What are shielded cable?
Shielded cables are used to connect the sensors or transmitters of Hazardous location with the device of measurement and control of the non-hazardous location.
The purpose of the shield is to create an equipotential zone around the capacitive coupling of the conductor with that of other conductors.
Need of shielding and isolation:
When there is no longer any insulation between the shield and the transmitter cabinet, there may be an excessive energy level in a hazardous location if the ground potential V1 is different from V2.
Because the fault current is limited only by the resistance of the shield and that which exists between V1 and V2 (from above pic), the generated spark could ignite the potentially dangerous atmosphere that surrounds it.
Grounding of the shielded cable:
The shield must be connected to ground at a single point, preferably at the ground point of the system. If the shield is connected to earth in two non-equipotential points, the current could circulate in the shield, impeding the functionality.
For intrinsically safe devices, the shield acts as another conductor between the hazardous and non-hazardous locations and could become the route of the fault current if the cable is damaged.
For passive barrier applications, the shield can be grounded locally if the galvanic isolation is not damaged by this connection. This means that the two shields on both sides of the isolation device must not be interconnected.
For applications where shielding is part of the segregation technique between different types of intrinsically safe circuits (ie multipolar cables), the reference ground connection of the shields must be the same as the ground connection of the passive barrier:
The shield S1 is connected to the same ground connection point as the measurement circuit. It must not be connected to the metal parts of the transmitter to avoid ground connection of the second circuit, which is not allowed by the intrinsic safety protection method.
The purpose of the field transmitter is to isolate galvanically. Therefore, there should be no connection between shields S1 and S2.
The shield S2 and S3 provide the shielding of the connection between the transmitter and the barrier. They are interconnected at an isolated point of the terminal block of the junction box.
S3 is also connected to the earth bus of the barrier which, by means of a separate conductor, is connected to the reference earth point.
The shield S4 completes the shielding of the system and is not very important from the point of view of security. It is connected to the reference point of the shield, which is represented by the ground bus.