According to site phenomenon, check as below:
1.check whether DG can work normally without any ECC500 control, and found DG can start normally on manual mode
2.check the DG start signal cable connection, found this cable connect firmly and correctly.
3.Check the alarm information on ECC500, found there is "Mains fail" alarm.
For this site, as described, the work mode was dual DG with no Mains input, so preliminary judge that this problem may caused by the ATS parameter setting.
As analyze above, frontline engineer check the ATS parameter setting on ECC500, when he check found that ATS work scenario set with Mains+DG, so ECC500 consider this system work with mains+DG scenario, when DG start, it consider it as Mains input, according to work logic, ECC500 will stop DG, so the self test will be fail.