Ansower from CHENZHU:

Isolated barrier does NOT require grounding.

Isoated barrier contains zener barrier funtion.

To be intrinsically safe (I.S.), Zener barriers must be connected to a dedicated I.S. ground. This ground wiring must be run separately, i.e. in separate cable trays – which costs money to create and maintain.

Unlike zener barrier, Isolated barrier has galvanic isolation between input and output. 

Take CHENZHU's isolated barrier for an example: The isolation voltage is above 2500 V, while the control cabinet is less than 500 V, cannot breakdown the I.S barrier.

In addition, the energy-limited unit of the I.S barrier forms a loop, independently from the control circuit.

Below drawing for your reference.