Could one use this to measure the phase difference if one had the voltage measurement?

I have looked at the below household power meter, and was baffled by (what seems to be) the current transformer to have both the neutral and live bus bars going through. Surely this would just cancel the magnetic field out?

here is some more information:

The power meter uses this mcu

current transformer:
enter image description here

  • 1
    \$\begingroup\$ It's not clear to me from the photo of the current transformer which direction each heavy wire is carrying the current. So I must assume that they start and finish the right way round to add up, rather than cancel, if this is indeed a sensing core, and not a common mode filter core. \$\endgroup\$
    – Neil_UK
    Jul 10, 2016 at 20:01
  • \$\begingroup\$ Welcome to EE.SE! While your picture is very informative, can you reverse engineer a schematic of the power connections in this device? It's difficult to tell where the connections actually go in terms of source/load for this metering application. \$\endgroup\$ Jul 11, 2016 at 4:15
  • \$\begingroup\$ If the white braid covered wires carry current in the same direction, the CT reads double the current. This may have been done to make the power meter more sensitive. \$\endgroup\$
    – tomnexus
    Jul 12, 2016 at 4:56
  • \$\begingroup\$ On the right side is the power company bus, and then that goes through that trip switch at the top, which then goes to the left side. the wire in the middle is the neutral line. \$\endgroup\$
    – Alex
    Jul 12, 2016 at 5:34

1 Answer 1


Based on the wires shown, this is part of a ground fault detection circuit or GFCI transformer. The red and blue wires only output the difference between 'Hot' and neutral current as an AC voltage.

The idea is that if the hot wire is leaking current to ground from bad insulation or if someone touches the hot wire or a shorted appliance, enough imbalance will occur to trip the circuit OFF. With balanced loads that return all source current back through neutral, the currents cancel each other out, so the red-blue pair have essentially zero volts AC on them.

Usually the trip point is 1mA of imbalance to trip these GFCI circuits. With a fast response time, it may also trip on 'arc' faults where a leakage in the insulation of downstream wiring is wet, or the wire is pinched by a staple or nail, etc, and starts to arc with small current spikes before the insulation burns enough to cause a fire.

Outside outlets that are not rain-proofed can be prone to arc faults. Sauna steam rooms do not have AC power outlets as the thick steam would create an arc fault on the first day of operation.

  • \$\begingroup\$ Perfect! That explains it, but according to the datasheet of the mcu that is doing the metering, it can measure 2 phases, each of which require a CT to be connected, however, besides this GFCI transformer, there are no other transformers. So how would one measure the current that is flowing to the load and its the phase? \$\endgroup\$
    – Alex
    Jul 12, 2016 at 5:38
  • \$\begingroup\$ It seems as though this might be possible through a kelvin bridge, if you look closely on the left side bus bar, you can see 3 connections (red white black and black again), these are the sense resistors it seems. \$\endgroup\$
    – Alex
    Jul 12, 2016 at 5:44
  • \$\begingroup\$ @Alex. Based on the wiring I could see, the dominate part was the current sense transformer set up to set leakage on the 'hot' wire. I could see bits of additional parts which meant other circuits were in use, but I could not tell what they did. \$\endgroup\$
    – user105652
    Jul 12, 2016 at 15:51

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