IS calculation for Simple Apparatus

Our client is asking for IS calculation for a "simple apparatus" (Murphy Vibration Switch VS2) installed with MTL Barrier MTL4016 . Now as I understand the IS calculation is not applicable for simple apparatus as long as associated apparatus/barrier is checked for respective zone operation and cable conformity.
Queries to the forum are :
a. Is my understanding correct as stated above ?
b. Is there any established method for IS calculations for simple apparatus ? If yes, may forum can kindly share ?
There are several apps available for the calculation. With products, sensors and barriers, of Endress+Hauser it's extremely friendly the use of ExiCalculator app, free download from App Store or Google Play. The app also generates pdf documents for safety records