Isolation, Electrical

WHENEVER ELECTRICAL EQUIPMENT IS CONNECTED TO A LIVING SUBJECT THERE IS A DANGER OF ELECTROCUTION due to malfunction. To minimize this risk, all connections to the subject should be electrically isolated from the main power lines, including the ground or neutral line. This means that there must be no electrical path between the subject and the line.

Optical isolation is usually the method of choice: If the device being connected to the subject is a monitoring or recording amplifier, for example, it is battery-operated so that no connection to the main power lines is needed. The isolator then converts the amplified signal from electrical to optical form, essentially by using it to drive a special Light Emitting Diode (LED) similar to those commonly used as indicator lights. The light is modulated so that its intensity is directly proportional to the signal amplitude.

The light then travels a short distance through an electrical insulator (epoxy plastic, for example) before hitting a linear photodetector that converts it back to electrical form. The detector part of the circuit is run from the main power lines, but is not connected electrically to the LED side (subject side) of the circuit in any way.

This maintains total electrical isolation of the subject from the power lines in the event of catastrophic failure of the equipment or the power system itself.

For electrical stimulation of the subject, the idea is the same but reversed: Here the stimulator must be driven by batteries, and receives a light signal from a powerline-driven modulator.

Although the isolator assures safety from powerline problems, the battery-powered amplifier or stimulator must still be carefully designed: Even small electrical currents can be lethal when directly connected to the subject, especially across the heart as in recording heart rate, etc.

For use on human subjects, always use properly designed and isolated equipment certified for the intended purpose. For small lab animals like rats, mice, and guinea pigs, however, the danger is usually considered minimal enough to use "home-made" preamplifiers without isolation. Consult the animal use committee at your institution for details.