Wherever possible bolts shall be fitted in such a manner that in the event of failure of locking resulting in the nuts working loose and falling off, the bolt will remain in position. 

25.13 All bolts/nuts/washers exposed to atmosphere shall be as follows

a) Size 12mm or below : stainless steel

b) Above 12mm : steel with suitable finish like electro gulvanzied with passivation or hot dip galvanized 

25.14 Each bolt or stud shall project at least one thread but not more than three threads through the nut, except when otherwise approved for terminal board studs or relay stems. If bolts and nuts are placed so that they are inaccessible by means of ordinary spanners, special spanners shall be provided.

25.15 The length of the screwed portion of the bolts shall be such that no screw thread may form part of a shear plane between members.

25.16 Taper washers shall be provided where necessary.

25.17 Protective washers of suitable material shall be provided front and back of the securing screws.

26 OVERLOAD CAPACITY: The transformers shall be suitable for loading as per IS 6600

what is perfect dtr for this requirement and AB switch for this.