Mains power isolation for ground loop solving

It is sometimes suggested that a mains power isolation transformer might be used to solve the ground loop problem. It is true, that many times a mains isolation transformer does help, but not always. A system with properly used isolation transformer is still safe, it can be even safer than system without isolation transformer. Remember that using a mains power isolation transformer can be a potential safety and liability risk if not properly used.

There are different types of isolation transformers and different situations where they are allowed to be used. Mains power isolation using isolation transformer is commonly used practice in electronics laboratories and in powering equipment in hospitals. Isolation transformers enable a variety of electronic systems to meet safety requirements. Such systems include medical diagnostic equipment, computer systems, and telecommunications equipment.

In those case the isolation is done for safety reasons, not for fighting against ground loop problems. There are safety isolation transformers that can safely isolate the output without ground connection going though, and they can be useful where they are allowed to be used. The idea that grounding problems or loops can be corrected using a mains power isolation transformer does not always work.

Ideally, an isolating transformer should be used to protect only one item of equipment at a time. With one item a fault in the equipment will probably not produce a dangerous situation. Sometimes those same safety isolation transformers are also used to fight against noise problems, but generally they are used for safety reasons in electronics laboratories. In electronics testing and servicing an isolation transformer is a 1:1 (under load) power transformer used for safety. With the transformer, as there is no conductive connection between transformer secondary and earth, there is no danger in touching a live part of the circuit while another part of the body is earthed. Bridging between either terminal and ground is safe because there is no circuit through which the current can flow as seen on the following picture from Sound on Sound magazine.

You can use the same isolation transformers to electrically isolate your A/V equipment from connected AC power and thus break any ground loops. I have successfully used safety isolation transformers that look like this to solve ground loop issues on audio/video systems.

A floating supply significantly reduces leakage currents and therefore in many cases minimizes noise and damage to equipment through leakage via signal cables. Please note that there are two inherent ‘dangers’ with floating systems, although neither ‘danger’ damages equipment operating on the floating supply. The first ‘danger’ is; Should a Live-Earth fault occur then it is highly likely that NO protection will trip as there is no return for the fault currents. The second ‘danger’ is now present as only on a second fault will any protection operate.

If you plan to use use a mains isolation transformer for ground loop solving in your audio/video system, be careful to choose a transformer that is rated as “safety isolation transformer” with high enough power rating for your equipment you plan to connect. Isolation transformers are specified in terms of the amount of isolation that they provide, the power rating, efficiency (in percent) and the tolerance of the voltage regulation (in percent). Power transformers with specified insulation between primary and secondary are not usually described only as “isolation transformers” unless this is their primary function. Safety isolation transformer can have safe ungrounded mains power output where you can safely plug ungrounded equipment and equipment with ground connection (which is left unconnected to anything). With an isolation transformer, you have the option of determining for yourself where the ground potential should be for “downstream” circuitry. Whether you establish a new earth ground or not, you are no longer connected to the neutral wire of the incoming line power.

Remember to connect only one equipment directly to the safety isolation transformer output! This configuration will be safe to use in all conditions I can think of. You can think that the combination of safety isolation transformer and the equipment as ungrounded properly dual insulated equipment and handle it as such. Connecting more than one equipment to output can lead to dangers (in case one of the equipment have faults in them). Using an isolation transformer that is not rated as “safety isolation transformer” can be also dangerous.

Electrical isolation is considered to be particularly important on medical equipment, and special standards apply. Isolating transformers of grounded equipment are used in medical rooms to isolate a load from the mains in medical applications. Virtually every piece of equipment destined for use in a medical environment needs isolation from the mains. In hospitals for example, there are many commercial PCs which are used with medical equipment and in the medical environment. In medical systems there are cases where there are more than one equipment connected to one isolation transformer output. Often the system must additionally be designed so that fault conditions do not interrupt power, but generate a warning.

EC standard EN 60950 and IEC 950 specify the electrical safety characteristics IT systems. Per IEC 950, leakage current should not exceed 3.5 mA for Class I machines and 0.25 mA for Class II machines. Class I electronic products that are designed for handheld use must be limited to 0.75 mA or less leakage current. There are also other details in the standard that touch the use of isolation transformers. International office product safety regulations including IEC 950 and UL 1950 require that an isolation transformer is only allowed to isolate the hot and neutral wires; the grounding wire must be passed straight through. Neutral-to-ground voltage and noise can be eliminated by the isolation transformer. This means that isolation transformer is not the right tool to solve computer systems ground loop problems in normal office environment for IT equipment, because the transformer type the standard knows does not help in solving ground loops and fully isolating transformer would not meet the standards. Transformer per IEC 950 and UL 1950 is not completely useless: Cleaning up the power can make the ground loops less problematic by reducing the amount of noise and EMI in the power lines.

