Question: Isolation transformer, Ground floating or Ground shorted to Neutral - Page 1
Forum section: electronics / beginners

author: Gtx21
author_rank: Contributor
timestamp: September 02, 2018, 03:56:36 am
content: For an Isolation transformer, I have disconnected the (edit) isolation side ground pin from the earth-ground.Now that the ground pin on the isolation side isn't connected to anything, should I leave it floating or should I short it Isolation-side-Neutral?Edit: The reason I ask this is because I have seenother transformersa transformer that has all 3, isolation-side neutral, earth-ground, isolation-side ground pin, shorted together.Edit: I'll try and be more clear.Code:| +-------------------------+| | | +-----+Wall |===>| Isolation Transformer | ----->| DUT || | | ^ +-----+| +-------------------------+ |+--+|Previously, Earth ground from the wall |went to the DUT with the scondary winding |output here: ------------------------------+It is this connection that I severed.

author: Monkeh
author_rank: Super Contributor
timestamp: September 02, 2018, 04:28:01 am
content: You should reconnect it properly.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 02, 2018, 04:30:10 am
content: It appears that you haven't got the concept of what the purpose of an isolation transformer is. There is no 'isolated ground' rather the one ground from the U shaped ground pin on the 3-wire input cord is connected to the transformer core and also to the U shaped pin on the 3-wire outlet connector on any isolation transformer that is designed to meet code. Only the secondary winding is isolated from ground.

author: Gtx21
author_rank: Contributor
timestamp: September 02, 2018, 10:02:09 pm
content: Quote from: Monkeh on September 02, 2018, 04:28:01 amI don't want the ground pin on the isolation side connected to earth ground.

author: IanB
author_rank: Super Contributor
timestamp: September 02, 2018, 10:05:49 pm
content: Quote from: Gtx21 on September 02, 2018, 10:02:09 pmThen you don't want to be safe. Why don't you want to be safe?

author: helius
author_rank: Super Contributor
timestamp: September 02, 2018, 10:11:04 pm
content: Why would you ever believe it would be safe to make the DUT chassis live by connecting it to "neutral" (which is not neutral at all, but simply one side of a transformer secondary)? This is manifestly unsafe, and makes shock more likely when you are manipulating live circuits in the DUT. If you are not manipulating live circuits in the DUT, you should not be using an isolation transformer.Earthing the DUT chassis is safer than leaving it floating, but it still won't protect you from insulation faults. If you believe that either choice will protect you from shock, you are wrong.

author: Gtx21
author_rank: Contributor
timestamp: September 02, 2018, 10:16:49 pm
content: Quote from: IanB on September 02, 2018, 10:05:49 pmThe transformer is grounded. The isolation transformer case is grounded. Only the DUT will not be grounded. By leaving it connected, I have an earth-ground on my DUT which can cause me probing issues.

author: Gtx21
author_rank: Contributor
timestamp: September 02, 2018, 10:20:32 pm
content: Quote from: helius on September 02, 2018, 10:11:04 pmThere is no DUT chassis in this case. Just a test board. I"m not sure what you mean by manipulating live circuits. I am probing live circuits.

author: IanB
author_rank: Super Contributor
timestamp: September 02, 2018, 10:34:31 pm
content: The isolation transformer has two outputs, L1 and L2, both of which are hot. If you connect either of L1 or L2 to the ground pin of the output receptacle, then you will be making anything connected to that pin become "hot". If you do that, then why are you using an isolation transformer at all? You might just as well connect the DUT directly to the mains.If you have an isolation transformer, then you should not mess with its internal wiring. You should leave it as provided. Otherwise, maybe you do not understand what you are doing and you should take a step back until you do.

author: Benta
author_rank: Super Contributor
timestamp: September 02, 2018, 10:39:53 pm
content: Umm... the purpose of an isolation transformer is to have a DUT supply setup that is potential-free.The input side, chassis and core of the isolation transformer should be grounded. The secondary side is supposed to float. Any grounding on that side will happen through your test equipment or DUT.

author: Gtx21
author_rank: Contributor
timestamp: September 02, 2018, 10:42:14 pm
content: Quote from: IanB on September 02, 2018, 10:34:31 pmThere are more than 1 kind of isolation transformer. I have seen 3 different internal configurations for them. Some leave the Isolation-side ground pin floating, some leave the isolation-side ground pin grounded to earth-ground. The 3rd I don't understand what they are doing but they have the Isolation Neutral pin shorted to the Isolation Ground Pin then shorted to Earth-Ground. So inconsistency exists.

author: Gtx21
author_rank: Contributor
timestamp: September 02, 2018, 10:43:33 pm
content: Quote from: Benta on September 02, 2018, 10:39:53 pmThis was my belief and why I disconnected the Isolation Side Ground pin from Earth-Ground.

author: Benta
author_rank: Super Contributor
timestamp: September 02, 2018, 10:46:03 pm
content: Quote from: Gtx21 on September 02, 2018, 10:43:33 pmAfter seeing your edit on the OP, we're on the same page.

author: Bud
author_rank: Super Contributor
timestamp: September 02, 2018, 11:20:18 pm
content: Quote from: Benta on September 02, 2018, 10:39:53 pmCan you post a maybe handwritten diagram ( as a quick measure) explaining where you will have a potential in the DUT if you connect its chassis ground to the mains ground. I am just trying to understand your concern.

author: Bud
author_rank: Super Contributor
timestamp: September 02, 2018, 11:25:25 pm
content: Quote from: Gtx21 on September 02, 2018, 10:43:33 pmSay you debug a TX circuit in a transmitter as the DUT. You key the transmitter in and its chassis become hot because of RF currents. You touch the chassis and it burns you. To remediate that you must ground the transmitter chassis. What do you do now ?

author: tautech
author_rank: Super Contributor
timestamp: September 03, 2018, 02:35:01 am
content: As Monkeh said, reconnect it.Think about what an isolation transformer primarily offersIsolation of each conductor WRT mains GroundThat's what keeps you safe-----r.The 2 current conductors are still lethal WRT each other but no longer to mains ground where N + E are tied together at the switchboard and in Powerco circuits.As has been mentioned the isolated output is now Live and Live but again only WRT each other.Play it safe out there in EE land.

author: sibeen
author_rank: Frequent Contributor
timestamp: September 03, 2018, 03:11:59 am
content: Quote from: Gtx21 on September 02, 2018, 10:42:14 pmIn this case the transformer is not being used for isolation but is being used to construct a clean Neutral - Earth bond.

