The principal of identification of isolation workability in relation to a ground was proposed. It is based on the electrosafety criteria usage and a minimum current flow to a ground. The conditions of workability of isolation of distribution network is calculated in look of changes of isolation workability restrictions in relation to regulations and standards.A mathematical model for the calculation of isolation workability in a look of a relation of workability indicators to the isolation parameters according to a ground. By analyzing the adequacy it as set that introduced formed models assumptions result in the appearance of a method error to 3,5 %.