53 Comments

  1. David Rokeby says:

    Thanks for all the info on grounding and avoiding ground loops. I am trying to sort out a situation where I have a network of motors where each motor has a simple local linear AC-DC converter. I am intending to distribute 24 VAC from a single transformer to the 31 motor locations and convert to 24VDC at that point. I am not running line voltage to each location because they are distributed over a very large articulated robotic arm. The motors are connected in a CANbus, and so I need to keep the grounds at each motor controller close to each other. Would it be advisable to run earth ground throughout with the 24VAC and attach it to each AC/DC converter’s 0 Volts output?

    Reply
    • tomi says:

      Thank you for your feedback.

      I would say that it would be advisable to run earth ground throughout with the 24VAC and attach it to each AC/DC converter’s 0 Volts output.

      Reply
  2. David Rokeby says:

    I greatly appreciate your input. One other small question in this regard: The structure that these motors and AC/DC converters are going to be installed in is essentially a series of aluminum square tubes. Each is electrically isolated in the sense that there is no direct metal to metal link from one tube to the next. Each aluminum tube has 2 ac/dc converters, on at either end. The question is whether it is best to connect earth ground to the each aluminum tube in one place, or at both power supplies or not at all. (This structure is 27 feet in the air so the safety function of the earth ground is not terribly relevant).

    Thanks again!

    Reply
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  5. Electronics lab electrical power « Tomi Engdahl’s ePanorama blog says:

    [...] of the body is earthed. Usually you do isolation for safety reasons, but sometimes you might need mains power isolation for ground loop solving, and the safety isolation transformer works for this as [...]

    Reply
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    This handy assessment has been very interesting to go through, I valued this considerably. I’m going how to e-mail the idea to be able to my co-workers to allow for all of them look greater than this also. Thanks definitely.

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  7. tecmax says:

    The Isolation Transformers are in high demand to render safety to the electronic devices at your home. We can save upto a large extent in the electricity bill with the aid ofvoltage stabilizers.

    Reply
  8. Tomi Engdahl says:

    I can agree your comment at “The Isolation Transformers are in high demand to render safety to the electronic device”

    You claim ” We can save upto a large extent in the electricity bill with the aid of voltage stabilizers.” is harder to swallow. I know that electronic statiblizers are commonly used for protecting electrical devices and making them work more reliably at locations where electrical power voltage can vary. I can’t immediately see how they would allow to save the electrical bill… Some stabilizers can actually increase the bill because losses on them. Can you give more details how the claimed savings on the bill can be realized and how much they can be?

    Reply
  9. Wilson Varghese says:

    Does Good Electrical Grounding can save electrical bills?

    Reply
    • Tomi Engdahl says:

      I don’t see how good electrical grounding could affect the electrical bill to any direction in any significant way.
      Good Electrical Grounding has a positive effect on reliable operation, electrical safety, noise and avoiding equipment damage.

      Reply
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  12. Tomi Engdahl says:

    Upcycle An Isolation Transformer
    http://hackaday.com/2016/10/16/upcycle-an-isolation-transformer/

    There are several reasons you should have an isolation transformer. They can prevent ground loops and also prevent a device under test from having a DC path to ground (or isolate an oscilloscope from DC ground, which can be dangerous in its own right, but that’s another discussion). [Tanner_tech] noticed that finding ballast transformers for sodium vapor street lights is getting easier as more street lights move to LED technology. What to do with these transformers? Build an isolation transformer, of course.

    Industrial Isolation Transformer From Trash
    http://www.instructables.com/id/Industrial-Isolation-Transformer-From-Trash/

    If you work with electronics projects that deal with unisolated mains electricity, than you may be in danger of being lethally shocked! In addition to being dangerous to you, mains electricity can be lethal to electrical test equipment -such as oscilloscopes- if they are used on any unisolated electronic devices. Luckily, there is a device called an isolation transformer that will supply 110 volts AC completely isolated from the mains. This isolation greatly decreases the probability of lethal shock while working with electronics projects. Most people are unable to afford this great safety device because of the one hundred dollar price tag attached to the majority of isolation transformers.

