I think general practice is to have everything protected with PME (Protected Multiple Earthing), where the rack, cold water system, warm water system, gas etc are all linked to the same earth point as the electrical system. That prevents earthing across phases from multiple phase inputs and for example, a potential difference between washing machine (electric earth) and your taps (cold/ hot water earths).
Although, if any of the devices in the unearthed rack has a 3 pin plug, then (assuming a good) a physical metal to metal connection will allow everything in the rack to be earthed.
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