All electrical equipment is categorised into classes according to the method of protection against electric shock that is used. For mains powered electrical equipment there are usually two levels of protection used, called "basic" and "supplementary" protection. The supplementary protection is intended to come into play in the event of failure of the basic protection.
4.1 Class I equipment
Class I equipment has a protective earth. The basic means of protection is the insulation between live parts and exposed conductive parts such as the metal enclosure. In the event of a fault that would otherwise cause an exposed conductive part to become live, the supplementary protection (i.e. the protective earth) comes into effect. A large fault current flows from the mains part to earth via the protective earth conductor, which causes a protective device (usually a fuse) in the mains circuit to disconnect the equipment from the supply.
It is important to realise that not all equipment having an earth connection is necessarily class I. The earth conductor may be for functional purposes only such as screening. In this case the size of the conductor may not be large enough to safely carry a fault current that would flow in the event of a mains short to earth for the length of time required for the fuse to disconnect the supply.
Class I medical electrical equipment should have fuses at the equipment end of the mains supply lead in both the live and neutral conductors, so that the supplementary protection is operative when the equipment is connected to an incorrectly wired socket outlet.
Further confusion can arise due to the use of plastic laminates for finishing equipment. A case that appears to be plastic does not necessarily indicate that the equipment is not class I.
There is no agreed symbol in use to indicate that equipment is class I and it is not mandatory to state on the equipment itself that it is class I. Where any doubt exists, reference should be made to equipment manuals.
The symbols below may be seen on medical electrical equipment adjacent to terminals.
Figure 6. Symbols seen on earthed equipment.
4.2 Class II equipment
The method of protection against electric shock in the case of class II equipment is either double insulation or reinforced insulation. In double insulated equipment the basic protection is afforded by the first layer of insulation. If the basic protection fails then supplementary protection is provided by a second layer of insulation preventing contact with live parts.
In practice, the basic insulation may be afforded by physical separation of live conductors from the equipment enclosure, so that the basic insulation material is air. The enclosure material then forms the supplementary insulation.
Reinforced insulation is defined in standards as being a single layer of insulation offering the same degree of protection against electric shock as double insulation.
Class II medical electrical equipment should be fused at the equipment end of the supply lead in either mains conductor or in both conductors if the equipment has a functional earth.
The symbol for class II equipment is two concentric squares illustrating double insulation as shown below.
Figure 7. Symbol for class II equipment
4.3 Class III equipment
Class III equipment is defined in some equipment standards as that in which protection against electric shock relies on the fact that no voltages higher than safety extra low voltage (SELV) are present. SELV is defined in turn in the relevant standard as a voltage not exceeding 25V ac or 60V dc.
In practice such equipment is either battery operated or supplied by a SELV transformer.
If battery operated equipment is capable of being operated when connected to the mains (for example, for battery charging) then it must be safety tested as either class I or class II equipment. Similarly, equipment powered from a SELV transformer should be tested in conjunction with the transformer as class I or class II equipment as appropriate.
It is interesting to note that the current IEC standards relating to safety of medical electrical equipment do not recognise Class III equipment since limitation of voltage is not deemed sufficient to ensure safety of the patient. All medical electrical equipment that is capable of mains connection must be classified as class I or class II. Medical electrical equipment having no mains connection is simply referred to as "internally powered".
4.4 Equipment types
As described above, the class of equipment defines the method of protection against electric shock. The degree of protection for medical electrical equipment is defined by the type designation. The reason for the existence of type designations is that different pieces of medical electrical equipment have different areas of application and therefore different electrical safety requirements. For example, it would not be necessary to make a particular piece medical electrical equipment safe enough for direct cardiac connection if there is no possibility of this situation arising.
Table 1 shows the symbols and definitions for each type classification of medical electrical equipment.
|B||Equipment providing a particular degree of protection against electric shock, particularly regarding allowable leakage currents and reliability of the protective earth connection (if present).|
|BF||As type B but with isolated or floating (F - type) applied part or parts.|
|CF||Equipment providing a higher degree of protection against electric shock than type BF, particularly with regard to allowable leakage currents, and having floating applied parts.|
Table 1. Medical electrical equipment types
All medical electrical equipment should be marked by the manufacturer with one of the type symbols above.