How often must I have our Portable Appliance Testing done?

I'm the Practice Manager in a dental practice and all our portable appliances were tested a year ago. Can anyone tell me the required interval for testing in a health care establishment as there appears to be little or no guidance on it available? Some companies are telling me one year, yet in the past we have been told 3. HELP!!
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It's a grey area but if the equipment is used on the public then 6 months Class 1, 12 months Class 2. If it's not used on the public then 24 months Class 1 with a formal visual inspection (fvi) at 12 months and an fvi for Class 2 every 24 months. We are talking here about portable appliances, ie that can be moved. If they are hand held then the only difference is that fvi is every 6 months for Class 1 and 2 with a full test every 12 months for Class 1 only.
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