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PORTABLE APPLIANCE TESTING
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TESTING MICROWAVE OVENS
In addition to PAT Testing microwave ovens need to be tested for leakage as well as heating power. Details on carrying out these tests are presented in this chapter.
Microwave ovens are always of a Class I construction. Carrying out a test on these is quite straightforward as one can quite easily get an Earth connection on the metal work to the rear of the oven. Alternatively the fixing screws can be used. Just make sure that the power switch is in the ON position when carrying out the test for Earth Continuity and Insulation Resistance.
Microwave leakage
Microwave ovens can be heavily used and sometimes abused in a busy kitchen catering for a number of staff and customers. They can in time become damaged and even corroded, if not looked after properly.
It is not unusual to see microwave ovens with broken door hinges, damaged or missing door seals, broken door catches or even cracks to the door glass.
While these ovens are designed and manufactured to operate normally and seal in the microwaves, it is important that there are no gaps in this metallic seal.
When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body, the same heating that cooks food. Exposure to microwave radiation can produce cataracts by affecting the lens in the eye. Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with the door open) can produce heat damage in other tissues as well, up to and
including serious burns which may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.
For these reasons, it is important to also check the leakage from
microwaves when they are in use. This is quite easy to do with a suitably calibrated Microwave Leakage Checker.
A large mug of water is placed in the microwave oven which is then put on maximum power for a minute. A Microwave Leakage Checker is then used to scan the door seals and the front glass. This will indicate if the level of Microwaves leaking out is
within an acceptable safe level which is 5 mw/sq. cm.
Microwave ovens are designed to switch off instantly as soon as the door is opened. It is very important to check that this is working.
Finally the oven has to be labelled appropriately to show that both the leakage and the interlock tests have been carried out.
Microwave heating power
Older microwaves often don’t deliver the power that is claimed in the user manual. It is quite easy to check the rated power output of microwave ovens using the following method.
• Fill a plastic measuring jug with about 300ml of tap water at a
known temperature and place it the centre of the lowest shelf in
the oven. Use a thermometer to measure the initial temperature. Please make sure not to leave the thermometer in
the beaker.
• The oven should then be operated at its maximum microwave
power setting for around 60 seconds. If the oven is a combination one, just use microwave power only.
• As soon as the microwave stops, open the door, give the contents of the beaker a very brief stir using the thermometer, then measure the final temperature of the water.
• Temperature Rise = (Final Temperature) - (Initial Temperature)
• The microwave heating power of the oven can be estimated as
follows:
Microwave Power (Watts) = 4.2 X (Volume in ml) X (Temp. Rise)
÷ Time (seconds)
User checks
Make sure that the user is aware of the potential problems with
microwaves. They should be asked to report any obvious signs of damage to the microwave oven immediately and should stop using it. If there is a noticeable drop in power, say food is taking significantly longer to heat up, then this should also be reported.
In addition to PAT Testing microwave ovens need to be tested for leakage as well as heating power. Details on carrying out these tests are presented in this chapter.
Microwave ovens are always of a Class I construction. Carrying out a test on these is quite straightforward as one can quite easily get an Earth connection on the metal work to the rear of the oven. Alternatively the fixing screws can be used. Just make sure that the power switch is in the ON position when carrying out the test for Earth Continuity and Insulation Resistance.
Microwave leakage
Microwave ovens can be heavily used and sometimes abused in a busy kitchen catering for a number of staff and customers. They can in time become damaged and even corroded, if not looked after properly.
It is not unusual to see microwave ovens with broken door hinges, damaged or missing door seals, broken door catches or even cracks to the door glass.
While these ovens are designed and manufactured to operate normally and seal in the microwaves, it is important that there are no gaps in this metallic seal.
When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body, the same heating that cooks food. Exposure to microwave radiation can produce cataracts by affecting the lens in the eye. Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with the door open) can produce heat damage in other tissues as well, up to and
including serious burns which may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.
For these reasons, it is important to also check the leakage from
microwaves when they are in use. This is quite easy to do with a suitably calibrated Microwave Leakage Checker.
A large mug of water is placed in the microwave oven which is then put on maximum power for a minute. A Microwave Leakage Checker is then used to scan the door seals and the front glass. This will indicate if the level of Microwaves leaking out is
within an acceptable safe level which is 5 mw/sq. cm.
Microwave ovens are designed to switch off instantly as soon as the door is opened. It is very important to check that this is working.
Finally the oven has to be labelled appropriately to show that both the leakage and the interlock tests have been carried out.
Microwave heating power
Older microwaves often don’t deliver the power that is claimed in the user manual. It is quite easy to check the rated power output of microwave ovens using the following method.
• Fill a plastic measuring jug with about 300ml of tap water at a
known temperature and place it the centre of the lowest shelf in
the oven. Use a thermometer to measure the initial temperature. Please make sure not to leave the thermometer in
the beaker.
• The oven should then be operated at its maximum microwave
power setting for around 60 seconds. If the oven is a combination one, just use microwave power only.
• As soon as the microwave stops, open the door, give the contents of the beaker a very brief stir using the thermometer, then measure the final temperature of the water.
• Temperature Rise = (Final Temperature) - (Initial Temperature)
• The microwave heating power of the oven can be estimated as
follows:
Microwave Power (Watts) = 4.2 X (Volume in ml) X (Temp. Rise)
÷ Time (seconds)
User checks
Make sure that the user is aware of the potential problems with
microwaves. They should be asked to report any obvious signs of damage to the microwave oven immediately and should stop using it. If there is a noticeable drop in power, say food is taking significantly longer to heat up, then this should also be reported.