How to send for repair when one of the 42030A is defective?

Send the whole set of 42030A to the nearest Keysight service center to repair. The reason is due to there is no individual serial number associated with individual standard resistor for 42030A, in other words, calibration must be done with the whole set of 42030A. The service center will then route the 42030A repair work to the hub which have repair capability to perform the replacement on the defective resistor and calibrate the whole unit of 42030A.