Date Code 20010731 Breaker Logic 3-21
SEL-352-1, -2 Instruction Manual
same input may be used for all three conditions if single-pole tripping is not used. The
programmable input settings are actually SEL
OGIC control equations. Use any combination of
Relay Word bits according to the SEL
OGIC control equation criteria in Section 5: Control Logic.
Breaker Status (52AA, 52AB, 52AC)
Scheme 2 also uses the three programmable inputs: 52AA, 52AB, and 52AC. Use SET G
<ENTER> to associate these with physical inputs.
Load Detector (50LD)
Range: 0.10–45.00 A in 0.01-A steps (5 A)
0.02–9.00 A in 0.01-A steps (1 A)
Use SET <ENTER> to access the 50LD overcurrent element. This element is used in the
failure-to-trip load current breaker protection scheme. The 50LD element should pick up when
the protected breaker is closed. This scheme detects failures of the breaker to open when breaker
current is lower than the 50FT setting, such as load-breaking operations.
If the protected breaker is in a ring-bus or breaker-and-a-half arrangement, set 50LD to pick up
for line-charging current of the shortest line serviced by that breaker.
Charging current for a given line can be calculated using the following equation:
I
c
= Vg • Bc A primary
where Vg is the line-to-ground voltage and Bc is the total line capacitive susceptance. Multiply
the line capacitive susceptance by the line length if it is specified on a per-unit basis.
For the example application, line-charging current I
c
is 350 A primary or 0.58 A secondary.
The 50LD element accuracy specification for a nominal 5 A relay is ±0.025 A secondary ±5% of
setting (±0.005 ± 5% for I
nom
of 1 A.) The error figure is calculated with the method used for
50FT. The maximum error for a setting of 0.58 A secondary is ±0.054 A secondary. To ensure
that 50LD picks up any time the protected breaker is closed, the 50LD element should be less
than the result of the following equation:
50LD = 0.58 A – 0.054 A = 0.53 A secondary
The relay also uses the 50LD element in the flashover protection logic. If you intend to use the
breaker pole flashover protection logic, please refer to the flashover logic descriptions before
finalizing your 50LD setting selection.
When the protected breaker is part of a ring-bus or breaker-and-a-half installation, load current
may be very low due to unequal current distribution between the two breakers. Failure-to-trip
load current logic may still be used to protect the breaker. Because current application may be
sequential, scheme timing is uncertain in ring-bus or breaker-and-a-half installations.
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