High Rise (100Yrs - Steel, Impedance of transformer secondary neutral to building Ground

[benjamin8761]

A separately derived system is provided on the 4th floor of a high rise.

We have connected the neutral and Ground at the transformer and then provided a Grounding Electrode Conductor from the Neutral terminal block of the transformer
to building steel.

The impedance from building steel (4th Floor) to the ground bar in the electrical room is 9.2 ohms. Is this acceptable, will this clear a fault and will it prevent
unacceptable transients during a lightening strike or intermittent secondary short on the secondary side of the transformer.

Please reference exhibit 250.14 for configuration.

Is there an article that dictates max impedance from these remote transformer to ground at the main service, what is the impact of a 9.2 Ohms impedance

Thanks in advnace

 

[infinity]

How are you measure 9.2 ohms? Also the GEC is not for fault clearing that is dependent on the connection of the SBJ (system bonding jumper).

 

[ron]

9.2 ohms of measuring resistance through direct conductive materials seems high. There is no code required max that I'm aware of for this application.

 

[infinity]

We have connected the neutral and Ground at the transformer and then provided a Grounding Electrode Conductor from the Neutral terminal block of the transformer
to building steel.

As far as the NEC is concerned this is all that's required.

 

[LarryFine]

Are you sure you're not actually reading a voltage?

 

[infinity]

IMO for a transformer on the fourth floor of the building the connection to the building steel is required but is doing little to nothing. The system would operate normally without the connection.

 

[LarryFine]

Don't you want/need the bond for line-to-structure faults?

 

[infinity]

Don't you want/need the bond for line-to-structure faults?

The building steel is already connected to the transformer through the EGC in the primary feeder. Admittedly that EGC is smaller than the required GEC so would a fault to the steel open the OCPD? Most likely yes because in a steel frame building there are so many parallel ground paths that it is unlikely that the GEC in this scenario is doing much of anything.

 

[don_resqcapt19]

Don't you want/need the bond for line-to-structure faults?

But that would only be for the steel in the area of the separately derived system. The resistance measurement was to the main grounding bar some distance away.

 

[electrofelon]

IT is a bit odd that the value is that high (assuming the figure is accurate). However I would not worry about it. Think of the millions of services off delta distribution systems that just have rods in dirt and that have dozens to hundreds of ohms