the other end.To derive 120 V power from a local 240 V outlet, you first plugged the US male into the transformer's 240 V side, and then the European male into the local outlet.Vice-versa for 240 V from 120 V, but we rarely did that.What could go wrong?

author: bdunham7
author_rank: Super Contributor
timestamp: July 20, 2021, 05:24:57 pm
content: Quote from: TimFox on July 20, 2021, 05:07:47 pm480VAC?

author: TimFox
author_rank: Super Contributor
timestamp: July 20, 2021, 05:29:37 pm
content: 480 V could happen, but think of what happens when someone trips over the cord and pulls it from the transformer socket...

author: bdunham7
author_rank: Super Contributor
timestamp: July 20, 2021, 05:32:06 pm
content: Quote from: ThermallyFrigid on July 20, 2021, 03:25:55 pmIsolation transformers do not remove the ground, they remove the ground reference from the mains voltage. Whether this is useful to you depends on a lot of additional circumstances.If your isolation transformer unit has ground pins on the input power cord and output socket, then it should pass the ground through without interruption. If your isolation transformer has only a two-prong cord and outlet, then it is not suitable for use with any device that requires grounding. Can you be specific about both devices--the isolation transformer and your SMPS--with either photos or model numbers?

author: bdunham7
author_rank: Super Contributor
timestamp: July 20, 2021, 05:37:09 pm
content: Quote from: TimFox on July 20, 2021, 05:29:37 pmYou mean the exposed live prongs on a double-male cord? No big deal. That's how a lot of people hook up their 240VAC generators to their homes--a double-ended dryer cord. You definitely want to do those things in the right order!

author: TimFox
author_rank: Super Contributor
timestamp: July 20, 2021, 05:42:11 pm
content: We had one client employee at a foreign site who regularly unplugged the transformer end first.Another problem we had was that sometimes during installation the power distribution (possibly from a generator, temporarily) was not properly grounded, so the metal of our stuff and tower computers was at roughly half the mains voltage, through capacitors in the usual line filters, with respect to “real ground”. Not enough current to kill you, unless you fell down.

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2021, 06:18:46 pm
content: Quote from: bdunham7 on July 20, 2021, 05:32:06 pmI would have said that they break the connection between neutral and ground, while passing ground through.

author: bdunham7
author_rank: Super Contributor
timestamp: July 20, 2021, 07:56:48 pm
content: Quote from: David Hess on July 20, 2021, 06:18:46 pmYou, I and the OP are all in the US, so that's pretty good description. However, there could be other arrangements--say 240VAC balanced power (a grounded center tap) or mains power that is designed as isolated but has leakage issues--and then my generic description covers those as well. Isolation transformers are a continually contentious subject around here an it's a worldwide forum, so I'm careful what I say.

author: TimFox
author_rank: Super Contributor
timestamp: July 20, 2021, 08:05:37 pm
content: If both sides of the transformer have a three-pole connector (male on one side and female on the other), with the PE/ground terminals appropriate to that series of connectors, the third (PE/ground) terminals should connect through and to the metal frame. I would not expect a direct ohmic connection between the primary and secondary windings, however.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 20, 2021, 08:18:40 pm
content: The Isolation Transformer is theBK Precision BK1604AThis informative video on YouTube is what I was watching. One of these videos on this transformer says to completely disconnect the Green ground wire inside.

author: TimFox
author_rank: Super Contributor
timestamp: July 20, 2021, 08:21:20 pm
content: I didn't watch the video, but the input side is a line cord with a US 3-prong male plug, and the output side is a panel-mount 3-prong female outlet, so I assume the "third prongs" are connected directly to each other and the case.

author: bdunham7
author_rank: Super Contributor
timestamp: July 20, 2021, 08:57:04 pm
content: Quote from: ThermallyFrigid on July 20, 2021, 08:18:40 pmI'm not watching a half hour video on basic topic, but Martin probably is at least worth a listen. I hope he isn't the one suggesting disconnecting the ground wire because I would consider that criminally negligent. If you want to interrupt the ground for some valid reason, like measuring ground leakage, you should use a 2-to-prong adapter, a "cheater" plug.I'm not sure exactly what you are trying to do, but be careful and think it through. Setting up all your measurements hands-off and then turn the power on is a good way to not get toasted.