    An isolation transformer works on the same principal that all other transformers use

    An isolation transformer has a winding ratio of 1:1, so the input voltage is the same as the output voltage. In normal mains current, the live wire is referenced to ground, so if the live wire is touched ,the person touching it will get shocked because they are capacitively coupled to earth ground. The isolation transformer fixes this issue.

    From comments:

    Cannot be sure of how much isolation you actually have though. This is really important to know.

    The way to check this is performing a Hi Pot test between winding’s and winding’s and core. You can use a simple Megger to do a basic Hi Pot test. But please don’t rely on this Transformer for serious safety use, especially for outside use as the housing is not sealed

    The most common reason to use a Isolation Transformers is to to provide Galvanic isolation to people from AC Utility supply ground returns for safety reason.

    ie. Domestic utility AC supply Neutral is usually close to or the same as the ground potential. So defective equipment with an isolation fault to Hot may lead to human contact and a flow through the body to another part of the body connected to ground. A good isolation transformer avoids this. Obviously it still has a dangerous potential on the output but it avoids the ground loop issue.

    Modern GFI’s do not isolate, they merely trip the power when a current from HOT to ground is exceeded. A shock will still happen, but due to it’s momentary nature it is unlikely to cause death or serious injury to most people.

    Until Modern GFI’s became available the use of isolation transformers for AC powered power tools inside was and probably for outside work is still highly recommended even mandatory for commercial professionals is some countries.

    Note: I used AC power tools as an example due the obvious physical contact using them involves, but it still will apply to any AC mains powered Test Equipment, and all other electrical equipment where physical contact is involved as well. Medical instrumentation is another very specialized field where electrical isolation is very important, this has other criteria beyond scope of a simple discussion like this.

    Reply
  13. Tomi Engdahl says:

    Test Gear: The Isolation Transformer from Salvage
    http://www.instructables.com/id/Test-Gear-The-Isolation-Transformer-from-Salvage/

    If you have worked much with 120 or 220 AC mains power you may know what an isolation transformer is, if not you should because it is important piece of safety equipment.

    So what is the point? There is no direct connection between the primary side and the secondary, so there is no ground connection between the two sides and a high dc resistance ( normally good for 100′s of volts ) between the two sides. This means that if you are grounded ( touch a ground, stand on a damp floor, use grounded test equipment ) that you can connect to either side of the isolation transformer without risk of a shock.

    So maybe you should have one, but do not want to lay out the money for one ( they are surprisingly expensive ). This instructable describes one I built and gives you tips to build your own. With a bit of luck you can find a salvage transformer ( or 2 ) and put it together for cheap.

    Reply
  14. Tomi Engdahl says:

    ToddFun.com: Isolation Transformer – Tripp Lite IS1000
    https://www.youtube.com/watch?v=11Yve2ijWyk

    Tutorial using an isolation transformer including what to buy, how to alter it for your tech bench and how to be safer when working with high voltage live mains. Also some helpful comments on how to use your oscilloscope safely when working with high voltage earth grounded mains.

    http://www.toddfun.com/2011/04/30/isolation_transformers/

    Reply
  15. Tomi Engdahl says:

    SDGEE #06 Mains Isolation Transformers and UK Earthing Systems
    https://www.youtube.com/watch?v=byrDzJtR7Fs

    Transformers & Earth Connections
    https://www.youtube.com/watch?v=fRhofcMyAyk

    Earth connections used with isolating transformers and building site transformers.
    An isolating transformer has no connection to earth at all, although the mains earth connection is often continued through to the outlet socket.
    A 110V site transformer has the centre of the output winding connected to earth, so that both output conductors are at 55V relative to earth.

    Reply
  16. Tomi Engdahl says:

    Isolated Electricity Supplies – Why they are not used for everything
    https://www.youtube.com/watch?v=Wx_v1-T1gE4

    An isolated supply is not referenced to ground, and so the only way to get a shock from it is to touch both conductors at the same time. They are not used everywhere as it is very difficult to ensure they remain isolated – any fault to ground anywhere would go unnoticed but convert the system to ground referenced.

    Reply
  17. Tomi Engdahl says:

    Mains Isolation Transformers
    https://www.youtube.com/watch?v=sfzy5IQMzyc

    What an isolating transformer does, and what they are used for.

    SDGEE #06 Mains Isolation Transformers and UK Earthing Systems
    https://www.youtube.com/watch?v=byrDzJtR7Fs

    Reply
  18. Tomi Engdahl says:

    Stray-Currents, compensating-currents and ground-loops in Transformers
    https://www.youtube.com/watch?v=YhDH7rOIuGg

    Why an isolation-transformer is not 100% isolating due to capacitive coupling. This effects haunts many HiFi-buffs and sometimes appears as hum even in normal audio/video connections. Roger explains the effect and makes a few measurements to demonstrate the effect.

