How should the instrument be earthed?
The complete instrument (ie both amplifier and sensor) should be earthed at one point only. This prevents possible induced voltages and errors of measurement which might occur if the screen were connected to earth at two points which had different potentials with respect to each other. The normal practice is to use 2 core screened cable for the 4-20mA signal loop, and to connect the cable’s screen to the power supply earth. This screen then terminates in the amplifier module’s gland, which is specifically designed for the purpose. The sensor cable also uses a similar gland and the cable is likewise terminated. The GRP housing features a special metallic / conductive coating inside to ensure earth continuity and, when correctly sealed, protects against RFI. Note that the sensor cable outer screen is NOT internally connected to the sensor metalwork. This is in accordance with the I.S.
