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. Tomi Engdahl says:

    Isolation Transformer. What you need to know

    What is an Isolation Transformer?

    An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety reasons. Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings.

    Typical Electrical Arrangement

    You probably don’t know it, but your mains supply is most likely provided to you via an isolation transformer. In the electrical substation that feeds your home lurks a huge chunk of copper and iron (the transformer) that takes relatively high voltage electrical power and converts this to our recognised 230-240V voltage that we all know. Your home is supplied with a cable from this transformer that has two conductors. One is the live conductor, and the other is a combined protective earth and neutral (PEN) conductor. (This is known as a TN-C-S system which is the most common in the UK. Other systems are available.)

    The Isolation Transformer for Safety

    So how can the isolation transformer be used for electrical safety? It all comes down to what a transformer actually is. In the simplest terms it is two coils of wire around an iron core. The incoming coil – called the primary – converts an electric field into a magnetic one. This magnetic field then induces an electric field on the second coil and hence a voltage appears on the output of this coil (called the secondary). By varying the number of turns in the coils the voltage can be stepped up or down, but in our case the number of turns are equal and so the output voltage is the same as the input voltage. However, the point to grasp here is that there is no electrical connection between the input and the output. The link is done by magnetism. This means that the output is “isolated” from the input and hence the term isolation transformer!

    The output of the isolation transformer still has a nominal output voltage of 230V between its output conductors, but there is no link to earth. This means that you can safely touch either conductor without risk of electric shock. You will still get an electric shock if you touch both conductors however!

    It is important to note that with an isolation transformer, a device that may have an earth fault that would trip a circuit breaker or blow a fuse will work just fine. In fact, isolation transformers are used for this very reason in certain applications where the sudden disconnection of power due to an earth fault may cause even larger hazards (such as in chemical plants, or in operating theaters). In such cases monitoring is usually provided so that an alarm is raised should this occur.

    Use of the Isolation Transformer for reducing electrical noise.

    The transformer, being a coil, has what is known as inductance. Inductance is a barrier to high frequency signals. Electrical noise is a high frequency signal and so the transformer acts as a block to this. Other power problems can also be reduced especially if there is an electrostatic screen in the transformer construction which is connected to earth. Any electrical transients between the power conductors and earth can be effectively reduced using this method.

    Disturbances between the power conductors can be reduced by the inductance but not eliminated. This is why in dedicated power conditioning devices that incorporate isolation transformers, further filtration is placed on the secondary side of the transformer to reduce this further.

    Redoing the N-E Bond

    In complex electrical installations, or some where the wiring may be old, have poor connections or otherwise has excessive impedance, the voltage between neutral and earth can increase, particularly at the furthest points from the distribution board and particularly where high currents are involved. This may, or may not be a problem for your electrical equipment. You could just rebond the neutral to earth again, but electrical codes do not allow for this. However since the secondary is isolated from the primary you can safely derive a new neutral and earth by bonding these together at the secondary of the isolation transformer. This is also done to eliminate noise between “neutral” and earth – as you are shorting it out.

    To protect operators working on equipment with the risk of water contacting live conductors the circuit has been fitted with an RCD.

    Transformer Regulation

    Transformers are not perfect and impedance exists in them that causes a volt drop within the transformer when current flows. The more current that flows the larger the volt drop and so the output voltage falls. The regulation of a transformer is the difference in the no-load voltage to the full load voltage expressed as a percentage. Poor regulation can introduce other problems into a circuit. For example, if the load is non-linear and takes current in high value chunks – such as in rectifiers, then the poor regulation can cause waveform distortion and introduce voltage harmonics into the system. Other problems include the voltage falling too low and causing under-voltage protection systems to operate.

    If we take our isolation transformer on which the output secondaries are not connected to earth. Try as we might there will always be some parasitic capacitance between the output phases to earth

    If we connect a piece of equipment to the transformer that contains an input filter, then we will find there are capacitors intentionally connected between the input phases and ground. Ignoring Zp (as Zc≪Zp), then Vm = Vo(Zc/(Zc+Zc)) Eg half Vo again.

    This is why the measured voltage between phase and ground tends to be around half the transformer output voltage. I can see why at first glance this may cause concern, as it appears that we have a high voltage to earth even via our isolation transformer. However no current will flow (and hence it is safe) if we make a connection between any phase and earth. All we do is now reference that phase to earth.

  2. Tomi Engdahl says:

    DC power isolation

    Joyo ZGP Noise Blocker

    Input Voltage Isolator
    Isolates hum, noise, crosstalk, and ground loops from non-isolated power supplies and signal patches
    Converts the multi power supply into a true independent daisy chain output power supply
    Filters ripple noise from the power adapter
    Breaks the ground loop and provides independent output with stable 9 V output voltage
    Input voltage: 9 V DC
    Output voltage: 9 V DC
    Output current: 800 mA
    Dimensions (W x H x D): 16 x 16 x 63 mm
    Weight: 17g
    Includes DC connection cable

  3. Tomi Engdahl says:

    eXtReMe iSoLaTiOn ( Low Leakage Power in Precision Electronics )

    00:00 Intro
    00:43 Normal transformers
    02:43 Fluke 732A
    04:11 Keysight 3458A
    04:55 Cheap switcher
    05:19 Good switcher
    06:02 CERN HPM7177
    06:52 UPS transformer
    07:33 PCB transformer
    07:48 Fluke 5700A
    08:18 Qi Charger
    09:18 Split bobbin transformer
    10:34 Keithley 182
    11:09 Keithley 617
    11:21 eXtReMe iSoLaTiOn
    12:57 Silly? ideas
    13:43 Solar cell
    15:10 Battery power
    15:48 Fluke patent
    16:27 Keithley patent
    17:34 Wavetek patent
    22:09 What I did
    24:06 Results

  4. Tomi Engdahl says:

    Tech Tips Tuesday, Isolation Transformer and Variac Safety

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

    Variac Isolation Transformer. Are Variacs Isolated?

    This Variac isolation transformer has stirred up controversy. Is it really isolated? Let’s do some tests and open it up.

  5. Tomi Engdahl says:

    Do isolation transformers degrade sound?

    AC isolation transformers are used as cleaning and purifying devices between the wall and our gear, but do they work?

  6. Tomi Engdahl says:

    Isolation transformer explained and demonstration

  7. Tomi Engdahl says:

    SDG #05 Medical Isolation Transformer Teardown and Modifications

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

    I dismantle the unit to add a twin 13A socket and 4mm banana sockets to make the device more user friendly for general use, and demonstrate the correct way to crimp ring terminals.

  8. Tomi Engdahl says:

    Isolation Transformer – Adds Safety or Not??

    Personal Safety at the Work Bench

  9. Tomi Engdahl says:

    What is the difference between a standard isolation transformer and a medical grade isolation transformer?

    The label and cost.

    Capacitance between windings, leakage current across primary and secondary, insulation resistance between primary and secondary.

    There are two types of isolation transformers, that is, two ways in which the primary and secondary circuits can be separated from each other. They are:

    Safety shield – Between the primary and secondary coils, a safety shield is placed with insulation on both sides. This ensures that the leakage current remains within prescribed limits. The thick safety shield complies with strict safety standards. In event of a faulty condition, the electrical path goes directly to ground, providing safety.

    Double (or reinforced) insulation – Instead of a shield, two or more insulation layers are packed between the primary and secondary sides. Each individual layer must pass the rigorous safety standards. If one layer breaks or fails, the next layer takes over and provides protection. In general, transformers that use insulation have lower leakage current levels when compared to shield type transformers.


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