    Reply
  19. Tomi Engdahl says:

    Isolation Transformers can save your life, and your equipment.
    https://www.youtube.com/watch?v=3rvEdHyAd2k

    Reply
  20. Tomi Engdahl says:

    Neutrals from transformers and in electrical circuits
    https://www.youtube.com/watch?v=AgCY_d98HF8

    What a neutral wire is and how it is derived at the transformer.
    Also how part of a circuit is a neutral, how it isn’t once disconnected and possible dangers of disconnecting wires which have apparently no voltage on them.

    Isolated Electricity Supplies – Why they are not used for everything
    https://www.youtube.com/watch?v=Wx_v1-T1gE4

    An isolated supply is not referenced to ground, and so the only way to get a shock from it is to touch both conductors at the same time.
    They are not used everywhere as it is very difficult to ensure they remain isolated – any fault to ground anywhere would go unnoticed but convert the system to ground referenced.

    Reply
  21. Tomi Engdahl says:

    ToddFun.com: Isolation Transformer – Tripp Lite IS1000
    https://www.youtube.com/watch?v=11Yve2ijWyk

    Tutorial using an isolation transformer including what to buy, how to alter it for your tech bench and how to be safer when working with high voltage live mains. Also some helpful comments on how to use your oscilloscope safely when working with high voltage earth grounded mains.

    Reply
  22. Tomi Engdahl says:

    Tech Tips Tuesday, Isolation Transformer and Variac Safety
    https://www.youtube.com/watch?v=XBsQ3sZ45Fk

    Some test bench safety tips, and a sneaky isolation transformer issue you may not know about.

    Comments:

    Hi Paul, wow I don’t understand why that transformer is officially out in the market as certified isolation transformer. In Europe this would never ever get the declaration of confirmatory to be sold as isolation transformer. Following VDE/ EN norm the secondary side never is attached to protective ground/ earth. But anyway what ever norms are telling at all it doesn’t make any sense…., or is there is something so much substantially different in Canada”s power distribution compared to Europa I don’t know?? Anyway very interesting video ! Thank you. 73 Peter

    There seems to be a lot of confusion about this even in europe. Mostly an “isolation transformer” has a earth connection on the secondary and a “safety transformer” has not, but has this clearly marked with a warning next to the outlet (and warns you not to hook up more than one device). But I have seen a “safety isolation transformer” (made by peaktech) that has a earth connection on the secondary, completely defeating its purpose (even false advertising imho).

    Hi Robert, yes very important issue and you are right a lot of confusion all over. I’m talking only about a galvanic protective separation transformer (Schutztrennung) according VDE 0570 Part 2-4, DIN EN 61558-2-4, EN 61558-2-4, IEC 61558-2-4

    I agree fully, but politics and bureaucracy gets in the way of sense once again. If they want to include that “Earth” they need to re-label the transformer, and remove the word “Isolation”

    It’s because isolation transformers such as the topaz was designed to protect electronic equipment from common mode noise. Often these types are sold as an AC line filter. It most likely has it’s own natural to ground bond inside. It was not designed to be a technician repair type isolation transformer. It’s satisfying to see Paul pointing this out. Too easy for a technician to not realize the danger.

    For starters you would normally use the isolation transformer to isolate the device under repair, not the scope.My isolation transformer the test plug is only connected to the secondary, the ground prong is completely open on the transformer.Mine is a Hammond manufacturing co, made in Canada.

    Reply
  23. Tomi Engdahl says:

    EEVblog #279 – How NOT To Blow Up Your Oscilloscope!
    https://www.youtube.com/watch?v=xaELqAo4kkQ

    How NOT to blow up your Oscilloscope.
    And how NOT to blow up your Arduino.
    Your oscilloscope ground clip lead can be a potential accident waiting to happen.
    Why? And how do you avoid it?
    The basics about measuring mains earth referenced equipment with your oscilloscope.

