Both NI and NIFW are strictly Division 2 protection methods. They share the common feature that any electrical or thermal energy contained within the instrument’s circuits is incapable of igniting the combustible material in the hazardous area under normal operating conditions.
Non-Incendive:
A rating of Non-Incendive means that, under normal conditions, the circuits of the instruments are unable to ignite the hazardous atmosphere. This is achieved without any consideration of special wiring assuming that the manufacturer’s published specifications are met. Consequently, control drawings are not required for instruments with NI classification.
Non-Incendive Field Wiring Apparatus:
A Non-Incendive Field Wiring Apparatus rating means under normal conditions the instrument’s circuits are incapable of causing ignition of the hazardous atmosphere.
This is a third party device that controls the current and voltage available to the instrument. In most cases, an ANFIW rated power supply located outside of the hazardous area is used for this purpose.
What are the differences between an Intrinsically Safe device and a Non-Incendive one?
They are dealing with different area classifications. The difference is that an intrinsically safe level or pressure sensor is tested for two fault conditions, where excess energy is sent to the sensor. To be classified as intrinsically safe, the sensor can not overheat or spark in fault conditions.
A non-incendive sensor is not tested to detect fault conditions. Excess energy is still avoided in the field with some type of power limiting device, but it is up to the design engineers to specify their own method. Using a certified barrier, in this case, is not necessary.