The three most common types of isolation are:
• Magnetic Coupling. In this form of isolation, a transformer creates a barrier. The mechanical properties of the transformer dictate the amount of isolation, but in general, this method provides excellent protection against hazardous voltages, particularly transient voltages.
• Capacitive Coupling. Although this method requires the least amount of space to implement, capacitive coupling is not as effective at protecting against transient voltages as the other types of isolation.
• Optical Coupling. This method provides a physical barrier by creating a space inside the measurement device that hazardous voltages cannot cross and by using light to send information across the gap. Optical coupling is commonly used to provide isolation in digital devices because the on/off signal can easily be turned into light by optocouplers. Similar to magnetic coupling, optical coupling provides excellent protection against transient voltages.
In most cases, you’ll purchase a product that uses one of these methods to provide isolation. The technical details of how these methods are implemented at the design level are not as much of a concern as understanding how to interpret isolation specifications and determining if a product is adequate for your application.