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.


  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?

    • 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.

  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!

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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 [...]

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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.

  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?

  9. Wilson Varghese says:

    Does Good Electrical Grounding can save electrical bills?

    • 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.

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  12. Tomi Engdahl says:

    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

    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.

  13. Tomi Engdahl says:

    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.

  14. Tomi Engdahl says: Isolation Transformer – Tripp Lite IS1000

    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.

  15. Tomi Engdahl says:

    SDGEE #06 Mains Isolation Transformers and UK Earthing Systems

    Transformers & Earth Connections

    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.

  16. Tomi Engdahl says:

    Isolated Electricity Supplies – Why they are not used for everything

    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.

  17. Tomi Engdahl says:

    Mains Isolation Transformers

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

    SDGEE #06 Mains Isolation Transformers and UK Earthing Systems

  18. Tomi Engdahl says:

    Stray-Currents, compensating-currents and ground-loops in Transformers

    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.


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