One common problem we run into from time-to-time is that of analog signals not operating properly. Whether it is a 0-5 VDC, 0-10 VDC, 0-20mA or a 4-20mA signal we have had problems with them all. Most times the problem is that the process is not operating as prescribed. In other words, let’s look at a variable frequency controller operating from a pressure transmitter. The system pressure is set for 90 pounds of pressure but the drive is either operating too fast, too slow or sometimes both. This in turn causes the system pressure to be unstable and to not be at set-point. This can be the result of improper routing of the signal wiring but if that is the case, it would show up from the beginning and should be dealt with at that time. This is of course unless some modifications have been made to the original conduit runs/wiring or the addition of a piece of equipment.
Most times what we see is that multiple grounds have been attached on the control or signal circuit. This causes a problem due to differing potentials in relationship to earth ground at each point of bond. Normally what we find is that the signal cable would have been grounded in the PLC cabinet or in the drive during time of commissioning. Then months later or even longer, someone may have had to replace a transmitter/transducer. At this time the installer grounds the signal cable at the location of the device. This is typically what happens causing the problem.
When you have a signal circuit grounded in more than one place, due to the differing potentials at the attachment points you get ground currents flowing through the shield. This causes an addition or subtraction of current or voltage on the signal cable causing an error to the controller. As the signal is traveling from the measuring device to the control device, the signal is being added to or suppressed by the ground currents flowing on the shield. This causes the error in the control unit and therefore confuses the entire process.
Some devices come with leads already hanging out of them as they have been “potted” at the factory. In the case of a pressure transducer or other metallic bodied device, one should measure the potential of the shield on the device in relationship to the body to make sure they are not connected. If they are, care should be taken to insure that the shield/ground is not connected at the other end of the run in the control device or where ever the run terminates. This is a major problem and is normally the cause of the error at most sites where water pumping/controlling is taking place.
As silly as this may seem, it is real and happens all the time. When installed correctly, signal cables should be grounded on one end only, it’s just that easy. But in practice, this mistake is very common and that is what lead to this write-up. It also helps to route your signal cables so they are not running parallel or in close proximity to your power leads. Never run signal cables in the same conduit as power leads as this is nothing less than a waste of time and money.