8.31
Date Code 20080213 Instruction Manual SEL-351A Relay
Breaker Monitor and Metering
Small Signal Cutoff for Metering
Small Signal Cutoff for Metering
The relay applies a threshold to the voltage and current metering quantities to
force a reading to zero when the measurement is near zero. Table 8.5 lists the
thresholds for nominal current and voltage values. These thresholds are above
the noise level for analog measurement by a digital system.
Without these thresholds, if a bus is deenergized or a circuit breaker is open,
noise-level values would cause erroneous energy accumulation.
These thresholds affect the metering functions for power and energy on a per-
phase basis. For example, if a load-side single-phase disconnect is open on
C-phase, the relay will force the IC metering value to 0.00, and the resulting
power and energy values for C-phase will be 0.00. In this example, the
A-phase and B-phase values are still calculated, provided the current and
voltage exceed the threshold values.
Table 8.5 Metering Thresholds (Secondary Units)
Current Channels: Nominal Current Input Rating
5A 1A 0.2A
a
a
Not available in SEL-351A-1.
0.05 A
a
IA, IB, IC 0.05 A 0.01 A N/A N/A
IN 0.05 A 0.01 A 0.002 A 0.0005 A
Voltage Channels: All Models
VA, VB, VC, VS
b
b
VS channel not present in SEL-351A-1.
0.1 V
Courtesy of NationalSwitchgear.com
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