We have no regulatory measures for electrical safety of highly sophisticated health care facilities, although only one tentative standard of manufacturing medical electrical equipments is, at the moment, been in effect. However, this situation must be overcome to realize a truly safe and reliable modernized medical system. From this point of view, we have developed the measures to describe electrical safety of health care facilities equipped with numbers of class I equipment that are concurrently used, since class I equipment play an important role in these facilities. A formula of defining most effective maintenance period has, also, been deduced.
In this paper, the following items are discussed.
(1) Two regular class I equipments can only be used, where single-fault conditions occur simultaneously under the condition that total patient leakage current shall not exceed the safety limit current.
(2) In connection with the above, it is possible to operate up to ten equipments simultaneously if 10-volt main supply voltage or doubled grounding system is used.
(3) Doubled grounding system and periodical inspection of it can decrease sufficiently the possibility of occurrence of single-fault conditions.
(4) Electrical safety of patient environment against occurrence of single-fault conditions can be classified into the following three cases : (i) 10≥n>5, (ii) 5≥n>2, (iii) n≤2, where n represents the number of engaged equipments.
(5) If we enforce periodical inspection every one week for case (i) and every one month for case (ii), the safety probability of the system can be raised to more than O. 999. As for case (iii), it is always safe against occurrence of single-fault conditions.
(6) The optimally effective period of inspection exists and it is deduced theoretically.3