this might help
The microwave ovens which are sold with a metal rack are combination
ovens which have a 'browning' feature.
"Why metal rack can be used in the combination mode?
In the combination mode, the microwave is used only for a short
duration in the microwave mode when metal rack cannot and shouldn't be
used. The rack has no sharp edges and the gap between the rods of the
rack is large enough to prevent sparking and to allow the microwave to
pass through.
Please note the grill rack is only to elevate the food to come closer
to quartz heater element for faster browning."
The above quoted explanation is from Bajaj Electricals - FAQ's On
Microwaves
(
http://www.bajajelectricals.com/faq/microwavefaq.htm )
When you are just microwaving, then remove the rack. While the
sentence from the website above is rather vague, the meaning is quite
clear, the rack is for browning and any exposure to microwaves is
minimal at best.
Your combination setting may look something like this:
Combination cooking (30% microwave plus 70% grill - or - 55% microwave
plus 45% grill)- it is used only for recipes which require both
microwave for cooking/ baking and grilling for browning.
As for damage to the magnito, if you operate an empty oven, there can
be damage. The microwaves, after reflection from the empty, cavity
will fall back on the magnetron to affect its life and will lead to
magnetron failure. Food in the oven absorbs the microwaves and
protects the magnetron. Once again, don't have the rack in when just
using the microwave.
Search - google
Terms - metal rack +in microvave ovens, combination microwave ovens
mike
ps copied and pasted from one site