• Before we shut off building power supply for MS PDU maintenance, we had the 7 nos. ATS tested and found L12 corridor ATS could not switchover normally from default power source to backup power source when we shut off the air switch (DB 12MAIN 400A/4P) above the ATS. After vendor (Sunland) completed L12 all of PDU tighten and cleaning work, the L12 corridor ATS resumed normal switching function. Suggest to have a closely observation for this situation during building power maintenance at the end of this year and make sure ATS could change over normally. In the meanwhile, vendor should prepare ATS spare part for next two-yearly power down activity in case of ATS is failure and need to be replaced.