It is easy to think that on a two-wire transmitter that the + terminal is power and the - terminal is reference. That is how power wiring works but it is not how current loop wiring works. In a standard two-wire transmitter the 24 VDC power comes from a power source, goes through the current loop, and returns back to the power supply. A device like a transmitter on the loop will use some of the voltage and manipulate the current. The remaining voltage will push the signal current through the input device so it can read the signal. One or more auxiliary devices can be added to the loop to also make use of the signal. It is important that all the electrons entering the wire at one end make the full loop and return back to where they started.
If either of the wires are grounded in more than one spot the electrons will jump off the current loop and flow through the ground. This can also occur if a loop is accidentally cross wired to another loop. Some of the electrons will go where they should and some will go through the wrong part of one of the loops. Grounds are almost never at exactly the same potential. If there is voltage drop or a difference in potential across a wire, current will flow. If stray current flows through the detector part of an input device the current will be read as signal. When a 4-20 ma signal goes through a 12 bit Analog to Digital converter then current as low as 0.0000039 amps is recognized. Any stray currents above that number will be interpreted as signal no matter where the current comes from.
The stray currents formed from a wire grounded in more than one location is called a ground loop. Ground loops are bad and they use to be very common. So to stop ground loops in modern systems the 24 VDC power supply is no longer automatically grounded to a reference ground. Some designs specifically ground the 24 VDC system and other designs ignore the ground and do not ground it. Also many DCS IO channels isolate each channel from ground. In these cases the current loop is not grounded anywhere. It is floating. These signals are all current limited and will not hurt you if you ground out the signal through your body. They will however give misleading values if you are troubleshooting and looking for voltage compared to a field ground. You will need to check for voltage verses another part of the same circuit. A field ground is not part of the current loop circuit.
This implies that if an old device that grounds the current loop is attached to a modern DCS that there will only be one ground in the loop instead of no grounds in the loop but it will work just fine. Modern systems allow you to just hook devices together and they almost always work. Many young engineers have never seen a ground loop. Older devices hooked to older DCS and PLC systems may ground the signal at both ends of the loop which creates ground loops and bad readings.
In AC (alternating current) applications a transformer is used to convert higher voltages to lower voltages and back again. The primary and secondary of the transformer are not automatically grounded together. In most AC applications the current is NOT limited below what would hurt you or kill you. It is important that the installation design choose to ground the secondary of the transformer and fuse it appropriately. The National Electric Code requires it. An accidental stray ground will only be detected if the first ground is properly installed. A bad ground could kill someone.
It is important to recognize why it is ok to have floating signals in current limited instrument signals and it is NOT ok to have floating signals in power wiring or non-current limited signals.
Until next month,
Self proclaimed Automation Expert, CAP, and PE