The Basics of Isolation Transformers and How to Select and Use Them

Traditional single-phase power wiring consists of a hot wire, a neutral wire, and a ground wire. When multiple, physically separated devices share a common power line, it is possible to create ground loops due to devices having different ground potentials. These ground loops are especially problematic in medical devices and can be troublesome during device test. For designers, it is difficult to measure ground loops with devices that use rectified line voltages. Grounded test equipment, like oscilloscopes, can inadvertently short power supplies in these devices. Also, high frequency noise can ride on AC power lines causing problems for sensitive transducers and instruments.

All these problems can be avoided by the proper application of isolation transformers between the power input and the device.

Isolation transformers provide separation from the power line ground connection to eliminate ground loops and inadvertent test equipment grounding. They also suppress high frequency noise riding on the power source.

Isolation transformers provide galvanic isolation between the AC power lines (mains) and the powered device. That means that there is not a DC path between the two windings. They serve three main purposes:

The first is isolating the secondary from ground (earth)
The second is to provide step up or step down of line (mains) voltages
The third is to reduce line noise being transmitted from the primary to the secondary or vice versa

Medical isolation
Isolation transformers intended for medical applications have to meet more stringent requirements in regard to leakage currents. There are maximum leakage current specifications for ground or earth leakage, enclosure leakage, and patient leakage. Ground leakage refers to leakage currents in the ground lead of a device. Enclosure currents describe currents that flow from an exposed conductive surface to ground via a conductor other than the ground lead. Patient leakage is current that flows through a patient to ground when connected normally to the device. Most devices in this category are certified under UL/IEC 60601-1.