PD IEC TR 63139:2018 Explanation of the mathematical addition of working voltages, insulationbetween circuits, and use of PELV, in TC 34 standards

Circuit analysis
Circuit elements
Electric circuits
Circuit theory
Insulation (electrical)
Insulation devices (electrical)
Voltage
Lamps
Lamp standards

Link: PD IEC TR 63139:2018 Explanation of the mathematical addition of working voltages, insulationbetween circuits, and use of PELV, in TC 34 standards
Source: BSI Standards