Gall fod ymyrraeth ar y system dydd Llun 28 Gorffennaf rhwng 18:00 a 23:30 a dydd Iau 31 Gorffennaf rhwng 18:00 a 23:30, ond bydd y gwasanaethau’n weithredol. / There may be disruption to the system on Monday 28 July between 18:00 and 23:30 and on Thursday 31 July between 18:00 and 23:30, but services will remain operational.