author: nctnico
author_rank: Super Contributor
timestamp: July 20, 2021, 10:01:34 pm
content: A CAT rated differential probe is a much better idea. A danger of an isolation transformer is that it stops the GFI from working and the DUT is likely to be grounded again through test equipment. In general using an isolation transformer is a very unsafe way to work on mains powered equipment compared to having a GFI and differential probe.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 20, 2021, 11:26:23 pm
content: Quote from: bdunham7 on July 20, 2021, 08:57:04 pmHow about 10 seconds ? At 25:45That's where he says to "snip" the green Earth wire and explains why.The man really sounds like he knows what he's talking about.

author: bdunham7
author_rank: Super Contributor
timestamp: July 20, 2021, 11:59:50 pm
content: Quote from: ThermallyFrigid on July 20, 2021, 11:26:23 pmWell, that's bad advice and there's absolutely no sane reason to do it that way, especially since the next user may not anticipate that the ground would be removed. One of the features of isolation units like that is the circuit breaker and the impedance of the transformer that limit fault current to a spark and a click, rather than an arc and fire--just in case you make an error hooking things up or there is an unanticipated fault. Removing the ground connection would change the result in some cases from a circuit breaker trip to a live chassis. Interestingly, Martin is in South Africa, although he apparently has lived in the US. Their power is completely different, AFAIK, so I'm not sure what he's doing with all of those NEMA plugs and such.

author: Benta
author_rank: Super Contributor
timestamp: July 21, 2021, 12:09:36 am
content: Quote from: ThermallyFrigid on July 20, 2021, 11:26:23 pm"The man" is an idiot. At the sequence you point to, he promotes using a battery powered oscilloscope for measurements on live connected equipment. The most surefire way of killing yourself.

author: bdunham7
author_rank: Super Contributor
timestamp: July 21, 2021, 01:02:49 am
content: Quote from: Benta on July 21, 2021, 12:09:36 amI didn't see that part. I use battery-powered scopes all the time on mains and HV--what's wrong with that?

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 21, 2021, 01:18:37 am
content: Quote from: Benta on July 21, 2021, 12:09:36 amI think nearly every expert would disagree.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: July 21, 2021, 01:20:28 am
content: Quote from: bdunham7 on July 20, 2021, 11:59:50 pmThanks.So I guess I'll reconnect the Green Earth wire.

author: pqass
author_rank: Super Contributor
timestamp: July 21, 2021, 04:49:18 am
content: Quote from: bdunham7 on July 21, 2021, 01:02:49 am1. exposed metal BNC shield which won't kill you if that's the only thing you touch; unlike fully shrouded multimeter leads.2. all channels have BNC shields connected together so you'll need to remember to use only one probes' ground lead or you might blow up your circuit (possibly with lead in hand).That's why differential probes are the recommended, yet expensive, method to properly use a scope on a live circuit.

author: bdunham7
author_rank: Super Contributor
timestamp: July 21, 2021, 05:23:26 am
content: Quote from: pqass on July 21, 2021, 04:49:18 amIf those things are present, then you have the wrong scope and probes, regardless of how it is powered. Except the common-BNC thing where it applies--that should be manageable if you aren't an idiot. But still, there are perfectly good CAT-rated battery powered scopes and probes for this purpose.QuoteRecommended by whom? The makers of CAT-rated isolated scopes or the purveyors of differential probes? I would mention isolation interfaces, but those are expensive and mostly obsolete, except for the economical but low performance Siglent ISFE.

author: dietert1
author_rank: Super Contributor
timestamp: July 21, 2021, 06:07:43 am
content: Don't know whether it has been mentioned:PE is a safety system for electrical devices running on mains and with metal on the outside that can be touched by a user. All those metal parts are connected to PE. In case a live wire inside the device comes loose and touches one of the conductive parts the user can reach, the PE connection will trip the mains fuse for that device and thus protect the user from touching a conductive part with live voltage on it, possibly saving his/her life.The PE wire can't be used under normal conditions, only in case of failure. The isolation transformers i know run PE from input to output. Of course, if you want to scope the primary side of a SMPS, you need to run it with an isolation transformer. Never disconnect PE of your scope, but run the SMPS through the isolation transformer. You need to be alert while doing this. Keep your left hand in the pocket. Don't go away with that setup live on your bench.Regards, Dieter