"On condition", means that items which are not normally energised in flight are tested at intervals, such as flotation switches on helicopters equipped with floats which initiate automatic inflation of the floats during ditching. Items such as relays and switches which are energised during flight are said to be "tested" on every flight and would not normally be replaced until they fail. They are normally sealed units as there is less chance of any corrosion affecting the contacts. Being sealed they cannot be inspected, replacing them all during a major aircraft check could be counter productive as there is no guarantee that a new unit is more reliable, if it's not broke don't try to fix it.