Goodknght436
Absolutely a floating ground has caused more problems than any other single item.
I have seen electricians spend months trouble shooting a intermittent electrical problem only to discover a floating ground.
Even thought you are using a 4-20 ma signal they do by default reject short duration spikes. But they are not without problems.
A basic rule of electricity the total sum current or voltage in an electrical circuit must sum up to = 0
I know that may not make sense but you have to look it up or just rust me.
The return path for the ma signal is the grounds or circuit commons .
The ma source is of course the OF4 output card then the signal passes through the wire to the input of the sensaphone . The current can't just stop there it has to return to the source the best available path is through the commons. That's why the path must have as low a resistance as possible. If either of those are floating then the resistance to the return current is constantly changing. And what's worse the voltage potential between these 2 points is also changing. If it goes high enough it could damage the internal circuit of either device. Electrical current will find a path back to it's source if possible and it may not be the path you want.
But if the commons are insulated from each other then there is no path for the current to take = no ma signal, but that dos not mean there is no voltage potential in fact the voltage potential could in fact be very high into the hundreds of volts (think static charge)
That’s why we ground everything.
No you cannot calibrate around a floating ground problem what you setup today may not be the same tomorrow or even this afternoon.
However there are times when you just can't ground or bond all the different power supplies together for whatever reason that where a signal isolator comes in. In a signal isolator the power supply , input signal and the outputs are all isolated from each other either by an isolation transformer coupling or optically coupled. Each has their own [input – com], [output – com],power supply High side - Low side or common. This allows you to connect across the different circuits without current passing between them.
Go out and lookup the data sheets on a few isolators and see how they are connected.
There some people that will claim that floating ground are a good thing. But if that was the case then why does the NEC require everything to be bonded (grounded) to a single electrical point of the building service. Ground conductor pulled with everything even the panel doors need to be bonded to the panel structure (not using the hinge as a conductor) and the panels must be bonded to the building structure.
Some of the worst shocks I have personally received while working in the industry were from floating grounds on circuits that were turned off. capacitance and inductive coupling on long wire runs. I have even heard or people getting electrocuted on power wires that are off but not properly grounded. I could write a book on the problems I have found due to a floating ground. And it seems to be getting worse not better with everything going to 24VDC input and outputs. Then they use multiple 24VDC power supplies (due to loading) but they forget to bond the power supplies together so the I/O works intermittently. And they spend hours trouble shooting a problem of their own creation.
I have actually seen a motor start and run with the PLC output off due to floating ground caused the contactor to pull in.
Floating grounds should never be used unless you have given the whole system a lot of thought and considered every possible current path and how you are going to handle a failure.