IEC 61439-2 Clause 8.101 contains information on the forms of Internal Separation.
The degree of internal separation is determined by the arrangement of the barriers and partitions inside the assembly and which parts of the assembly are separate. The result is recorded as Form X, on a scale of Form 1 – Form 4b.
The internal arrangement will be classified according to the criteria in Table 104. Further information can be found in Annex AA.
If you are familiar with testing to BS EN 61439, please be aware that the Internal Separation guidelines in IEC 61439 are not the same as BS EN 61439.