ADE7761B
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 sec on the Logic Output FAULT after the fault
event. The FAULT logic output is independent of any activity on
the F1 or F2 outputs. Figure 28 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 30. This means that
current passes through only the phase or neutral circuit. Figure 30
shows current being passed through the phase circuit. This is
the preferred option because the ADE7761B starts billing on the
input V1A on power-up. The Phase Circuit CT is connected to
V1A in Figure 30. Because 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.
V
R
1A
F
IB
CT
FAULT
FILTER
V
V
1A
1A
AND
RB
RB
V
COMPARE
C
A
1B
V
F
F
1A
V
V
TO
1A
1B
AGND
MULTIPLIER
0V
V
V
1N
V
V
1N
1B
IB
0V
AGND
C
B
TEST
CURRENT
V
< 93.75% OF V
1A
1B
CT
R
F
1B
1
RA
FAULT
<0
C
F
1
1
RB
V
V
2P
>0
VR
ACTIVE POINT – INACTIVE INPUT
R
F
2N
6.25% OF ACTIVE INPUT
V
C
T
Figure 28. Fault Conditions for Active Input Greater Than Inactive Input
240V rms
1
RB + VR = RF.
Fault with Inactive Input Greater Than Active Input
Figure 30. Conditions for Calibration of Channel B
Figure 29 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 becomes active and a swap over to the V1B
input occurs. The Analog Input V1B becomes the active input.
Again, a time constant of about 3 sec is associated with this swap.
V1A does not swap back to 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 indi-
cator becomes inactive as soon as V1A is within 6.25% of V1B. This
threshold eliminates potential chatter between V1A and V1B.
If the neutral circuit is chosen for the current circuit in the
arrangement shown in Figure 30, this may have implications for
the calibration accuracy. The ADE7761B 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 may differ slightly. Because under no-fault conditions
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.
FAULT
FILTER
AND
V
V
1A
1A
MISSING NEUTRAL MODE
A
V
COMPARE
1B
V
V
TO
1A
1B
The ADE7761B integrates a novel fault detection scheme that
warns and allows the ADE7761B to continue to bill in case a
meter is connected to only one wire (see Figure 31). For correct
operation of the ADE7761B in this mode, the VDD pin of the
ADE7761B 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.
MULTIPLIER
0V
V
V
1N
1B
AGND
B
V
< 93.75% OF V
1B
1A
FAULT + SWAP
<0
>0
ACTIVE POINT – INACTIVE INPUT
6.25% OF INACTIVE INPUT
Figure 29. Fault Conditions for Inactive Input Greater Than Active Input
Rev. 0 | Page 18 of 24
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