On site we do lots of temporary setups for shows & etc. This building is 40 years old & as such the distribution has no grounds other than the conduits. When I set up a temporary 12 circuit 100amp 208v 3 phase Panel for use, it is done with 20 amp receptacles mounted on the panel. A 6/4 cable is plugged into a 70 amp 3phase 208v disconnect which in turn feeds the panel. Should the panel be bonded at the neutral since there is no ground for the safest application? Rather than running a separate ground wire with it & tied to what? the disconnect? Do you understand my dilemma?