This study reports on experience gained during factory testing on a pilot project digital substation. Testing IEDs in a digital substation is dependent on how test and simulation functionality as defined by the IEC61850 is implemented by the vendors, and also requires a different approach to testing compared to testing on a conventional station. This study highlights limitations imposed on testing due to missing IED test facilities, as well as the need for the end customer to fully specify IED functionality to ensure vendor interoperability.