We are a county jail also-the folowing housing units all have microwaves: 3- 60 man direct supervision units/1- 34 direct dupervision unit and a 26 man inmate worker unit. We sell microwaveable items in the commissary (popcorn, soups, etc...) . While the inmates are in the dayspace they may use the microwaves for heating these items, if it is abused the imate receives a rule violation--burning things in the microwaves whether by accident or on purpose is rare, as we will remove the microwaves from the unit, so the other inmates discourage 1 or 2 from screwing it up for everyone. The inmates clean the housing units 2x daily, this includes the microwave area--if the micro is abused in anyway, it is removed for a period of time. If it is broken just from wear and tear it is replace as the budget allows. They are all industrial strength microwaves so they are pretty tough. Since it is direct supervision and the housing officer is touring constantly, abuse is very rare.
For the most part we haven't had too many incidents of inmates heating items up to use in an assault, we've had a few hot coffee/waters thrown around though.