Splicing Equipment Grounds together from Separately derived systems 💜

[Cindysparky]

Let me preface this: the majority of my background in Residential. So, there aren’t a lot of separately derived systems

I’m working commercial now and I’m pulling wire. To set the scene, it’s a giant high rise I’m helping build. There’s emergency panels, uninterrupted power panels, panels for just the smoke dampers, normal power panels, a lightning control system….

So, in a few of the mechanical spaces they opted not to control the lights by the lighting control system but rather locally with a switch. It’s about a 50/50 split in these room of lights emergency powered lights to “normal” powered lights.

The lights in this room are controlled by a relay and there’s a “mixing” box for the EM power and the normal power.

Finally, here’s the question: if I tie all the equipment grounding conductors together in the mixing box and there is a ground fault on (let’s say) the EM… does the current flow back to the EM panel only???

Now I know that electricity takes all paths. However, is it more true to say that electricity takes all paths in its circuit (and the normal panel isn’t part of it’s circuit)? Could you say that it wouldn’t flow to the normal panel because the system are separately derived? Or would you say that because everything is bonded together, even though it would be a very resistive path, some electricity would take a weird route to the normal before returning the the EM?

Basically, what happens when you splice the EM equipment grounds with the normal ones???

Of course, I’m talking if a ground fault or short circuit would occur.
Thanks

 

[don_resqcapt19]

The current can only flow to its source. If there are multiple Separately Derived Systems, there will be parallel paths for the fault current because each SDS, assuming they are grounded systems, will be connected to the building grounding electrode system.
So there will be some current on the EGCs for the other systems, but the path via those systems will have a much higher impedance and the path via the EGC for the system with the ground fault will carry the vast majority of the fault current. No matter what the path is, that ground fault current will flow though the overcurrent protective device for the faulted circuit and the OCPD will open.

 

[Cindysparky]

The current can only flow to its source. If there are multiple Separately Derived Systems, there will be parallel paths for the fault current because each SDS, assuming they are grounded systems, will be connected to the building grounding electrode system.
So there will be some current on the EGCs for the other systems, but the path via those systems will have a much higher impedance and the path via the EGC for the system with the ground fault will carry the vast majority of the fault current. No matter what the path is, that ground fault current will flow though the overcurrent protective device for the faulted circuit and the OCPD will open.

Ok great, that’s what I was picturing. So really, despite the fact that they are connected in that mixing box…. They are actually always connected through the larger grounding system of the building even when they aren’t connected through splicing. Never thought about that.

Thanks for the reply