thanks guys for all your opinions. as Elevation had explained, boeing's QRH 'claim' the BUG system as an AC source. from what i understand from that, if you have one main AC plus the BUG system, there is NO need to land ASAP !!
boeing is quite clever by making things muddy !! as i mention, we operate using a single MEL/DDG. i believe certain company opt to have a non-ETOPS MEL/DDG and an ETOPS MEL/DDG, i would be interested to know what their MEL/DDG says.
HS