24 VDC system grounding

[V0N_hydro]

Here are some more points on the pros and cons of grounding DC negative

ken moore:

"Back in the day when we all used linear power supplies, the common was always grounded. Since we've moved on to switch mode power supplies, we always leave the common ungrounded, with a switched power supply you have true isolation. If you use NPN inputs, I recommend you NEVER ground the common"

Sparky:

"Safety - Most people think of 24VDC as safe (including myself) and have no problem touching anything with 24V. Now lets say, the common is not grounded and some where in the circuit a DC Cmmon wire comes in contact with a high voltage like 480V, a fuse will not blow nor a breaker trip. The next time someone touches +24VDC, they will find themselves in contact with anywhere from 300-500 Volts."

BobLfoot:

"I think we must distinguish between Controls DC and Instrumentation DC. In Cotnrols DC it makes a lot of sense to ground the 24 - for the reasons listed. In Instrumentation DC there are an equal number of arguments (isolation, noise, cross-talk) for not grounding either side of a 24 vdc supply. "

BobBBB -

"I work with genertaor systems all the time.
The battery supply negative is grounded - it supplies the hard wired stuff and generator controllers.
Secondary supply to the PLC is through a 24VDC/24VDC switch mode power supply. Negative is NEVER grounded, for isolation purposes. Battery chargers can go high, surges damage the PLC etc. The switch mode affords protection to the PLC but if the negative is grounded there is no isolation between one side of the switch mode and the other. May as well not use one.

Another reason for not grounding the negative to a PLC: - NPN inputs - grounding an input wire in the field can turn the input on - if it is not grounded there is no ground reference and the input will not inadvertently turn on. Typically found on Japanese machines. I normally use PNP inputs and do not really care if the output is NPN or PNP. Usually use NPN outputs."

Roy Matson

"If the negative is not grounded and the positive gets grounded by accident, no fuse will blow but all the signals that were floating around zero will now be at -24 which some input cards won't be able to read, i.e,. the signal input will be -23 to -19 instead of 1 to 5. If the card shares the power supply it's probably OK however quite often they are separate.

If the short to ground is only intermittent, it might be very hard to find the fault. wheras if you blow a fuse or pop a breaker on just the faulty loop, it's much easier to find"

Curt Wuollet:

"Pro grounding the DC negative: you won't get too far along before you hook up a piece of gear that ties the negative side to ground and that may be a bad place to ground for noise and ground loops. It makes chasing a problem a lot easier if you know where the ground is and you return everything within reason to that point."

 

[Eric Nelson]

Definitely a case of "We agree to disagree"...

Back in the 80s, we had a handful of assembly machines that were built in Germany (Heino ilsemann). Pre-PLC days. DIN-rail mounted logic gates, all wire-wrapped to create the 'program'.

Anyway, they used a floating DC power supply, but there was a terminal block which seemed to be purpose-built as a ground detector. Looked like a regular terminal block, but it had 2 lights on it. DC + and - was connected to it. Their service guy told me that the lights indicated when there was a short to ground.

I'm guessing it worked similar to a 3-phase ground fault indicator. The 2 lights probably shared a common ground connection. If there was a short, one of the lights would get brighter, and the other one would go out.

This item from Siemens appears to work similarly, but with a relay output.

https://eb.automation.siemens.com/mall/en/WW/Catalog/Product/3RK1408-8KE00-0AA2



-Eric

 

[boxofrelays]

I've had a recent panel assembled by one of my techs. All power supplies went to a common negative (0 volt return terminal) there was one single link from that common return bus that was suppose to go to an isolated ground. It was left off by accident. With all field devices terminated and power turned on BANG! I lost several intrinsically safe barrier terminals. Cost me a few thousand dollars. A really embarrassing oversight. I provide a common return on my power supplies and ground it. I don't want my ground to be referenced at a field device.

-Chris

 

[Andybr]

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.

There is no reason at all why you should see 12V between either side and ground. If you have a truly floating system then you will measure 0V between either side and ground because the resistance of your meter will create a reference for the system. If you are seeing 12V then the reference is being created somewhere else and the resistance of the reference connection is much lower than that of your meter. This is not surprising because many components have centre tapped LRC filter arrangements which would do this. The reality is that any system will only remain truly floating as long as there is nothing connected to it. Once you add components you create all sorts of leakage paths to ground.

 

[BobB]

For generator jobs I normally ground battery supply negatives but then feed my PLC through a 24VDC/24VDC switch mode power supply. The negative from the switch mode is never grounded - you lose your isolation through the switch mode for a start. Never had an issue with this.