FUNCTION CHARACTERISTICS
29
NA016 - Manual - 05 - 2022
Phase overcurrent - 50/51
Preface
Three operation thresholds, independently adjustable (
I
>,
I
>>,
I
>>>) with adjustable delay
(
t
>,
t
>>,
t
>>>).
The first one may be programmed with inverse time according the IEC 60255-3/BS142 standard.
The second and third thresholds have a definite time characteristic.
For the definite time thresholds a reset time can be set (
t
>>RES
,
t
>>>RES
) useful to reduce the clearing
time for intermittent faults.
Operation and settings
Each phase fundamental frequency current 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 re-
stored.
The first threshold (
I
>) may be programmed with definite 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
/
I
>
inv
)
0.02
- 1]
• Very Inverse Time (IEC 255-3/BS142 type B or VIT):
t
= 13.5 ·
t
>
inv
/ [(
I
/
I
>
inv
) - 1]
• Extremely Inverse Time (IEC 255-3/BS142 type C or EIT):
t
= 80 ·
t
>
inv
/ [(
I
/
I
>
inv
)
2
- 1]
Where:
t
:
operate time
I
>
inv
:
threshold setting
t
>
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
/
I
>
inv
≤ 20
For all definite time elements the upper limit for measuring is 40
I
n
for traditional CT input versions or
12.5 kA (primary current) for LPCT input versions (e.g. 25
I
n
with
I
n
= 500 A).
All overcurrent elements can be enabled or disabled by setting the relative start and/or trip output to
a selectable relay inside the
Set \ Relays
menu.
The first overcurrent element can be programmed with inverse time characteristic by setting the
I>Curve
parameter (
DEFINITE, IEC/BS A, IEC/BS B, IEC/BS C
) available inside the
Set \ 50/51
menu.
An adjustable reset time delay is provided for second and third threshold (
t
>>
RES
,
t
>>>
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
I
>>
def
I
>>>
def
I
>
inv
t
>>>
def
t
>>
def
I
>
inv
t
General operation time characteristic for the phase overcurrent elements - 50/51
TRIP
Timers-F50-51.ai
I>> Start
I>> Trip
t
>>
def
t
>>
def
INPUT
t
>>
RES
t
>>
RES
t
>>
RES
t
I>> element - 50/51 - phase overcurrent timers
