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.