Medical Isolating Transformers: Principal Types
In medical settings it often is necessary to have several devices connected at the same time to a medical electrical system. In order to achieve this safely, the devices should be connected through an isolation transformer. This provides galvanic isolation between primary and secondary windings, which has the benefits of limiting leakage of currents to earth, isolating the electrical supply to life-saving devices, protecting supply in case of a circuit fault and guarding against electrical surges, spikes and noise.
The basic criteria that must be met by a medical isolation transformer are specified in the BS EN 60601-1 standards. In practical use, however, different designs are required to meet different applications that exist in medical settings.
Type B Transformers
Devices commonly used in patient care, such as computers and video monitors, can present the risk of tripping the electrical supply to whole room due to their natural earth leakage. Type B medical isolation transformers are used to provide a pseudo-mains supply to medical equipment, where the secondary winding is earthed, forming a quasi-neutral. This type of transformer can be applied to medical devices which predate the BS EN 60602 standards to allow them to meet the new standards for earth leakage, extending the service life of expensive equipment.
Type F Transformers
This type of transformer is used when power supply systems need to be physically insulated from earth in order to reduce leakage from the secondary winding to values of less than 5mA. The use of toroidal transformers by Siga and other manufacturers in this setting creates an extra level of protection for vulnerable patients, who could suffer fatal effects from even small amounts of earth leakage.
Electrical noise and interference can present a problem for sensitive equipment such as computers and lab instruments. Isolation transformers with specially designed electrostatic Faraday shields are used to block electric fields without interrupting the flow of current, reducing electromagnetic interference. Toroidal transformers typically generate very low electromagnetic interference, making them useful for these applications.
An additional use of isolation transformers is to increase safety. Transformers that isolate an electrical device from mains circuits reduce the risk from grounded objects which could be hazardous around high voltages. Isolation transformers employing a 1:1 ratio between primary and secondary windings can be used to protect patients and medical professionals from electric shocks.