We have PLC system with dual power supply 24V dc with OR gated diode to make 1oo2 for the loop voltages which connected to the digital input of the PLC. Surprising, thing is that when we try to measure the Power Supply voltage across the +ve and -ve points, I can read 24Vdc without any problem. But when the same measured points with respective to ground by the High end FLUKE multi-meter, I mean Red Lead at +ve of PS and Black lead to the ground, initially it shows 15Vdc and slowly it becomes 0Vdc. Why this is happening so?. I expect it should show +12Vdc with respective to GND with on measurement of +ve leg and -12vdc when we measure on -ve leg. Kindly reply asap.

 

My guess is that your system is ungrounded so the only reference to ground is via capacitance created by the wiring. the meter is slowly discharging whatever charge has built up there.

--
Bob

 

Simple, your power supply and it's load are floating. That is the
negative is not connected to ground. So there is only leakage and capacitance providing your readings.

Regards
cww

 

> Simple, your power supply and it's load are floating. That is the
> negative is not connected to ground. So there is only leakage and capacitance
> providing your readings.

Is it good to leave like this or I need to do some thing to avoid future damages. Will it cause any fault alarms due to noises?. Some times we receive the fake alarms from the Bentley Nevada sensors which are installed on the Compressor.

 

The noise issue depends on a lot of other factors besides grounding, in fact it's hard to even define noise without a ground reference. In some cases grounding a floating system can cause problems. I always ground one side of the poser supply, I think that with the narrow exception of differential signals, it you have to float a system to make it work, its broken.

Grounding and bonding is one of the least understood subjects in automation. I ground one side to avoid having house power leakage and inductive effects from raising my signal grounds to some AC potential and so that ground faults trip protective devices. If that causes problems, they are problems that should be solved whether you ground the DC or not. There are exceptions, but they are rather obvious. And as you've seen it can be really interesting checking things out. And even more interesting chasing down noise problems and parasitic leakage and ...............

Regards
cww

 

Dear CWW,

As replied the noise is not clear to any one in automation. I feel this one of the toughest task to catch this fish by the net in the field of instrumentation. I confused with other split power systems with this floating voltage system. Then, I feel my power supply is ok as per floating system. Then I am not going to do any thing at this moment.

Regds..

 

Grounding and bonding are clearer to some of us, especially those with RF or precision analog background. Judging by the number of questions on this list they should be better addressed in documentation and education, but that kinda conflicts with the "easy to use", "anyone can do it" marketing for automation gear. From what I've looked at, certainly, just anyone _has_ done it. And rework is often impractical and always expensive. This contributes to the high percentage of failed automation projects. The devil is in the details. To their credit, many manufacturers have done an amazing job at making the stuff easy to use, but the underlying physics haven't changed and ignoring the subject can really come back to bite you. For example, I'm truly amazed that they can make 12 bit analog stuff reasonably repeatable in an industrial environment, albeit at low rep rates. But, it doesn't take very much to reduce that to eight bits you can believe and some noise. And I've seen installations with dozens of thermocouples, none of which you can believe. I've even seen discrete logic that flips bits and you really have to work to screw that up with volts of noise margin. With some of the stuff I've seen (and been asked to fix) the amazing part is that it works at all.

Regards
cww