Analog wiring from a sensor to I/O point is prone to certain types of problems than can disrupt accurate readings. Ground loops are particularly troublesome, since they are capable of serious signal disruption and their effect can be intermittent.
　　For a ground loop to occur, two things must be present: at least two different grounds that are at different potentialswww.cechina.cn, and a galvanic path or circuit established between those grounds. Usually the circuit is completed when the process signal wire is connected from the transmitter (ground potential #1) to the receiving device (ground potential #2).
　　Ground loops cause problems by adding or subtracting small current or voltage levels to or from the process signal. The I/O point receiving the signal can’t differentiate between the desired and corrupted signalCONTROL ENGINEERING China版权所有, so the readout or DCS will not reflect true process conditions. There are two solutions: Eliminate all but one ground控制工程网版权所有, or isolate each ground from the others.
　　Eliminating grounds may not be possible for some instruments (e.g., thermocouples) because they require a local ground for accuracy, or they must be grounded for personnel safety.
　　When multiple grounds cannot be eliminated, the solution is to use signal isolators. These devices break the galvanic path (the dc continuity) between grounds that are at different earth potentials. Without this path, there is no way for any stray current or voltage to reach the receiving device. Moreoverwww.cechina.cn, an isolator also eliminates another problem: ac continuity noise, otherwise known as common mode voltage.
　　There are two basic types of signal isolators:
　　Two-wire (loop-powered); and
　　Four-wire (line- or mains-powered).
　　Two-wire isolators draw power from the 4-20 mA process signal loopCONTROL ENGINEERING China版权所有, so they require no additional power supply or lines which can save on wiring costs. Most two-wire isolators are output loop powered (OLP), but there are two-wire isolators that are input-loop powered (ILP) too. Output loop powered isolators a