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82
NC20 - Manual - 01 - 2015
FUNCTION CHARACTERISTICS
Phase overcurrent - 50/51 RMS
Preface
One alarm and one operation threshold, independently adjustable (
I
>
AL
,
I
>) with adjustable delay
(
t
>
AL
,
t
>).
The fi rst one, useful for alert approaching to the capacitor overvoltage, have a defi nite time char-
acteristic.
The operation threshold may be programmed with defi nite or inverse time according the IEC and
ANSI/IEEE standard, as well as with CAPACITOR curve (optimum compliance to ANSI and IEC stan-
dards).
A reset time can be set (
t
>RES
) useful to reduce the clearing time for intermittent faults.
The fi rst alarm threshold may be inhibited by start of the operation threshold (
I
>).
Operation and settings
Each phase RMS current (including up to eleventh harmonic) is compared with the setting value.
Currents above the associated pickup value are detected and a start is issued. After expiry of the
associated operate time a trip command is issued; if instead the current drops below the threshold,
the element is restored.
The fi rst threshold (
I
>) may be programmed with defi nite or inverse time according the following
characteristic curves:
Standard Inverse Time (IEC 255-3/BS142 type A or SIT):
t
= 0.14 ·
t
>
inv
/ [(
I
RMS
/
I
>
inv
)
0.02
- 1]
Very Inverse Time (IEC 255-3/BS142 type B or VIT):
t
= 13.5 ·
t
>
inv
/ [(
I
RMS
/
I
>
inv
) - 1]
Extremely Inverse Time (IEC 255-3/BS142 type C or EIT):
t
= 80 ·
t
>
inv
/ [(
I
RMS
/
I
>
inv
)
2
- 1]
Moderately Inverse (ANSI/IEEE type MI):
t
=
t
>
inv
· {0.01 / [(
I
RMS
/
I
>
inv
)
0.02
- 1] + 0.023}
Very Inverse (ANSI/IEEE type VI):
t
=
t
>
inv
· {3.922 / [(
I
RMS
/
I
>
inv
)
2
- 1] + 0.098}
Extremely Inverse (ANSI/IEEE type EI):
t
=
t
>
inv
· {5.64 / [(
I
RMS
/
I
>
inv
)
2
- 1] + 0.024}
CAPACITOR:
t
=
t
>
inv
· {50000 / [1.1 · (
I
RMS
/
I
>
inv
)
17
-1]} + 0.1
Where:
t
: operate
time
I
RMS
:
RMS input current
I
>
inv
: threshold
setting
t
>
inv
: operate
time
setting
The alarm threshold (
I
>
AL
) has defi nite time characteristic.
For all inverse time characteristics, following data applies:
Asymptotic reference value (minimum pickup value): 1.1
I
>
inv
Minimum operate time:
0.1 s
Range where the equation is valid:
[1]
1.1 ≤
I
RMS
/
I
>
inv
≤ 20
If
I
>
inv
pickup ≥ 2.5
I
n
, the upper limit is 50
I
n
For all defi nite time elements the upper limit for measuring is 50
I
n
.
All overcurrent elements can be enabled or disabled by setting
ON
or
OFF
the
I>AL Enable
and/or
I> Enable
parameters inside the
Set \ Profi le A(or B) \ Phase overcurrent-50/51 RMS \
I>AL Element
(
I> Element) \ Setpoints
menus.
The fi rst overcurrent element can be programmed with defi nite or inverse time characteristic by
setting the
I>Curve
parameter (
DEFINITE, IEC/BS A, IEC/BS B, IEC/BS C, ANSI/IEE MI,
ANSI/IEE VI, ANSI/IEE EI, CAPACITOR
) available inside the
Set \ Profi le A(or B) \ Phase over-
current-50/51 RMS \
I> Element \ Setpoints
menu.
The operation of
I
>AL element may be inhibited by:
The start of the fi rst element (
I
>) by setting
ON
the Disable I>AL by start I> (
I>ALdisbyI>
) parame-
ters available inside the
Set \ Profi le A(or B)\ Phase overcurrent-50/51 RMS \ I> Element
\ Setpoints
menu.
The start of second and/or third element (
I
>>,
I
>>>) of the fundamental frequency phase overcurrent by set-
ting
ON
the Disable I>AL by start I> (
I>ALdisbyI>>, I>ALdisbyI>>>
) parameters available inside
the
Set \ Profi le A(or B) \ Phase overcurrent-50/51 Fundamental \I>> Element, (I>> Element)
\ Setpoints
menus
Note 1 When the input value is more than 20 times the set point , the operate time is limited to the value corresponding to 20 times the set point
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t-int-F50-51.ai
I
R MS
I
>
t
>
A L
t
>
I
>
A L
t
TRIP
General operation time characteristic for the phase overcurrent elements - 50/51 RMS
