problems can be expected on an un-grounded switched power supply

Which ground? Input ground? Output ground? Is your switcher isolated? Are you talking about the -24V ground?

earth ground on the output side of the power supply, the -24 vdc connection, giving it a 0v potential.

OK, now where does the ground go, and what are you disconnecting the ground from?

If the ground is simply the return on the -24V, the answer is obvious.

the ground is connected from -24 connection on the power supply to the earth ground bar on the main sub-panel.

By removing this grounded connection, how would this cause noise on the power supply?

So, you've got two wires coming out of the power supply - one is -24 Volts and the other is -24V return (0V). Which of these is connected to earth ground?

no, one wire is +24vdc the other is -24vdc (ov), the ground is connected to the -24vdc connection.

OK - now is the earth ground connected to anything that you're driving with the +/- 24V? In other words, does the thing you're driving see +/-24V, or 0 and 48V?

If it's the former, then I don't think disconnecting the ground from your -24V will have any negative affect on noise.

We have had to connect the negative DC terminal to ground before due to voltage unbalance on the + to ground and the - to ground. This problem was inherrent to the particular power supply / battery charger we were using.

Some manufacturers use a floating - which will allow loads to continue running without interruption upon + or - groundfault. If the supply is grounded the short circuited load will be interrupted by upstream protection i.e. fuse / ckt bkr..

For a test I would lift the ground and measure the + to ground vs. the - to ground, the + & - voltages should be somewhat balanced.. The downfall with no ground reference is if you have a + or - ground on consumer you will never know untill you have a short circuit. Some power supplies / battery chargers have + & - DC ground alarms of which will somewhat flag problem.

It sounds like there is huge confusion.

If you take a typical bench power supply it will have three terminals for the output (we'll ignore sense lines) minus zero and plus. There will also be a chassis ground which will generally be connected to the zero.

A supply like this can be run with plus and minus floating (eg neither connected to zero)

Say it is giving 12v out...

a) If you connect the minus to zero you have a 0 to +12v supply.
b) If you connect the plus to zero you have a -12v to 0 supply

It is all relative.
If you have two such supplies wired as a) and b) and you connect the zeros (and hence the minus of a) to the plus of b) you now have a -12v 0 +12v supply.

I hope this helps..I remember having to explain this to an electronics graduate who thaught it would blow up the supplies. (I showed him which end of the soldering iron to pick up too)

Del

I do understand all of the connections with a DC power supply, and thank you for all the responses.

The problem I was having, is when I remove the ground from the secondary side of the PSU, I introduce a noise on the DC line while the digital drive was running. I did found a solution on the RA website, "System Design for Control of Electrical Noise". I tied a ceramic capacitor from +24 to ground and another from -24 to ground and I wrapped the dirty side around a ferrite sleeve. This did reduce my spike from 5.84v to 47mv.

Removing the ground will render the switched power supply to malfunction because it is the reference point at 0 volts potential, i.e. +24 volts terminal grounded to provide a -24 volts potential.

It looks like you have a ground loop problem. The 0V of your supply is grounded both at the source and the load. The problem with this is that if you have a noisy device with lots of power nearby (your AC drive), you will have all kind of noise flowing through the grounding system. Electrons don't care if they flow in the intended ground wire or an auxiliary supply or the shield of your analog signal. If the path is continuous, some of the ground current will flow through.

This is why you never ground a power supply, a communication link, a cable shield, or an analog signal at both ends. In general, we ground only at the source. This prevents ground current from noisy source or faults from affecting the low voltage equipment.

Sometimes equipment 0V is already grounded internally. This forces us to leave the source floating. I don't like doing this for safety reasons and usually try to find another way. Adding an isolator usually fix the ground loop problems.

If -24v volt ground is removed and the IO is driven by the same power supply then there is a chance of fluctuating input and ouput and floating voltage could be found in the power supply section.

Your question it make some one confuse.

would tell us what is your 24 VDC system, Is it floating or not.

floating it meas there is CT (center Tap) of sourcce transformer power.

(-12 to 0 to +24 ) Vdc. this is no 24 volt grounded except casing of instrument equipment or cahses. the ground body equipment expected will detection of current leak by ELB relay.(earth leak breaker). this relay setting depent on insulation cable or equipment, some time in orde Kohm only. the relay will detect which line was leak is +12 Vdc leak to ground or -12 vdc. lead of relay will show also the line was grouded.

ofcouse if your leak current on the TX or to the I/O PLC leak it mean mA (4 - 20) not same with expected reading or measurement this will distrube your system.

you can measure also what line was leak to ground. The way is measure negative line to ground is it still 12 volt or less. the same way to positve line.

other system is battery like vehicle negative line connect same point with ground.

You received a lot of answers to mull through. Here is my take.

The term switching powers supply is pretty wide open and interpreted differently by different people. Some people think of a bench power supply while others may be considering a separate buck regulator and still other will think of a DC to DC converter. They are all a little different. The ground you refer to could be a system ground or the bench supply ground. It is hard to give good advise without clear understanding of the problem. Regardless ground and/or common are terms used for a reference point. Earth potential is often used for various reasons; safety, keeping the equipment all reference to the same point etc... Some people have a tendency to confuse the power supply return with ground because the return is typically tied to ground but not always. Anyway a signal ground reference should be designed to minimize current flow on the reference and avoid ground loops (complete enclosed ground paths of relatively low resistance). We often float our power supplies but connect them to the circuit ground. That is one of the advantages of using isolated supplies.

If the output of the power supply you are using is ground isolated and has three terminals, +v, -V and ground then it can operate perfectly well without the ground connected. Most bench supplies we use can be operated without the ground connection. I do it all of the time and I select specific points in my circuit to connect to ground to avoid ground loops and current flowing on the ground reference.

Might have repeated a few things but I stopped in the middle and started back up.

Hope this helps