A meter stator contains a potential coil and one or more current coils. These coils provide both voltage and current signals to the meter. The stator must be able to compare these two signals at any moment in time. Therefore, to establish forward rotation of the meter disk, polarity marks on instrument transformers must be observed. Incorrect polarity connections will result in reverse rotation of the meter disk. Instrument transformers have polarity marks to aid in establishing forward rotation for each stator.
Correct phase relationships between voltage and current signals must be matched up. For example, in a form 9S meter; phase "A" voltage and current signals; phase "B" voltage and current signals; phase "C" voltage and current signals are specifically matched up to feed common stators in the meter.
The task of establishing correct polarity as well as matching up voltage and current signals from the appropriate phases is much easier with the use of wiring made with color coded insulation. It is very difficult to check an instrument rated metering installation that has been wired with all black wires!
The color code for instrument transformer rated installations used throughout this web site was established by Wheatland Electric Cooperative in the 1960’s. Red, orange, and yellow are bright colors and are therefore used for potential wires. Black, blue, and brown are dull colors and are used for currents. Green is used for grounding. These colors are not used randomly but rather in a specific order. For example; red potential is paired up with a black current for a given phase; orange potential is paired up with a blue current; and yellow potential is matched up with a brown current. The use of this color code or a similar code established by the utility greatly enhances the opportunity for success when wiring instrument rated metering.
Factory Pre-wired Meter Sockets
Meter sockets may be ordered pre-wired with color coded test switch handles to match the utilities color code. A factory wired meter socket should always be checked with a continuity meter to confirm that it is wired correctly. Linemen often go to great lengths to correctly wire instrument metering only to have problems because of a miss-wired factory wired meter socket.
Separate Current Circuit Return Paths
Some utilities use separate current return wires for instrument metering installations. For example, a CT with a black wire connected from X1 to the top of a current coil would need a white wire with black stripe connected from the bottom of the current coil back to X2 on the CT. By utilizing separate return paths rather than using a common (green) wire for all CT returns, transients, additional return conductor voltage drop, etc… may be minimized.
Single Point Grounding
Many linemen believe the old rule "The more grounds the merrier". In other words, you can not have too many grounds. This is not partially true for instrument metering. Meter socket and instrument transformer case grounds should always be present. But generally speaking, a single point for grounding instrument transformer (X2) return wires should be placed near the instrument transformers at a common point. Do not ground the same wires a second time in the meter socket. To do so would establish a parallel path in the return wires which would make the installation more susceptible to metering errors caused by transient currents that do not belong in the metering circuit paths.