Earth Ground; Isolating Ground And Neutral From Conduit - Siemens SIMATIC PCS 7 OSx Hardware Manual

Power Requirements (continued)

Earth Ground

1-12 Hardware Overview
Ensure that the AC power receptacle is properly grounded to earth. In some
installations, a metal conduit that connects to the neutral phase at the
circuit-breaker box supplies the earth ground of the power receptacle.
Depending on site conditions, this metal conduit may conduct current from
other sources that can interfere with the operation of a computer or printer
plugged into the socket.
To minimize this possibility, isolate earth ground and neutral from the
conduit (except at the service entrance housing); see
receptacle wired in accordance with National Electrical Manufacturers
Association specification, NEMA 5-15R. Also, follow these guidelines.
Ensure that the protective ground wire resistance is less than 1 ohm.
Ensure that the resistance to earth ground does not exceed 1 ohm;
ideally it should be 0.1 ohm or less.
Power
Company
Transformer
Service
Entrance
Hot
Neutral
Power Earth
Company Ground
Earth Rod
Ground
Figure 1-2 Isolating Ground and Neutral from Conduit
Circuit
Breaker
Panel
Conduit
Conduit
Protective
Ground
SIMATIC PCS 7 OSx 4.0.0 Hardware (PA RISC)
Figure 1-2. Use a
Data Terminal or
Computer Equipment
Receptacle