I read and article Mike Holts wrote on bonding and grounding for 3 phase systems. I went to our outlet that is a dedicated run to the subpanel and measured the voltage between ground and neutral. .2 to .4vac. Based on the article I was expecting to see 4.5vac. 3vac for the panelboard and 1.5vac for the branch. Can you let me know if I am missing something?
Also the owner let me know that nearly all the electronic thermostats in the complex for the residents HVAC unit have been replaced due to failure and some of the replaced ones have gone out again.
One possibility, since you mentioned the substation across the street, is that there is a substantial voltage gradient from one side of the building to the other.
One thing this can cause is strong earth currents near the surface going into the substation.
Any electronics which is not connected to a solid thick EGC could pick up substantial voltage from local earth contact. This contact could be from metal plumbing stubs which transition to non-metallic for the long runs, from incidental contact with structural metal which is not effectively bonded, etc. or from misguided attempts to connect the equipment to a local ground instead of the EGC.
This could in turn cause large end-to-end currents in data cables like RS-232 or coax which has no dielectric isolation, control wiring, etc.
One way to test for this is to disconnect the data cable at one end and measure the voltage from the ground or shield wire(s) to the ground of the equipment you disconnected from.
As some of our members along with Mike Holt have opined, a good safe ground system can be built with one or more good earth electrodes which are bonded together and a solid EGC network above ground, and only ONE point at which the two are connected together.
That avoids current flowing in the EGCs.
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