3 phase transformer bonding / grounding question.

[Mrdave714]

I'm working on a restaurant that has 2 services. One is a new 240/120 1ph. I just installed.
When I was tracing out all the existing circuits to label the new service, I noticed all the receptacles on the 2nd service were reading "Open Ground"


The 2nd service is an old 240v 3ph. corner grounded supply. That service is used to supply a customer owned 208Y/120 3ph transformer
That transformer supplies a 3 phase switchboard. I checked in the transformer and in the switchboard and there is no bonding between ground and neutral.

The transformer case is bonded to the 240v service and has an additional ground bar connected to the case.
The ground wire runs through the transformer to the ground bar in the switchboard.

Obviously one of the 240v service phases is connected to ground w/ two rods and CWpipe.
I had already replaced all the ground clamp connections on the 240v panel as they were corroded.

In the 208Y/120v switchboard I'm reading phase to ground voltages of A-55v / B-44v / C-55v.
Phases to neutral are all good at 120v
Phase to Phase are all good at 212v

Shouldn't there be bonding jumper between neutral and ground somewhere?

Not much 3 ph experience and 1st corner grounded service I've seen in 40 years. I don't want to do anything stupid!

 

[zbang]

Shouldn't there be bonding jumper between neutral and ground somewhere?

Definitely. Usually the X0 terminal (of the 208/120v transformer) jumps directly to the transformer frame and to the GES somehow. (Check out the code sections on Separately Derived Systems.)

ETA- it helps to think of how a fault current will flow and thus trip the overcurrent. Without that bond, there's no path so the breaker won't trip.

 

[ActionDave]

250.30 the code section. The X0 can bond in the transformer or the first disconnect.

 

[Mrdave714]

250.30 the code section. The X0 can bond in the transformer or the first disconnect.

So just screwing down the bonding screw in the panel would be compliant?

 

[ActionDave]

Your Grounding Electrode Conductor would need to land in the panel also. I would put the screw in today GEC or no GEC.

 

[Mrdave714]

Your Grounding Electrode Conductor would need to land in the panel also. I would put the screw in today GEC or no GEC.

On the way there now.
Looks like the previous tenant that had the transformer installed was using like this for many years.
It's a 100a panel. There is a 6ga ground conductor that runs from the main service ground bar, through the transformer (connected to drop in lug bolted to transformer frame, continues to the panel inside with a ground bushing, then connected to ground bar in panel. All in EMT.
Looks good except for missing jumper ?