From what I can see, a SELV power supply shall not exceed 60Vdc and have maximally 120V peaks of less than 200ms.
Is there any requirement or something, that mandates that a DC power supply does not become an AC supply when the rectifier fails?

 

Selv regulations are max 120V DC, 50V AC,
No Earth, totally isolated output (ie Transformer ), Double insulated cables.

****************************************
SELV" stands for "separated extra-low voltage" in installation standards (e.g., BS 7671) and for "safety extra-low voltage" in appliance standards (e.g., BS EN 60335).

A SELV circuit must have:

• protective-separation (i.e., double insulation, reinforced insulation or protective screening) from all circuits other than SELV and PELV (i.e., all circuits that might carry higher voltages)
• simple separation from other SELV systems, from PELV systems and from earth (ground).

The safety of a SELV circuit is provided by

• the extra-low voltage
• the low risk of accidental contact with a higher voltage;
• the lack of a return path through earth (ground) that electric current could take in case of contact with a human body.

 

The values you quote appear to be from IEC/EN 60950-1 (Information technology safety standard).

The standard specifies similar limits for a.c. voltages – under normal operation the a.c. SELV limit is 42.4Vac and under single fault conditions is permitted to reach 71Vac.

The problem with using the term ‘SELV’ is that different product safety standards have differing definitions for SELV.

 

The question is related to industrial products, where the argumentation of safety seems to rely on the fact that reverse voltage on the power supply input can only happen during the installation of the product. But it seems to me not too far fetched to imagine a standard tranformer with a bridge rectifier, where the rectifier fails such that there is pure AC on the output. Yes big old transformers are almost obsolete now, and a smps is very unlikely to fail in such way due to the amount of energy coming throught the transformer in one cycle and the amount of output capacitance, but that for me doesn´t remove that possibility.
There was some thought that a SELV power supply should not do that, but I don´t see such case in any of the definitions.

 

Using a linear transformer to generate 60Vdc would require an input to the output rectifier of 42.4Vac (assuming no diode voltage losses) – should the rectifier fail (somehow) such that the a.c. input voltage appeared at the output, the 42.4Vac would be at the SELV limits for normal operation (and well below 71Vac permitted under single fault conditions).