Building Safe And Reliable Electrical Systems With Optocouplers White Paper

Designers must consider many factors when selecting an isolation technology. The primary factor, of course, is the safety of equipment and personnel. Industrial equipment typically operates using signals of several hundred to several thousand volts. Yet the threshold of human safety can be as low as 42 V DC or 60 V AC. Electronic equipment can be even more sensitive when composed of integrated circuits that can typically be damaged by even a few tens of volts applied across the wrong pins.

To prevent humans and electronics from harm they must work in the safety extra low voltage realm even though other parts of the electrical system uses high voltages. Keeping these two voltage realms separated while also passing information between them is the job of the isolation device. These isolation devices must be able to operate with a continuous stress across their isolation barrier of hundreds of volts.

A second factor to consider is the isolation device’s insulation rating. There are three levels of insulation rating: functional, basic, and reinforced or double. Functional insulation is that needed for the device to operate and implies nothing about safety. Basic insulation provides protection for users from electrical shock, as long as the insulating barrier remains intact. Reinforced, or double, insulation provides failsafe operation in that should one level of insulation fail a second level will continue to protect the user. All signal lines going from the high voltage realm to electronic circuits driving interfaces that a user might touch, such as switches and displays, require isolation with a reinforced insulation rating. One of the prime considerations in achieving a reinforced insulation rating is the distance through insulation that a high-voltage signal must traverse in order to reach a human.

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