
2-15
SIMATIC 545/555/575 System Manual
Pre-installation Guidelines
The second approach to handling noise problems is to isolate the problem
device and its wiring from the electronics and associated signal wiring. You
can accomplish this by increasing the physical distance from some types of
noisy devices. For extreme cases, electrostatic (metal) shielding may be
required. This is true for noise sources outside as well as inside the
mounting cabinet (NEMA-type recommended).
Two kinds of field wiring warrant special attention: wiring that enters the
harsh noise area to enable monitoring and control of devices, and TTL or
low-level wiring (less than 24 V). In these cases, supplement the physical
separation between control and noise-prone wiring with shielded,
twisted-pair wiring for the control signals.
Process transmitters should normally be grounded at the transmitter end.
Use a single-point shield ground, as shown in Figure 2-10.
Shielding
Twisted pair
Shielding
Transmitter
Single-point ground
To controller I/O
Figure 2-10 Grounding Shielded, Twisted-Pair Cables
Noise Isolation
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