# why are non isolated buck converters a safety risk with 120V DC?

So i've come to understand that buck converters CAN be used for 120V to 5V BUT they are not becuase they are often a safety risk. I also understand that they can be used if they are away from a consumer OR if they are used in an industrial scenario.

for example: it can be used with a ceiling fan becauase a consumer does not interact directly or near to the buck converter circuit.

why is a non-isolated buck converter unsafe with specifically a 120V connection? It is considered safe to move from 40V DC to DC , seemingly regardless of current flow.

looking to understand this better

Thanks

EDITTED (to specify use of non isolated buck converter)

Because 120V is considered dangerous because of the risk of electrocution. The fact that it's direct current and not alternating current makes it much more dangerous. 40VDC is the limit at which it's considered safe.

It's simply because it is an unisolated voltage at high enough levels to be hazardous. So if you're working with the low voltage end in the safety of the knowledge it is only 5V and something fails, it could easily end up being a dangerous 120V instead of the 5V you were expecting.

simulate this circuit – Schematic created using CircuitLab

Figure 1. A 5 V DC output is used for this example. (a) Using a non-isolated AC/DC converter in a somewhat safe manner. (b) Inadvertent reversal of mains connection makes this circuit live.

Ideally the neutral wire is "neutralised" at the supply transformer or where the supply enters the building. This means that the the voltage between the neutral and earth should be zero but it could be several volts if high currents are flowing in the neutral wire. That means that the voltage on a non-isolated AC/DC converter should be close to 0 V on the COM output and close to 5 V on the DC+. These voltages will vary with the amount of current in the neutral wire caused by other loads in the building.

The problem occurs if the non-isolated AC/DC converter is connected to the mains in reverse. The COM wire is now live and the +5 V output is +5 V above whatever voltage the live happens to be at any instant.

Your question suggests that you understand that this may be acceptable in a completely insulated system where there is no danger of contact by a user. If a user touches either the DC COM or +5 V in Figure 1b then electrocution is a real risk.