ADE7761B

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

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

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 swap.

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 ADE7761B starts billing on the

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 ADE7761B powers up with the

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.

MISSING NEUTRAL MODE

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

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.