Grounding and the Use of the Signal Reference Grid in Data Centers

Overview

The SRG became standard practice in 1983 through FIPS PUB 94; that guideline was withdrawn in 1997 but is still widely referenced. The paper argues that modern IT equipment communicates over transformer-isolated Ethernet or fibre, so the grid is unnecessary. Data interfaces are ranked by susceptibility to inter-system ground noise: total immunity (fibre, wireless), high (Ethernet), partial (RS-485, Modbus etc.) and low (parallel, RS-232, SCSI, video, proprietary backplane).

The numbers carry the argument: low-immunity interfaces can be disturbed by 0.1 V or less of ground noise, partial-immunity ones by 10 V or more, while Ethernet has an interface breakdown voltage above 1000 V. Above 1 MHz, noise follows the lowest-impedance path, which is nearby cables rather than the grid. Each claimed SRG benefit (interference, damage prevention, noise discharge, ESD, safety) is examined in turn and mostly rejected.

The cost case is explicit: a budgetary cost of about $6.50/ft2 ($70/m2) becomes $182 per rack at 28 ft2 (2.6 m2) per rack. Re-torquing and testing every two years at $2/ft2 ($22/m2) adds $280 per rack over 10 years, for a TCO of approximately $460 per rack. The practices offered instead are rack-to-rack bonding, bonding cable trays to racks, and consolidating copper data cabling entry at one surge-protected point. Baying kits do not reliably bond racks because they do not cut through paint or plating, so dedicated ground interconnection kits should be used and trays bonded to racks at both ends. Related standards named are IEEE 1100, TIA-607 and FIPS PUB 94.

Key points

1. The SRG became standard in 1983 via FIPS PUB 94; withdrawn 1997 but still cited.
2. Noise immunity classes: total (fibre, wireless), high (Ethernet), partial (RS-485, Modbus), low (RS-232, SCSI, video).
3. Low-immunity interfaces can be disturbed by 0.1 V or less, partial by 10 V or more; Ethernet breakdown voltage exceeds 1000 V.
4. Above 1 MHz noise takes the lowest-impedance path, nearby cables.
5. SRG budgetary cost about $70/m2, $182 per rack.
6. Biennial maintenance adds $280 per rack over 10 years; TCO about $460/rack.
7. Baying kits do not reliably bond racks; use ground interconnection kits and bond trays at both ends.

Using it in design

• Do not include an SRG by default in new designs; offer it as an optional line item with $/m2 install and biennial maintenance cost.
• Require a rack-row bonding conductor back to the PDU ground.
• Flag cable trays and ladders not bonded to racks at both ends.
• Warn when a low-immunity interface (RS-232, SCSI, video) crosses rows, and consolidate copper telecom entry at one surge-protected point bonded to the main power ground.

Who should read it

Electrical designers writing grounding, bonding and rack-infrastructure specifications.

Related standards

• IEEE 1100
• TIA-607
• FIPS PUB 94

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2012

Document number

WP 87

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation