SinglePhaseEnergyMeterIC
with Integrated Oscillator
BL6502A
ꢀ
FAULT DETECTION
The BL6502A incorporates a novel fault detection scheme that warns of fault conditions and
allows the BL6502A to continue accurate billing during a fault event. The BL6502A does this by
continuously monitoring both the phase and neutral (return) currents. A fault is indicated when
these currents differ by more than 12.5%. However, even during a fault, the output pulse rate on
F1 and F2 is generated using the larger of the two currents. Because the BL6502A looks for a
difference between the signals on V1A and V1B, it is important that both current transducers are
closely matched. On power-up the output pulse rate of the BL6502A is proportional to the product
of the signals on Channel V1A and Voltage Channel. If there is a difference of greater than 12.5%
between V1A and V1B on power-up, the fault indicator (FAULT) will go active after about one
second. In addition, if V1B is greater than V1A the BL6502A will select V1B as the input. The
fault detection is automatically disabled when the voltage signal on Channel 1 is less than 0.5% of
the full-scale input range. This will eliminate false detection of a fault due to noise at light loads.
If V1A is the active current input (i.e., is being used for billing), and the signal on V1B (inactive
input) falls by more than 12.5% of V1A, the fault indicator will go 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 one second on the logic output FAULT after the
fault event. The FAULT logic output is independent of any activity on outputs F1 or F2. Figure 8
illustrates one condition under which FAULT becomes active. Since V1A is the active input and it
is still greater than V1B, billing is maintained on VIA, i.e., no swap to the V1B input will occur.
V1A remains the active input.
V1A
V1B
FAULT
to ADC
V1A
V1B
V1N
current
sampling
0V
V1B < 87.5% V1A
Figure 8. Fault Conditions for Inactive Input Less than Active Input
Figure 9 illustrates another fault condition. If V1A is the active input (i.e., is being used for billing)
and the voltage signal on V1B (inactive input) becomes greater than 114% of V1A, the FAULT
indicator goes active, and there is also a swap over to the V1B input. The analog input V1B has
now become the active input. Again there is a time delay of about 1.2 seconds associated with this
swap. V1A will not swap back to being the active channel until V1A becomes greater than 114%
of V1B. However, the FAULT indicator will become inactive as soon as V1A is within 12.5% of
V1B. This threshold eliminates potential chatter between V1A and V1B.
V1B
V1A
FAULT
to ADC
V1A
V1B
V1N
current
sampling
0V
V1A < 87.5% V1B
Figure 9. Fault Conditions for Inactive Input Greater than Active Input
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- 11 -
3/15/2007
Total 14 Pages
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