    Reply
  24. Tomi Engdahl says:

    Isolation Transformers: Internal Design and Safe Use with Oscilloscopes
    https://www.youtube.com/watch?v=5SjVm-rF39A

    In this video we will 1.) Attempt to build an economical Isolation Transformer (I.T.) 2.) Discuss Medical I.T.’s and how to convert them into a Tech I.T. 3.) Using actual testing and measurement in a live amp, we will develop a set of rules for safe use of I.T.’s with oscilloscopes. 4.) State the rules for safe use of I.T.’s with oscilloscopes, which include the benefits of an un-grounded chassis. In addition, a new cast member is introduced, Jack’s good friend, Ollie the Feral Cat, whose craving for catnip knows no bounds :)

    Reply
  25. Tomi Engdahl says:

    Mains Isolation Transformers
    https://www.youtube.com/watch?v=sfzy5IQMzyc

    What an isolating transformer does, and what they are used for.

    Reply
  26. Tomi Engdahl says:

    ISOLATION TRANSFORMERS FOR TESTING CIRCUITS WITH AN OSCILLOSCOPE.
    https://www.youtube.com/watch?v=NoR9wrgh3wc

    ISOLATION TRANSFORMERS. USE AT OWN RISK.

    OTHER GREAT VIDEO EXPLAINING THE CONCEPT.

    Isolation Transformers can save your life, and your equipment.
    https://www.youtube.com/watch?v=3rvEdHyAd2k

    Reply
  27. Tomi Engdahl says:

    Isolation Transformers
    https://www.youtube.com/watch?v=vz70GSBhYjA

    This video presents a thorough discussion of why isolation transformers are able to provide protection against shock hazards while working on electronic equipment, using transformerless amp circuits, and/or operating an oscilloscope. In the next video, we will build a basic, inexpensive isolation transformer…..so stay tuned.

    This video shows 120VAC (mains) circuitry. The general principles of ground referencing for 240VAC mains circuitry are basically the same.

    Reply
  28. Tomi Engdahl says:

    ISOLATION Transformer “MOD”
    https://www.youtube.com/watch?v=n92RLRYcT3Q

    WARNING: DO THIS AT YOUR OWN RISK, UNLESS YOUR QUALIFIED DO NOT ATTEMPT IT!
    The first thing I have to do is make this isolation transformer safe.
    It had a BONDED neutral on the secondary winding, UNSAFE for use on a line connected hot chassis radio.

    Reply
  29. Tomi Engdahl says:

    Transformers & Earth Connections
    https://www.youtube.com/watch?v=fRhofcMyAyk

    Earth connections used with isolating transformers and building site transformers.
    An isolating transformer has no connection to earth at all, although the mains earth connection is often continued through to the outlet socket.
    A 110V site transformer has the centre of the output winding connected to earth, so that both output conductors are at 55V relative to earth.

    Reply
  30. Tomi Engdahl says:

    Big Dim Bulb Tester with Variac and Isolation Transformer
    https://www.youtube.com/watch?v=51mjt9nFoeA

    Here is a large dim bulb tester and variable AC power supply that will provide up to 240 Volts. It current limits at “Around” 1 amp at 120V, and 500mA at 240V with the bulbs switched in. This is a dangerous build. If you build this, it’s at your own risk.

    Reply
  31. Tomi Engdahl says:

    TTT155 The Death Difference
    https://www.youtube.com/watch?v=w3tgY3s47f4

    The importance of understanding and using isolation transformers whenever working on AC powered equipment.

    Reply
  32. Tomi Engdahl says:

    A Look At My Isolation Transformer Setup
    https://www.youtube.com/watch?v=ZWT3bkukxuQ

    I needed an excuse to try out my new audio recorder, so I thought I’d show how I salvaged and repurposed some isolation transformers for my ‘experiments’.

    Reply
  33. Tomi Engdahl says:

    The Power Grid, Isolation Transformers and Earthing
    https://www.youtube.com/watch?v=OBYzvkY2-eA

    In order to explain the idea behind some isolation transformer units I’m working on at the moment, I explore the power grid, safety features of electric installations, Earthings Systems and RCDs.

    Types of Earthing System for Electricity Supplies (UK)
    https://www.youtube.com/watch?v=AWxeb2MI37c

    The three main types of earthing for mains electricity supplies to buildings in the UK. TT, TN-S and TN-C-S.

    Reply
  34. Tomi Engdahl says:

    How to test an isolation transformer
    https://www.youtube.com/watch?v=ZrCCoxEuZl0

    In this video I show how to test an isolation transformer to see if it is truly isolated. This test can be used on any type of isolation transformer.

    Some valuable isolation transformer information.
    https://www.youtube.com/watch?v=Xf8xTRVXwmI

    A couple of things that can save you injury or even your life.

