earth fault only occurs whilst testing devices?

interesting one which we fixed by changing an s90 apollo smoke for an xp95 apollo smoke in the end but i am at a loss to explain it:

advanced 1 loop panel using apollo xp95 devices. earth fault only occurs when testing devices (any devices). earth fault always cleared when panel was reset after testing and never occurred in normal use. earth fault hovering at 1.7 v (normal, just about) and dropping to 0.4v when any device tested. nothing to do with sounders as they are conventional via a sounder card and earth fault occurred even when sounders were disabled.

any ideas as to an explanation?