ADE7761A

Rev. 0 | Page 18 of 24

Fault with Active Input Greater than Inactive Input

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

outputs F1 or F2. Figure 28 shows one condition under which

AGND

FILTER

AND

COMPARE

TO

MULTIPLIER

FAULT

A

B

>0

<0

ACTIVE POINT – INACTIVE INPUT

6.25% OF ACTIVE INPUT

0V

FAULT

Figure 28. Fault Conditions for Active Input Greater than Inactive Input

Fault with Inactive Input Greater than Active Input

Figure 29 illustrates another fault condition. If the difference

Again, a time constant of about 3 sec is associated with this

AGND

FILTER

AND

COMPARE

TO

MULTIPLIER

FAULT

A

B

>0

<0

ACTIVE POINT – INACTIVE INPUT

6.25% OF INACTIVE INPUT

0V

FAULT + SWAP

Figure 29. Fault Conditions for Inactive Input Greater than Active Input

Calibration Concerns

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 ADE7761A

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.

AGND

CT

CT

RB

RB

0V

IB

IB

V

TEST

CURRENT

240V rms

Figure 30. Conditions for Calibration of Channel B

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 ADE7761A powers up with the

causes a fault to be flagged and the active input to be swapped

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.

MISSING NEUTRAL MODE

The ADE7761A integrates a novel fault detection that warns

and allows the ADE7761A to continue to bill in case a meter is

connected to only one wire (see Figure 31). For correct

ADE7761A 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.