Some of you might remember that back in 1975 Paul Simon had a hit song entitled “50 Ways to Leave Your Lover.” Well, coming in a close second are the number of ways (47 of them) that you can wire a 3-phase power meter incorrectly! In this article we’ll briefly discuss why there are so many ways to incorrectly wire a 3-phase power meter and how you can try to insure that your wiring is correct.
Six Different Inputs to Deal With
A 3-phase power meter has 6 different input signals which must be present and connected correctly in order to measure power accurately:
- There are 3 voltage inputs (we will refer to them as L1, L2, L3) which are connected to the three “hot” wires of the power system being monitored.
- There are 3 current inputs (we will refer to them as CTA, CTB, CTC) which are connected to 3 “Current Transformer” sensors (CTs). The CTs have holes through their centers and the L1, L2, L3 hot wires pass through the holes in the CTs. The CTs measure the currents flowing in the hot wires.
A picture may help to clarify our word description:
Note that a “Neutral” power wire is also shown on the drawing . This wire will be present on a 3-Phase Wye power system and absent on a 3-Phase Delta power system. This article is valid for both scenarios.
The L1/L2/L3 wiring is straightforward. A single wire is run from each “hot” wire to its corresponding L1/L2/L3 input terminal on the power meter. The CT installation and wiring are a bit more complex, however.
Note that each current transformer has two wires on its output which run to the power meter, and the power meter has 2 screws labeled “X1” and “X2” for each CT input. Normal convention is that the wires from the CT are colored white and black, and the white wire connects to the X1 screw while the black wire connects to the X2 screw.
The body of the CT has one side designated “H1” and the other side is “H2.” This could be done with labels or molded directly into the plastic CT body. The CT should be installed with the H1 side facing the power source and the H2 side facing the load.
How to Get It Wrong – Cross Wiring the CTs and L1/L2/L3 Wires
In the drawing below, current transformers CTB and CTC have been cross-wired with L2 and L3:
Note that CTB is around the L3 wire and CTC is around the L2 wire. In this scenario, the power meter will calculate Phase A power correctly, but Phase B power and Phase C power will both the incorrect, resulting in the total power also being incorrect.
Here is a diagram showing the different ways that CTA, CTB, CTC can be paired with L1, L2, L3. Each diagonal line represents an incorrect cross-wiring between the CTs and hot wires:
How to Get It Wrong – Reversing the CT Polarities
In the drawing below, current transformer CTC has been installed over hot wire L3 with the “H1” side facing the load instead of the power source:
With CTC’s H1 facing the wrong direction, the power meter will either read a Phase C power of zero (if meter is not capable of bi-directional power measurement) or it will read a negative power (if meter is capable of bi-directional power measurement). Either way, the total power measurement is going to be incorrect.
With three CTs, each capable of being installed with plus or minus orientation, there are 8 possible combinations of CT polarities:
Combining CT Cross-Wiring and CT Polarity Possibilites
Below is a diagram showing the possible CT cross-wiring combinations and the possible CT polarity assignment combinations. Note that out of all the possible combinations, there is exactly one combination that measures total power correctly!
Symptoms of Incorrect Meter Wiring
Incorrect CT-hot wire matching or reversed CT polarities will give lower-than-expected power readings or even negative power readings. Power factor will also read unusually low on the cross-wired phases.
What’s A Poor Installer To Do?
To have a chance of getting it right, you need to pay scrupulous attention to wire assignments. Use different wire colors and/or use stick-on wire tags to unambiguously designate the wire functions at both ends of the wire runs.
Determining Correct Wiring Configuration On An Installed Meter
If you suspect that your meter wiring might be wrong, the best way to determine correct wiring is to physically trace everything out. However, this may be difficult or impossible to do on some installations (for example CTs are sometimes buried inside switchgear which is locked for safety reasons).
If physical wiring inspection isn’t possible then “in theory,” if you are monitoring a constant load, you could rearrange the wiring to try every combination in the table above looking for the highest total KW reading. This isn’t very practical as it would require a tremendous amount of physical wire swapping and it’s unlikely that the load would remain constant during the length of time it would take to do all the wire rearranging.
Some power meters such as the Kele endicator meter will allow the user to electronically rearrange the wiring without physically moving the wires on the terminal blocks. Both the endicator LCD/keypad and endicator Desktop Software can be used to change the CT-hot wire matching and CT polarity assignments without picking up a screwdriver.
Even with electronic wire-switching capability in the meter, it would still be quite a challenge to keep track of which combinations had been tried and which combinations were still to be tested, all the while hoping that the load remained constant.
Auto-Configure To The Rescue
The Kele endicator power meter was designed with a special Auto-Configure feature to aid the meter installer in determining proper meter wiring. Auto-Configure can be invoked from the endicator LCD/keypad or endicator Desktop Software. When invoked, Auto-Configure analyzes all 6 of the incoming signals in a 3-step, 30 second process.
At the end of 30 seconds, Auto-Configure will return either a Pass or Fail message. If Auto-Configure passed, endicator will use the new wiring configuration automatically. You don’t have to do anything further! If Auto-Configure failed, endicator will continue to use the configuration that was active before Auto-Configure was run.
Does Auto-Configure Work Every Time?
We’d love to say yes, but the honest answer is that it works most of the time. There are a few reasons that Auto-Configure might not work on your installation:
- Insufficient or missing CT signals: all CT signals must be present and the load should be at least 5% of full scale. If Auto-Configure fails, make sure all the CT wires are connected and there is a sufficient load, then try again.
- Missing L1/L2/L3 signals: all L1/L2/L3 hot wire voltage signals must be present. If Auto-Configure fails, use a voltmeter to verify that L1/L2/L3 voltages are all present. If any voltage is missing, make the connection and try again.
- Load changes: the load should be stable during the 30 seconds that Auto-Configure is running. If Auto-Configure fails, try several more times. If loads change dynamically during one part of the day but are more steady at another time, try Auto-Configure at the more stable time.
- High levels of power system noise: some equipment like variable-frequency drives (VFDs) can inject high levels of noise into the power system. If you have VFDs as part of the load, try turning them off and retrying Auto-Configure.
There are many ways to wire a 3-phase power meter wrong and only one way to wire it correctly. Use color coded wire and/or wire tags to clearly identify each wire at the power system connection points and at the meter connection points.
If the power system connection points will be inaccessible later (locked up inside switchgear for example), try to do you meter testing early when you still have access to the power system connections.
If you have installed a Kele endicator power meter, give Auto-Configure a try and let it check the wiring for you in 30 seconds!