There has been some discussion on a few forums and G+ surrounding possible grounding issues if Rackstuds™ are used to mount the equipment and we wanted to address that and provide a few excellent resources on proper grounding techniques.
After discussions with an Electrical Engineer and from gathering information from industry experts and resources, we’ve highlighted what we believe to be a summary of the key points when it comes to relying on standard cage nuts for grounding equipment to 19″ racks and cabinets. This may be a common practice but if intentional grounding is mandated, relying on standard cage nuts (and therefore Rackstuds™), would not meet the requirements of the common grounding system.
The grounding section of TIA-942 offers guidelines that meet the basic principles and adds additional details specific to the modern data center environment. One important issue is the creation of electrical continuity throughout racks and cabinets
"Because most racks and cabinets are made of painted components that are bolted together, there is no guarantee that electrical continuity exists from one rack component to the next."
In the data center, rack and cabinet continuity is important for safety; electrostatic discharge (ESD) protection; and the proper grounding of switches, servers, and power strips.
"Continuity may not exist if the installer fails to scrape paint between sections of rack or use specially designed paint-piercing hardware."
According to standards TIA-942, J-STD-607-A-2002, and IEEE 1100 (the Emerald Book), a properly designed grounding system as shown below has the following characteristics:
Is intentional: each connection must be engineered properly, as the grounding system is only as reliable as its weakest link.Is visually verifiable.
Is adequately sized to handle fault currents.
Directs damaging currents away from sensitive electronic equipment.
Has all metallic components in the data center bonded to the grounding system (e.g., equipment, racks, cabinets, ladder racks, enclosures, cable trays, water pipes, conduit, building steel, etc.).
Ensures electrical continuity throughout the structural members of racks and cabinets.
Provides grounding path for electrostatic discharge (ESD) protection wrist straps.
Based on the above with reference to using Rackstuds™ in a system that mandates a common bond:
The third prong (power lead earth) should not be relied upon as the only form of earth.
Always follow the grounding/earthing recommendations of the manufacturer and if equipment provides for it, use the ground lug for common bond rather than relying on an incidental bond through the mounting ‘ears’ of the equipment. From an EMC standpoint, it is equipment dependent. If grounding is required, it should be explicitly stated by the manufacturer.
Hardware typically supplied with bolt-together racks is not designed for grounding/earthing purposes.
Bottom Line
If a proper earthing system is to be deployed, the equipment must be independently grounded using the manufacturers recommendations. If grounding through the equipment mounting ‘ears’ is desirable (and the only option), then the paint must be removed from all metal surfaces and ideally mounted using a specialist Bonding Cage nut. In relation to Rackstuds™, these can still be used if the equipment is bonded independently or if the mounting hardware is supplemented with at least one bonding cage nut.
References:
Network Bonding and Grounding Seminar: TIA-607-B – Generic Telecommunications Bonding and Grounding (Earthing) for Customer Premises, Tom Turner, Product Manager, Power & Grounding available here
Grounding and bonding questions answered – Cabling Installation & Maintenance Magazine by by John Carnevale. Article here.
Facility Considerations for the Data Center – Cisco White Paper available online here.