    Reply
  35. Tomi Engdahl says:

    Every maker should have…[Pt.28] an Isolation-Transformer and a Variac
    https://www.youtube.com/watch?v=FuEmGR8bBEc

    Roger explains why you should have an isolation-transformer for your personal safety.
    How does it work, what can you do with it and what is the difference to a Variac.
    It´s cheaper and quite easy to build one of your own.

    Reply
  36. Tomi Engdahl says:

    Deadly isolation transformer flogged on ebay
    https://www.youtube.com/watch?v=hFTVYwvn114

    They sell anything on Ebay. Buyer beware, don’t get burned like this person did.

    Reply
  37. Tomi Engdahl says:

    SDGEE #05 Medical Isolation Transformer Teardown and Modifications
    https://www.youtube.com/watch?v=Kkb6QCMjfic

    In this video I teardown my benchtop medically rated isolation transformer by REO Inductive Components. (The REO-MED 1000).

    Reply
  38. Tomi Engdahl says:

    Isolating transformer, portable, testing
    https://www.youtube.com/watch?v=YdZy41gGUY4

    Portable Isolating transformer testing. Caution the standards for testing change from time to time. Check for validity in your area. The principles of testing are the same all over the world. The wires that are supposed to be joined together should have continuity (low resistance). The wires that are not supposed to be joined together have insulation (very high resistance) in excess of 1 million Ohms- 1M Ohm. There is no requirement to open an appliance for testing.

    Reply
  39. Tomi Engdahl says:

    Isolated Electricity Supplies – Why they are not used for everything
    https://www.youtube.com/watch?v=Wx_v1-T1gE4

    An isolated supply is not referenced to ground, and so the only way to get a shock from it is to touch both conductors at the same time.
    They are not used everywhere as it is very difficult to ensure they remain isolated – any fault to ground anywhere would go unnoticed but convert the system to ground referenced.

    Reply
  40. Tomi Engdahl says:

    Comments from https://www.youtube.com/watch?v=Wx_v1-T1gE4 :

    Richard O’Keeffe
    2 vuotta sitten
    Hi JW, there are some large isolated terre systems used in the UK, Railway signalling power supplies are IT systems. IT is used because it is deemed safer to live with the first earth fault and not blow a fuse. There is monitoring equipment installed which alarms at an IR value less than 100k ohms and the fault is supposed to be fixed with 3 days. Very specialist field I know but IT networks do exist, the modern trend is to use class II switchgear and transformers with a special cable.

    ben kerr
    2 vuotta sitten
    seen IT system in cranes too, were you really don’t want a fault causing a supply to trip fpr something like an electromagnetic pick-up.

    Marcel Germann
    2 vuotta sitten
    IT systems are common in hospitals too.

    Tim Boler
    1 vuosi sitten
    Same on most electrical installations on ships

    BersekViking
    2 vuotta sitten
    With a ground reference system, you also have a low impedance path to ground. In a floating system you could have a high common mode voltage of several thousand volts on the system that could cause damage to the cabling or even kill you.

    Mark Horton
    2 vuotta sitten
    That could be the case if you had something like a van der graph generator, but the isolated supplies mentioned here are just 240v transformers. So they will only have that voltage plus a possible and tiny DC component which will not have any significant energy to it. It could only be tiny because there is nothing to drive the voltage differential higher, left to themselves static voltages float down not up.

    Sebastian Tunem
    2 vuotta sitten
    Here in Norway, most end users are supplied with electrical power from IT transformers (3 phase 230 volt 50 Hz) with no neutral wire. Most new transformer circuits are TN (3 phase 400 volt 50 Hz). I have heard that Norway is maybe the only country with widespread use of IT transformers for end users.

    Terry Clark
    2 vuotta sitten
    Interesting observations. I’ve often wondered about the reasons for earthed supplies and assumed that the reason was to mitigate the danger of insulation leakage from primary to secondary at the substation which could otherwise bring the whole house supply up to 11kV.

    rtel123
    2 vuotta sitten
    The north americans got around the unknown-ground-reference safety issue by grounding a secondary centre tap of the supply transformer on the street. That way, we have a choice of supplying 240 volts or 120 anywhere in the house, and the max shock hazard is only 120-to-ground regardless. To ground one side as they do in europe and thereby creating a shock hazard of 240-to-ground everywhere creates an unnecessary increase in risk where 120 would have been sufficient.

    Reply
  41. Tomi Engdahl says:

    Stray-Currents, compensating-currents and ground-loops in Transformers
    https://www.youtube.com/watch?v=YhDH7rOIuGg

    Why an isolation-transformer is not 100% isolating due to capacitive coupling.
    This effects haunts many HiFi-buffs and sometimes appears as hum even in normal audio/video connections.

