Why Are Ceiling Plates, Filters and Shelves So Important in Microwave Ovens?


The microwave oven ceiling plate (also called stirrer cover, grease shield or splatter shield) serves at least two essential purposes in the operation of a commercial microwave oven.

  • It prevents grease and food particles from accumulating in and around the oven ceiling and antenna assembly, which, after repeated exposure to microwaves, will carbonize and begin arcing. Once the arcing process starts, it perpetuates itself with each cook cycleConsequences  of operating without ceiling plate.
    • As shown in the photo to the right, arcing will rapidly damage the antenna assembly and, if left unchecked, will destroy the magnetron(s) and eventually burn a hole right through the metal oven cavity.
    • Manufacturers recommend that the customer routinely remove these protective covers for cleaning. Unfortunately, in the hustle and bustle of a commercial kitchen, for any number of reasons, the covers are often not reinstalled. By the time the customer calls to report the arcing, the damage has already occurred.
    • Warranty: Manufacturers warn that operating an oven without this protective cover properly in place can void the oven warranty.
  • In air-driven models, raised air guides that extend from the upper surface of the grease shield cause the circulating air supplied from the cooling motor to swirl around the antenna. Plastic fins on the antenna assembly catch the swirling air, providing turning power. If the grease shield is missing, the antenna will not rotate. A stalled antenna creates direct beams of energy resulting in severely localized heating, which causes arcing and burning that can damage the inner door, the cooking shelf, and eventually the magnetron(s).


The microwave oven air intake filter prevents grease, dust and other air-born particles from collecting and blocking the airflow passages within the oven. Operating the oven without the filter can lead to a buildup of gunk and other debris in the air ducts and blower assembly as shown in the illustration to the right. This condition is a recipe for an overheating situation and can ultimately cause damage to the magnetron(s).

The microwave oven cooking shelf (or tray), like the ceiling plate, provides at least two critical functions. Similar to the ceiling plate, it serves to protect the antenna assembly beneath it in models so equipped. The shelf also holds the food a set distance above the cavity floor, which enables the microwaves to penetrate the food from all sides, top and bottom.

If the shelf gets cracked or the seal around the outer edges becomes compromised, it will begin to allow seepage of moisture-laden grease down into the floor area below the cavity shelf, including into the lower waveguide in models so equipped. After repeated and prolonged exposure to microwaves, this agglomerate of grease soon becomes charred, eventually carbonizing and thus becoming a catalyst for arcing during cook cycles.

The gruesome photo on the right vividly illustrates what happens when the cooking tray is not properly maintained.

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Results of missing air filter
Results of using a microwave oven with a defective shelf