That is an irregular voltage spike from backplane1 .  Save the hardware log and then clear the hardware log. If voltage status does not go away, then, shutdown system and drain the flea power to clear the state. If feasible to do so at that time, i would also reseat all connections to backplane1 . If the problem returns, then there may a problem with backplane1 which requires replacement. More rare of case would be a Motherboard issue where the backplane1 power source is coming from. I would reseat that connector also. Let us know how that goes. 
Hello, We have a Dell Power Edge R720XD, We noticed an error on our alerting system that the voltage was unknown, so when logged into open manage to investigate the issue, this is what I seen,  Voltage sensor 21 [System Board BP1 5V PG] is Unknown reading It is not erroring in the Dell logs because its state is Unknown? We have another one of the same server and on that one state is good!  Does anyone have any idea what this means?  Thank You. 
Voltage Problem - Dell Power Edge R720XD