In automation industry, it is desirable, even essential, that a sensor not have a direct (galvanic) electrical connection with the system to which it is interfaced, in order to avoid either dangerous voltages or currents from one half of the system from damaging the other half. Such a system is said to be “isolated”, and the area which passes a signal without galvanic connections is known as an “isolation barrier”. Isolation barrier protection works in both directions, and may be needed in either half of the system, sometimes both.
Common applications requiring isolation protection are those where sensors may accidentally encounter high voltages, and the system it is driving must be protected. For example 48 VDC or 110 VDC input to DI card, 4-20 mA transducer input to AI card.
Serial communication channels are excellent candidates for two way isolation. Isolation also alleviates the problem of ground loop currents by isolating field and system ground. Opto-isolators are commonly used to provide system isolation.
In addition to isolation, surge protection (form lighting / ESD) is included in electronic cards to prevent damage to critical card components. MOVs, TVS and fuse are some common type of components used to provide protection.