Hello, good day. I just wanna ask advise regarding this. Suppose there are two service in a two-storey industrial plant building. One from a medium voltage and it terminates in the ground floor main switchgear room. The other service is from a low voltage system (ungrounded system) and terminates at the second floor electrical room. It is expected that there will be electrical ground bus in each electrical room. My question is, do we have to connect the two electrical ground bus?

I know that NEC 250.2(B) requires that equipment grounding is required in ungrounded system in order to "limit the voltage imposed by lightning or unintentional contact with higher-voltage lines, limit the voltage to ground". Also my concern here is, if there's an unintentional contact from the medium voltage. Since it is ungrounded system, the breaker won't trip until there's a second line-to-ground fault. The low voltage equipment might also be damage by the high voltage. Also, for a case like this, do we also need to have bonding connection from ground bus to the building steel?

Thank you in advance for your comments/sharing.