IEC 61755-2-4:2015(E) defines a set of prescribed conditions that should be maintained in order to satisfy the requirements of non-angled polished reference connections. The prescribed conditions include dimensional limits and optical fibre requirements of the optical interface to meet specific requirements for reference connection (plugs and adaptors) used for attenuation measurements. Two different grades for reference connections are defined in this standard. The use of each of these grades depends on the application and on the targeted attenuation measurement uncertainty. The model uses a Gaussian distribution of light intensity over the specified restricted mode field diameter (MFD) range. This standard is intended to be used for shipping and acceptance inspections. The reference connector plug is specified for B1.1, B1.3 and B6 fibres as specified in IEC 60793-2-50. The use of the reference connector plug would not be recommended where classification of fibre is difficult, for example construction and maintenance of cable plant. Keywords: non-angled polished reference connections, plugs and adaptors, attenuation measurements