ADE7761
Fault with Active Input Greater than Inactive Input
Calibration Concerns
If V1A is the active current input (that is, being used for billing),
and the voltage signal on V1B (inactive input) falls below 93.75%
of V1A, the fault indicator becomes active. Both analog inputs are
filtered and averaged to prevent false triggering of this logic
output. As a consequence of the filtering, there is a time delay of
approximately 3 s on the logic output FAULT after the fault
event. The FAULT logic output is independent of any activity on
outputs F1 or F2. Figure 26 shows one condition under which
FAULT becomes active. Because V1A is the active input and it is
still greater than V1B, billing is maintained on V1A, that is, no
swap to the V1B input occurs. V1A remains the active input.
Typically, when a meter is being calibrated, the voltage and
current circuits are separated as shown in Figure 28. This means
that current passes through only the phase or neutral circuit.
Figure 28 shows current being passed through the phase circuit.
This is the preferred option, because the ADE7761 starts billing
on the input V1A on power-up. The phase circuit CT is con-
nected to V1A in the diagram. Since there is no current in the
neutral circuit, the FAULT indicator comes on under these
conditions. However, this does not affect the accuracy of the
calibration and can be used as a means to test the functionality
of the fault detection.
FAULT
FILTER
AND
V
R
F
1A
V
V
1A
1A
IB
CT
COMPARE
V
A
B
1B
V
V
RB
RB
TO
C
C
1A
1B
V
F
F
1A
MULTIPLIER
0V
AGND
V
V
1N
1B
V
V
1N
AGND
IB
0V
TEST
CURRENT
V
< 93.75% OF V
1A
1B
CT
FAULT
R
F
1B
RA*
RB*
VR*
C
F
<0
>0
ACTIVE POINT – INACTIVE INPUT
V
V
2P
6.25% OF ACTIVE INPUT
R
F
2N
V
Figure 26. Fault Conditions for Active Input Greater than Inactive Input
C
T
240V RMS
*RB + VR = RF
Fault with Inactive Input Greater than Active Input
Figure 28. Fault Conditions for Inactive Input Greater than Active Input
Figure 27 illustrates another fault condition. If the difference
between V1B, the inactive input, and V1A, the active input (that is,
being used for billing), becomes greater than 6.25% of V1B, the
FAULT indicator goes active, and there is also a swap over to the
V1B input. The analog input V1B becomes the active input. Again,
there is a time constant of about 3 s associated with this swap.
If the neutral circuit is chosen for the current circuit in the
arrangement shown in Figure 28, this may have implications for
the calibration accuracy. The ADE7761 powers up with the V1A
input active as normal. However, because there is no current in
the phase circuit, the signal on V1A is zero. This causes a fault to
be flagged and the active input to be swapped to V1B (neutral).
The meter can be calibrated in this mode, but the phase and
neutral CTs might differ slightly. Because under no-fault condi-
tions all billing is carried out using the phase CT, the meter
should be calibrated using the phase circuit. Of course, both
phase and neutral circuits can be calibrated.
V1A does not swap back to being the active channel until V1A is
greater than V1B and the difference between V1A and V1B—in this
order—becomes greater than 6.25% of V1A. However, the
FAULT indicator becomes inactive as soon as V1A is within
6.25% of V1B. This threshold eliminates potential chatter
between V1A and V1B.
FAULT
FILTER
AND
COMPARE
V
V
1A
1A
MISSING NEUTRAL MODE
V
A
B
1B
The ADE7761 integrates a novel fault detection that warns and
allows the ADE7761 to continue to bill in case a meter is
connected to only one wire (see Figure 29). For correct
operation of the ADE7761 in this mode, the VDD pin of the
ADE7761 must be maintained within the specified range (5 V
5%). The missing neutral detection algorithm is designed to
work over a line frequency of 45 Hz to 55 Hz.
V
V
TO
1A
1B
MULTIPLIER
0V
V
V
1N
1B
AGND
V
< 93.75% OF V
1B
1A
FAULT + SWAP
<0
>0
ACTIVE POINT – INACTIVE INPUT
6.25% OF INACTIVE INPUT
Figure 27. Fault Conditions for Inactive Input Greater than Active Input
Rev. A | Page 20 of 28
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