II - DCE FEL ČVUT v Praze

Shielding of
intrinsically safe
signal lines
Grounding system
of intrinsically safe
circuits
Mechanical Configuration of an Automation System with SIMATIC S7 Ex Modules
I/O Modules with Intrinsically-Safe Signals
C79000-G7076-C152-04
In Section 5.3.3, DIN VDE 0165 stipulates general equipotential bonding in
hazardous areas to avoid different potentials and sparking as a result.
Equipotential bonding must be rated and implemented as laid down in DIN
VDE 0100.
In accordance with DIN VDE 0165, Section 6.1.3.3, intrinsically safe circuits
are generally not grounded. However, they must be grounded if required for
safety reasons. They also may be grounded if required for functional reasons.
Grounding may only take place at one point by connection to the equipotential
bonding conductor.
Intrinsically safe signal lines and cables are shielded for measurement
reasons and in order to avoid inductive coupling as, in most cases, no signal
level is applied.
The following procedure must be implemented in the planning of the
equipotential bonding with intrinsically safe signal lines:
– Metallic housings whose mounting arrangement provide reliable
contact to structural components do not require a separate ground as
they are incorporated in the equipotential bonding arrangement of the
system.
– The shielding is grounded at only one point in order to avoid looping.
This is implemented for systems of Zone 1, 2 and 11 outside the
hazardous area, preferably in the control room.
The shield must be insulated at the device in the hazardous zone. The
measured value is routed via a twisted pair signal line (single cable) to a
distribution board and via a multiple cable to the control room. The shield is
insulated at all intermediate points.
In zone 0, the shield is connected directly at the apparatus adapter box
(mostly zone 1) to the general equipotential bonding system. The apparatus is
grounded directly via the ground conductor.
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