24v DC on a 480v ground

[LoganB]

Because of the physical design of the machine in question, we are being forced to ground our 24 V DC hardware onto a ground being used by 480 V AC equipment. I imagine I can use a diode to protect the 24 V DC from the large amount of potential noise on the ground? Is there a better way to do this? If a diode is the answer, is there anyway that our PLC can be alerted to the diode being blown on the ground?

Thanks in advance for any input or advice!

 

[James Mcquade]

LoganB,

BAD idea!
What if you somehow end up with a floating ground?
don't laugh, we had a plant with a 75 volt ac floating ground due to electric company miswiring the 3rd stage winding in the main plant transformer.


bad neutral, bad phase wiring, loose connections.

your diode will only protect on one side of the sine wave only and conduct on the other.

james

 

[rupej]

For people that ground their 24VDC power supplies, I usually see them tie it to the same ground that is shared with the 480VAC feed, control transformers, etc.

Others just don't ground the DC.

 

[BobB]

For people that ground their 24VDC power supplies, I usually see them tie it to the same ground that is shared with the 480VAC feed, control transformers, etc.

Others just don't ground the DC.

Yes. I am one of the ones who do not ground the DC negative and let the supply float. Grounding the negative removes the inherent isolation in a switch mode power supply.

 

[damica1]

Yes. I am one of the ones who do not ground the DC negative and let the supply float. Grounding the negative removes the inherent isolation in a switch mode power supply.

This man knows what he is talking about!!

 

[willxfmr]

In an ideal world, all grounding points would be tied together so there is only one, unbroken, path back to the earth at the service. In the real world, this is seldom the case, so you just have to decide for yourself what side of the "ground the power supply" fence you want to be on. There are pros and cons for each position, and both sides will argue for their side until the cows come home.

Bubba.

 

[BobB]

In an ideal world, all grounding points would be tied together so there is only one, unbroken, path back to the earth at the service. In the real world, this is seldom the case, so you just have to decide for yourself what side of the "ground the power supply" fence you want to be on. There are pros and cons for each position, and both sides will argue for their side until the cows come home.

Yes but...... I do a lot of swimming pool work (commercial) and all circuits must be protected by RCDs set at 30ma - even VSD circuits. We have worked long and hard with a local VSD manufacturer to use low loss filters in the drives and we now have drives running on 30ma common, cheap RCDs up to 75 kW without cutting out the earth connection on filters and the like - the filtering is fully intact.


Further 24VDC and 24VAC control circuits must have isolation - hence 24VDC negative from a swithc mode is NEVER grounded or isolation is lost. Similarly 240VAC/24VAC transformers are all double wound for isolation - groung the output neutral and isolation is again lost. 24VAC double wound tranny output is also allowed to float or we break the rools! Simple as that.


These are valid reasons to not ground these supply negatives or neutrals.


That being said another advantage in not grounding the negative 24VDC output is that if you are switching negative in the field there is ground reference if BUBBA shorts a PLC input wire to ground accidentally turning on an output and creating hell.


I also SUSPECT that if say 240VAC was accidentally connected to a 24VDC PLC input wire there would also be no ground or neutral reference to cause a big bang in the input card but I have not tried this and have no wish to do so.

 

[Highland Controls]

What really sucks is working hard to make sure 0VDC is not connected to earth, only to find out a supplier of a signal conditioner or motion controller tie them together internally.

 

[BobB]

Yep!

 

[LoganB]

The 24 VDC circuit has a Prosoft wifi radio that seems to generate a decent amount of noise, so I was thinking we would need to have the negative grounded. Is this going to be a case of picking your poison? Noise from the 480 if it's grounded or noise from the radio if it isn't?

 

[James Mcquade]

can you put the wifi radio on a power supply of its own?

james

 

[LoganB]

Yes, that might be the best course of action. The customer won't like having more hardware tacked on this late in the game, but they will likely find it preferable to replacing L16s every few months.

Thanks again for your help everyone!

 

[Kataeb]

What really sucks is working hard to make sure 0VDC is not connected to earth, only to find out a supplier of a signal conditioner or motion controller tie them together internally.

in this case, it is safer to connect the 0vdc to earth...