I am installing a new ground bus bar for telecom equipment grounding in an existing building. There is an existing ground cable coming from and existing transformer and down into the slab. The grounding point is nowhere to be found. Hence, the question whether it is acceptable to splice tap the new ground bus bar to the existing ground wire.

Ideally, the equipment you ground should be using this transformer, as it is a separately derived system, assuming this is a small transformer inside a building, far from the main service.

Can you find that the transformer is connected to building steel , in addition the ground wire entering the slab?

Thanks for your response. Yes, the equipment to be grounded will be using the transformer. I have no way of confirming where the ground wire is terminated- to the building steel or to ground rod. Between the transformer and ground slab, there is a splice box where we can tap the ground wire. As the conduit and ground wire penetrates the slab, it is already untraceable. So, is it acceptable to tap the existing ground wire or should the new ground bus bar be connected to the existing grounding point (which may be impossible to locate already). Or perhaps install a separate ground rod and tie into both ground rod and the existing ground wire? I appreciate your prompt response. Thanks.

Technically, you should install a minimum of #6 copper back to the grounded neutral bar at the Main service. Does not matter whether the actual grounding electrode can be found.

Grounding to a local dry transformer in a building is not appropriate, unless, you can prove it is connected to building steel, which is a common grounding electrode, that would insure you are connected back to system ground.