Why do we usually short ground and negative together on a 24 VDC power supply?

It is quite common to earth one side of a DC supply - all telephone networks earth the positive, but the idea is the same.

The advantages are that it allows ADS in much the same way as the earthed neutral on the AC mains - a fault from your DC wiring to earth can be detected and cleared automatically by blowing a fuse or whatever.  It also provides protection against contact with a higher voltage, either due to a failure of the mains transformer,  or contact elsewhere - if there was no earth reference the entire DC rail pair could be flapping up and down at 230V AC and no-one would realise, as it would all appear normal, in that loads connected between the rails would be unaffected.

(This is why the insulation requirements for SELV  (floating) are rather tighter than PELV (earthed) or FELV (non-isolated from mains) )

The disadvantage is when it needs to connect to other things that may already be earthed, or that go outside, where you may accidentally bridge between mains earthing systems through the DC wiring, which may not be rated for the fault current that could flow.

At higher DC voltage, where shock becomes a serious risk, then the whole question of of earthing versus floating supplies also have to be looked at from the perspective of safety, and again depending where it is used and what sorts of fault conditions are most likely, the balance of earth or not may fall either way.

There is no clear winner for all cases but there are situations when PELV trumps SELV, and you are looking at one.

Mike,

It is quite common to earth one side of a DC supply - all telephone networks earth the positive, but the idea is the same.

The advantages are that it allows ADS in much the same way as the earthed neutral on the AC mains - a fault from your DC wiring to earth can be detected and cleared automatically by blowing a fuse or whatever.  It also provides protection against contact with a higher voltage, either due to a failure of the mains transformer,  or contact elsewhere - if there was no earth reference the entire DC rail pair could be flapping up and down at 230V AC and no-one would realise, as it would all appear normal, in that loads connected between the rails would be unaffected.

(This is why the insulation requirements for SELV  (floating) are rather tighter than PELV (earthed) or FELV (non-isolated from mains) )

The disadvantage is when it needs to connect to other things that may already be earthed, or that go outside, where you may accidentally bridge between mains earthing systems through the DC wiring, which may not be rated for the fault current that could flow.

At higher DC voltage, where shock becomes a serious risk, then the whole question of of earthing versus floating supplies also have to be looked at from the perspective of safety, and again depending where it is used and what sorts of fault conditions are most likely, the balance of earth or not may fall either way.

There is no clear winner for all cases but there are situations when PELV trumps SELV, and you are looking at one.

Mike,