Should I Earth the DC Output of a MEAN WELL DIN Rail Power Supply?

We recently had a customer ask if we recommend earthing the 0VDC from the V- DC output terminal of MEAN WELL DIN rail power supplies. Or, would we recommend leaving the DC output floating, so it is isolated from protected earth? 

In terms of safety, the customer thought it safer to earth the 0V, but was concerned that earthing may introduce noise which may deteriorate the signal.  

Unfortunately, there is no definitive answer.  

MEAN WELL DIN rail power supplies are safe to use either way, with or without the DC output being connected to earth.  

There is an isolation transformer between the input side and the output side of the power supply, so they are safe to use without earthing the output.  

There is no obvious reason for the signal to be any noisier when the output is earthed. In fact, earthing the output will stop stray leakage voltage from appearing on the DC output. 

Because there are EMI suppression capacitors built into MEAN WELL power supplies, a small AC leakage current will appear on the DC output lines. Earthing the negative line will cause this current to flow into the earth.  

This current is very small, less than 1 mA, too small to be unsafe, but if there are enough power supplies with the negative output connected to earth, the resulting current flowing into the earth line may cause an earth leakage breaker to trip out.  

Most earth leakage breakers will trip at around 30mA, so if there are more than about 50 power supplies on the same circuit with their negative outputs earthed, you may find nuisance tripping of the earth leakage breaker. 

Earthing the negative output may have unexpected consequences, such as earth loops causing noise in low voltage signals.  

Typically, industrial equipment uses 4-20mA signals which are quite immune to noise, but anything with a low voltage signal will need careful consideration of earthing with respect to earth loops. In this case, it may be better to leave the power supply negative unearthed. 

Finally, in the case of a failure in the isolation transformer’s insulation, yes, it would be safer to earth the DC output. However, Power Supplies Australia has never seen an instance of the transformer insulation breaking down. 

If you have any questions about using or installing power supplies, do not hesitate to contact Power Supplies Australia.