This from the FAQs for Microwave Ovens at the SANYO website...
"As a general rule you are advised not to place metal objects in a microwave oven when microwave cooking. However there are exceptions. If a utensil has been supplied by the manufacture specifically for this purpose then you can be sure it is suitable for use in the way recommended in the instruction book. Similarly small quantities of foil can be used, provided the instructions are followed.
It is important however that there is no contact between two metal items, such as grill rack and metal drip tray, or foil and oven wall, or arcing may occur."
http://www.sanyo.co.uk