    Roger explains the effect and makes a few measurements to demonstrate the effect.
    Another video explaining the effects even in switch-mode power-supplies:
    A multi-stage EMI-Filter for DC Power-Supplies Pt.1: Noise sources and noise-coupling
    https://www.youtube.com/watch?v=lRMnaHw0SzA&list=PL6W_lVCKeS07KqeSAzN0Y3YehfSb3EKhH

    Reply
  42. Tomi Engdahl says:

    How to test an isolation transformer
    https://www.youtube.com/watch?v=ZrCCoxEuZl0

    In this video I show how to test an isolation transformer to see if it is truly isolated. This test can be used on any type of isolation transformer.

    Reply
  43. Tomi Engdahl says:

    Sujit Gupta this looks isolated, but it still has capacitor between mains side low voltage side.
    “To suppress the high frequency common mode is is necessary to put capacitors between the input and output side of the power supply with a capacitance substantially higher than the capacitance in the flyback transformer.”
    “When desinging a class 2 (unearthed) PSU we have no choice but to connect these capacitors to the input “live” and/or “neutral”. Since most of the world doesn’t enforce polarity on unearthed sockets we have to assume that either or both of the “live” and “neutral” terminals may be at a sinificant voltage relative to earth”
    “That means on a class 2 PSU we have a difficult tradeoff between safety and EMC. Making the capacitors bigger improves EMC but also results in higher “touch current” (the current that will flow through someone or something who touches the output of the PSU and mains earth).”
    https://electronics.stackexchange.com/questions/216959/what-does-the-y-capacitor-in-a-smps-do

    Reply
  44. Tomi Engdahl says:

    Is An Isolation Transformer A Safety Measure Or Death Trap?
    https://sound-au.com/articles/iso-xfmr.htm

    Introduction

    Before I start, I must point out that most common transformers provide isolation. The transformer used for a power amp or preamp will always provide galvanic isolation between hazardous voltage (the mains) and the lower voltages needed by most circuitry. Galvanic isolation simply means that there is no electrical connection between the mains and secondary windings, and power transfer is provided by a magnetic field. Other forms of galvanic isolation include optocouplers, pulse (small, high frequency) transformers and capacitive couplers (using a very low capacitance). While these provide isolation, they are not the subject of this article. All galvanic isolation products will show an open circuit between the mains and secondary (or input and output) when measured with an ohm meter or insulation tester.

    This article is based on the use of a 1:1 mains transformer (120V or 230V), used to isolate the mains input so there is no neutral (earthed) conductor on the secondary side. These are commonly used as a ‘safety’ measure, but as described here, this is often inappropriate and can be dangerous. It has long been believed by many people that using an isolating transformer on the test bench is a good safety measure, and will somehow protect the service tech from harm. In reality, for most products, this is false. If you always use an isolation transformer, it can let dangerously faulty equipment pass undetected.

    There are cases where an isolation transformer is absolutely essential. It’s still very common, and circuitry connected directly to the mains is dangerous to work on. An isolating transformer is essential if you need to work on the electronics. In the ‘old’ days when so-called ‘hot chassis’ equipment was popular (it was cheaper to build), you couldn’t work on the gear without an isolation transformer. These products were common, especially in the US, but also in Australia and elsewhere. TV sets were the most likely to have a hot chassis, but there were also radio receivers that used the technique.

    The most common requirement now is switchmode power supplies, but most aren’t designed to be repaired. For those that can be fixed, you definitely need an isolation transformer, but it should be left disconnected and out of circuit for any other gear you work on. While many people think that isolated mains are ‘safer’, it’s an illusion (and a dangerous one!).

    The safest way to power mains operated equipment (but where you don’t need to work on live mains) is from the normal mains via a safety switch. These go by many names, such as core balance relay, RCD (residual current device), earth leakage circuit breaker (ELCB), ground fault (circuit) interrupter, or just plain old ‘safety switch’. A full explanation of how these work is outside the scope of this article, but is easily found on-line via a web search. A brief and simplified description is provided below. Note that I will use the term ‘safety switch’ as a matter of course.

    Of course there are situations where use of an isolation transformer is essential, with one being an unexpected requirement – testing portable safety switches.