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2018, 03:59:49 am
content: Quote from: Gtx21 on September 02, 2018, 10:43:33 pmThe idea of the ground pin on a socket/receptacle is that any exposed metal parts connected to it should be tied to the earth ground in your local environment, putting them at the same potential as you, and therefore making them relatively safe to touch. This remains as true on the output side of an isolation transformer as it does on the input side.None of the live parts of the device under test should have any connection to this safety ground. They should be completely isolated, and the isolation transformer exists to preserve this isolation.

author: Gtx21
author_rank: Contributor
timestamp: September 03, 2018, 10:44:08 pm
content: Quote from: IanB on September 03, 2018, 03:59:49 amThis is an excellent point. I think I’m getting tripped up on specific instance based needs.My goal at this very moment is to reduce probing risk.Now, if the earth ground for the DUT only goes to the casing, I think I’m on board. If the earth ground for the dut is shorted to the DUT neutral, or used in some way internally, then I have a problem.The particular instance I ran into is where a heat sink was earth-grounded via a support screw to the case bottom, which was connected to the Emitter of a BJT, thus bring the Neutral to Earth Ground. In this situation, I got lucky and realized it before I started probing. Had I not, poof goes something.Maybe I’ll just spend the $300 for some differential probes.

author: Bratster
author_rank: Frequent Contributor
timestamp: September 03, 2018, 10:49:19 pm
content: The micsig dp10013 probes are around $130 on eBay right now.Sent from my Fi Moto x4 using Tapatalk

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2018, 10:55:29 pm
content: Quote from: Gtx21 on September 03, 2018, 10:44:08 pmIf the situation you describe occurs, then the device you are looking at has an electrical fault. No part of the live wiring of a device is permitted to be connected to earth ground. There are insulation testers that exist to check for this issue. They apply a high voltage between the combined L/N conductors of the device and the earth/ground conductor/chassis/casing and test for leakage current. A high resistance of 1 MΩ or more must exist for the device to pass the test, otherwise the device is unsafe and must not be used.Rather than investing in a differential probe, it might be better to invest in an insulation tester so you can check for device faults before working on them.

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 04, 2018, 03:26:42 pm
content: Ground shorted to neutral, NEVER! Bad bad practice.Saw this in Romania.Envoyé de mon iPad en utilisant Tapatalk

author: Monkeh
author_rank: Super Contributor
timestamp: September 04, 2018, 03:40:40 pm
content: Quote from: perieanuo on September 04, 2018, 03:26:42 pmPonder how neutral becomes neutral. It is, in fact, normal practice - when you want a neutral. There is no neutral without it.

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 04, 2018, 03:49:39 pm
content: Quote from: Monkeh on September 04, 2018, 03:40:40 pmNeutral is never neutral.Usual in triphasic systems every phase is distributed to 1/3 of the customers hoping it gets the network balanced and the neutral to 0, but every triphase network is unbalanced, so Neutral is not 0,never ever.It's almost zero Ground is earth plus 2-4 ohms, end of story,protects you from dying.Ground is mandatory imho everywhere, not only in wet rooms and dangerous electrical equipment.So keep the ground connection continuous.Regards,pierreEnvoyé de mon iPad en utilisant Tapatalk

author: Monkeh
author_rank: Super Contributor
timestamp: September 04, 2018, 03:54:13 pm
content: Quote from: perieanuo on September 04, 2018, 03:49:39 pm

author: SparkyFX
author_rank: Frequent Contributor
timestamp: September 04, 2018, 06:28:12 pm
content: The discussion around connecting ground and neutral strongly depends on the question if we are talking about anIT/TN/TT grid. The ops question was about an IT grid.Connecting the chassis of a device to protective earth only protects you as long as- the voltage source is grounded as well (should a mains wire have a short to the chassis, it trips the breaker by high current, therefore shuts off mains)- the person is grounded and only connected to one lead at a timeThe purpose of the isolation transformer is to protect by transforming voltage potential WRT ground to voltage potention not related to ground, thus not have the (carefully isolated) secondary side connected to ground. Therefore a grounded person will not have to suffer potentially dangerous current flowing through to earth should some part with input voltage be touched.If earth and neutral on the secondary side are connected all of this protection would be gone - there would be no benefit of using an isolation transformer. Use it to be able to inspect potentially defective devices with a faulty live chassis.There are of course special cases, e.g. as you put the DUT on the bench it could touch something that is grounded and through a ground fault short the setup becomes dangerous, so you got to know what you are doing at all times.QuoteBefore the (properly installed) GFCI it is quite common to have Neutral and Ground connected near the house distribution panel (in TN-C-S grids), it will however trip the GFCI all the time if it was connected behind the GFCI, as the return current is split up between neutral and ground and exceeds the GFCIs value. It is not common in TT-grids.It might also lead to live chassis all over the place should neutral and ground be interrupted one day (which is more likely if they were connected to save a wire). So never mixing up both is a good practice.