Earthing instrument body

Please reply by referring relevant IEC or equivalent standard

That's funny I was going to tell you the same thing, it is interesting that you don't seem to know what the relevant standards are (are you an electrical consultant?).

Do the instruments require earthing? Are they designed to be earthed? Does the manual tell you to earth them?

If the answer is yes to any of the above questions then you need to earth the instruments for safety and/or correct instrument functionality.

If in doubt read the standards (or at the very least the instrument manual).

It is good practice to earth it, necessary or not.

If the output is a current signal, earthing it should make no difference to its performance.

If you are talking about the single pair cable shield it should be only connected at one end so that you dont get any eddy currents within the loop

Depending on the installation method the device is sometimes grounded to the piping or to the conduit if that method of installation is used. This connection is only a mechanical ground not that of the wiring as indicated in the comment above which as indicated prevents ground loops in the system.

Site standards may specify the bonding of all elctrical apparatus, but regulatory standards usually do not.

The main purpose of bonding the instrument body is for hazardous area applications to reduce ignition source potential from induced or fault potentials.

If it is in a hazardous area, get a hazardous area consultant to assess the installation.