    To be able to work on the hot chassis receivers, it is essential to use a 1:1 isolation transformer. Without it, it’s impossible to use an oscilloscope, and simply touching the chassis could be lethal. These have now all but vanished from the mainstream, but many pieces of vintage gear is still restored by hobbyists all over the world. Now, in their place, we have SMPS (switchmode power supplies). These (mostly) solve the isolation problem on the secondary side, but of course the SMPS primary circuitry itself is at mains potential. Also, be aware that some LED lighting power supplies are not isolated, so everything is at mains potential! This means that touching the wiring to the LED panel may be dangerous, but fortunately, such light fittings are generally fully enclosed to prevent contact.

    For additional safety, use a Variac with the isolation transformer if you can. Many switchmode supplies are designed to operate from 90-260V, so using the lowest voltage possible for the SMPS to work means that some of the voltages you are exposed to are lower, and have a lower risk factor. This doesn’t always work of course – some SMPS only work over a limited voltage range. Others generate 400V DC or more before it’s switched to the transformer (active power factor correction) and the voltage is the same regardless of input voltage within its operating range.

    Reply
  45. Tomi Engdahl says:

    Somewhat related to isolation is balanced power:

    110V Centre Tap Earth (55V-0V-55V)
    https://www.gstransformers.com/technical/110v-centre-tap-earth-55v-0v-55v.html

    A centre tap is a connection made from the centre of the secondary winding of a transformer. It is labeled as Earth, Common or 0V. Grounding secondary winding of a transformer is usually done by connecting the earth to the neutral lead. There exists another system in which earth is connected to the centre tap of the transformer instead of the neutral. This is known as Centre Tap Earth (CTE). CTE systems are very common in electrical distribution systems and site safety operations.

    Some common CTE operating voltage
    110V CTE 55V-0V-55V
    220V CTE 110V-0V-110V
    230V CTE 115V-0V-115V
    240V CTE 120V-0V-120V

    Before selecting a transformer for a system, it is important to understand its earthing system whether it is neutral earthing or centre tap earthing. By having a wrong transformer for the application may cause damage to the connecting devices.

    110V Site Transformer Repair
    https://www.youtube.com/watch?v=2t_JR0CxctQ

    Reconnecting the mains input lead and replacing the cable gland on this 110V site transformer.
    These are used to power 110V tools in the UK. The output of the transformer has a centre connection to earth, so the voltage between live parts and earth is only 55 volts, much safer than either 110 or 230V.
    These transformers have a very short lead by design, as they are located indoors. A 110V extension lead is used to connect to tools and equipment outside, so the 230V remains inside the building.

    Reply
  46. Tomi Engdahl says:

    Shaver Sockets for UK Bathrooms – With Isolating Transformer
    https://www.youtube.com/watch?v=hyaaPt6UuEU

    An older example of a shaver socket outlet as used in UK bathrooms. Normal socket outlets are only permitted where they are at least 3 metres from the edge of the bath, so in reality they are never installed.
    These shaver sockets contain an isolating transformer and are limited to 20VA, so are only of use for very low power items like shavers and toothbrushes.
    Modern examples are very similar, although usually riveted together so more difficult to take apart for demonstrations like this.
    The black 2 pin plug in the video is a Europlug, the UK shaver plug is similar but has slightly shorter parallel pins.

    Comments:
    The newer ones (last 30/35 years) didn’t have the red manual switches to select the voltage and to enable turning it on or off as the British Standard had changed to remove the shock risk from someone using the switches with wet hands. I’m pretty sure that the on/off switch operated the primary winding, therefore if wet hands and condensation had managed to form a conductive path to the switch contacts, then a shock risk would be created. Those MK Logic shaver sockets were all very well made items like all all older MK items, especially before the introduction of Logic Plus in ’96.

    The 20 VA limitation is the result of the transformer being designed with only sufficient iron in the core to support limited magnetic flux which will only produce 20VA in the secondary. Any attempt to draw more power will just saturate the core. The waveform will go to a square wave (well, clipped and distorted sine) with reduced voltage and no more power will be transferred. So the transformer is both isolating and power limiting.

    only kind-of. The output is power-limited, but the input side will still overheat quickly because the core no longer helps limit the current when saturated. At that point, only the resistance of the windings will limit the current. You do need a fuse, breaker and/or thermal fuse for it to make it safe.

    Reply
  47. Tomi Engdahl says:

    Tech Tips Tuesday, Isolation Transformer and Variac Safety
    https://www.youtube.com/watch?v=XBsQ3sZ45Fk

    Some test bench safety tips, and a sneaky isolation transformer issue you may not know about.

    Reply

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