FUNCTION CHARACTERISTICS
37
NC020 - Manual - 05 - 2022
—
Phase overcurrent - 50/51 RMS
Preface
One adjustable operation threshold, with adjustable delay.
The element, useful for alert approaching to the capacitor overvoltage, may be programmed with
definite or inverse time according the IEC and ANSI/IEEE standard, as well as with CAPACITOR curve
(optimum compliance to ANSI and IEC standards).
A reset time can be set (
t
>RES
) useful to reduce the clearing time for intermittent faults.
Operation and settings
Each phase RMS current (including up to twenty-third 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 threshold (
I
>) may be programmed with definite or inverse time according the following char-
acteristic 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
I
>
inv
:
operate time setting
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 definite time elements the upper limit for measuring is 50
I
n
.
The overcurrent element can be enabled or disabled by setting the relative start and/or trip output to
a selectable relay inside the
Set \ Relays
menu.
The overcurrent element can be programmed with definite 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 \ 50-51 RMS \ I> Setpoints
menu.
An adjustable reset time delay is provided for the operation threshold (
t
I
>
RES
).
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
t-int-F50-51.ai
I
RMS
I
>
t
>
t
TRIP
General operation time characteristic for the phase overcurrent elements - 50/51 RMS
Timers-F50-51.ai
I> Start
I> Trip
t
>
t
>
RESET
INPUT > threshold
t
I
>
RES
t
I
>
RES
t
I
>
RES
t
I> element RMS phase overcurrent timers - 50/51
