Rules for Grounding PLC I/O Racks

Guidelines for setting up grounds for PLC I/O racks containing analog and digital signals are:

Each I/O rack should have an isolated signal ground bus, 1‐inch wide by ¼‐inch thick, running from top to bottom next to the rack. The bus should have tapped holes to accommodate ground connections from various devices in the rack.

The signal ground bus should connect to the system ground plate at only one point, via a stranded, insulated copper wire of #8 AWG or larger. If the rack contains analog and digital signals, a separate bus should be provided for each.

The rack frame must be connected to the system ground, via a connection that is kept isolated from the signal ground bus.

Each chassis or panel in a rack should have internal grounding lines that connect to the signal ground bus. Connections should be via ring tongue connectors that bolt to the bus. If the chassis has several types of signals that need to be grounded, such as low level sensor signals, high‐level output modules, or noisy switching circuits‐each should have a separate line to the signal ground bus. Only circuits of the same voltage level should share the same ground return line.

Rack power should be supplied through an isolation transformer or UPS. This prevents ground loops between the instrument and electrical power grounds.

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