24 VDC system grounding

[NetNathan]

I have done about (3) 24VDC systems and have a late question...
I have been grounding the DC negative and have not seen any issues.

I am supplying 24VDC to PLC Power, PLC analog an ddigital IO, and Instrumentation.
I use 2 24VDC power supplies, 1 PS for all the Instrumentation and Analog IO and the other PS for Digital IO.

Gounding?

 

[boxofrelays]

Just be sure to tie all the negative power supplies to one common bus. Don't daisy chain the negative terminals, of each of the power supplies. Then run one ground to a good solid ground, from the common negative bus, and not the 120/240 VAC mains ground.

-Chris

 

[KB1GNI]

^ Agreed.

Very good practice to tie DC zero volts to ground. A floating power supply can do some really weird things, especially to analog signals.

I still remember THAT lesson from the school of hard knocks!

 

[Mickey]

^ Agreed.

Very good practice to tie DC zero volts to ground. A floating power supply can do some really weird things, especially to analog signals.

Disagree, I don't ground the 24vdc common especially for analog devices because ground can be a very noisy environment.

There are systems that require a grounded common though. Such as ignition systems.

I went to a different school of hard knocks.

This has been talked about before. See link.

http://www.plctalk.net/qanda/showthread.php?t=23355&highlight=grounding+24vdc

 

[NetNathan]

Thanks for the answers so far..

I have ran into pressure transducers that would not work unless negative was grounded.

This is the procedure I use.....each PS has its own Positive and Negative. Even though the Negatives are grounded, I don't use a common negative for both PS.

You might say I treat the negative the same as the white neutral on 120VAC system.

 

[Mickey]

Added a link to my above post, here it is again.

http://www.plctalk.net/qanda/showthread.php?t=23355&highlight=grounding+24vdc

 

[V0N_hydro]

we normally have 120V DC battery bank and 24VDC battery bank and both are floating.

Our latest job is using 120-24VDC converters instead of the 24VDC battery bank, with 24VDC- grounded, but still 120V DC is floating.

One nice thing about the floating DC supply is that it is possible to detect a single ground fault and fix it. two ground faults are required to blow a fuse.

It is an effort to make sure that none of the equipment internally grounds the DC- or to mount it in such a way as to keep it isolated.

 

[Donnchadh]

Hi

I must have done to same school Of hard knocks as mickey as we never ground the common either. Must but not all panels I see would be the same but here anyway if you do the wire should be blue and white in colour and if you don't it should be just blue.

Donnchadh

 

[WineryPLC]

I always float my power, I don't want my cabinet becoming a neutral for power.

 

[bkottaras]

+1 for floating common.

 

[Lancie1]

Very good practice to tie DC zero volts to ground.

Careful, the only reason it measures "0 volts" is BECAUSE you tied it to ground. Otherwise it will measure -24 volts or -120 VDC, or whatever. Connecting (-) to ground does not mean that is is no longer negative relative to the + side of the power supply, but that it is no longer negative relative to your ground point. It is a hard lesson to learn, and a subtle point that is often missed.

 

[NetNathan]

Added a link to my above post, here it is again.

http://www.plctalk.net/qanda/showthread.php?t=23355&highlight=grounding+24vdc

I can now see I am not the only one with this question...

 

[Mickey]

I can now see I am not the only one with this question...

You can also see that opinions goes both ways.

 

[Mickey]

Careful, the only reason it measures "0 volts" is BECAUSE you tied it to ground. Otherwise it will measure -24 volts or -120 VDC, or whatever. Connecting (-) to ground does not mean that is is no longer negative relative to the + side of the power supply, but that it is no longer negative relative to your ground point. It is a hard lesson to learn, and a subtle point that is often missed.

On ungrounded 24vdc systems if you use ground as a reference you would see
about 12vdc on the common or plus side. I do not use ground as a reference with a ungrounded system, I use the common of the 24vdc supply.

I have had electrians/technicans tell me they only have 12vdc from the power supply, I then ask what they are using for a reference.
I can't remember if you would see a negative 12vdc on the common side reference ground.

 

[brendan.buchan]

I ground the negative of all my 24VDC power supplies here and typically labelled 0VDC.