ADE7761ARSRL datasheet(20/28 Pages) AD

20 / 28 page

ADE7761

Rev. A | Page 20 of 28

Fault with Active Input Greater than Inactive Input

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.

V1B

V1N

V1A

AGND

FILTER

AND

COMPARE

MULTIPLIER

FAULT

V1A

V1B

V1B < 93.75% OF V1A

ACTIVE POINT – INACTIVE INPUT

6.25% OF ACTIVE INPUT

FAULT

V1A

V1B

Figure 26. Fault Conditions for Active Input Greater than Inactive Input

Fault with 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.

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.

V1B

V1N

V1A

AGND

FILTER

AND

COMPARE

MULTIPLIER

FAULT

V1A

V1B

V1A < 93.75% OF V1B

ACTIVE POINT – INACTIVE INPUT

6.25% OF INACTIVE INPUT

FAULT + SWAP

V1A

V1B

Figure 27. 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 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.

AGND

V1B

V1N

V1A

*RB + VR = RF

VR*

RB*

RA*

V2P

V2N

TEST

CURRENT

240V RMS

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

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.

MISSING NEUTRAL MODE

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.

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