Thytronic NA20, Manual, Page: 69 / 208

FUNCTION CHARACTERISTICS
69 NA20 - Manual - 04 - 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 definite or inverse time according the IEC and ANSI/IEEE
standard, as well as with rectifier, I
2
t or EM curve.
The second threshold may be programmable with independent or dependent time according to the
I
2
t curve and the third threshold with independent time.
For each threshold a reset time can be set ( t
>RES
, t
>>RES
, t
>>>RES
) useful to reduce the clearing time
for intermittent faults.
The first threshold trip may be inhibited by start of the second and/or third threshold ( I >>, I >>>).
Similarly the second threshold trip may be inhibited by start of the third threshold ( I >>>).
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]
• Long Time Inverse (IEC 255-3/BS type B LTI): t = 120 · t >
inv
/ [( I / I >
inv
) - 1]
• Moderately Inverse (ANSI/IEEE type MI): t = t >
inv
· {0.01 / [( I / I >
inv
)
0.02
- 1] + 0.023}
• Very Inverse (ANSI/IEEE type VI): t = t >
inv
· {3.922 / [( I / I >
inv
)
2
- 1] + 0.098}
• Extremely Inverse (ANSI/IEEE type EI): t = t >
inv
· {5.64 / [( I / I >
inv
)
2
- 1] + 0.024}
• Rectifier (RI): t = 2351 · t >
inv
/ [( I / I >
inv
)
5.6
- 1]
• I-squared-t ( I
2
t = K): t = 16 · t >
inv
/ ( I / I >
inv
)
2
• Electromechanical (EM): t = t >
inv
· {0.28 / [-0236 · ( I / I >
inv
)
-1
+ 0.339]}
Where:
t : operate time
I >
inv
: threshold setting
t >
inv
: operate time setting
The second threshold ( I >>) may be programmable with definite or inverse time according to the I
2
t
curve: t = 16 · t >
inv
/ ( I / I >
inv
)
2
The third threshold ( I >>>) with definite time.
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
• 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
.
All overcurrent elements can be enabled or disabled by setting
ON
or
OFF
the
I> Enable
,
I>>
Enable
and/or
I>>> Enable
parameters inside the Set \ Profile A(or B) \ Phase overcurrent-50/51
\ I> Element ( I>> Element , I>>> Element) \ Setpoints menus.
The first overcurrent element can be programmed with definite or inverse time characteristic by set-
ting the
I>Curve
parameter (
DEFINITE, IEC/BS A, IEC/BS B, IEC/BS C, IEC/BS B LI, ANSI/
IEE MI, ANSI/IEE VI, ANSI/IEE EI, RECTIFIER, I2t, EM
) available inside the Set \ Profile A(or
B) \ Phase overcurrent-50/51 \ I> Element \ Setpoints menu.
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 >> I >>>
t >
t >>
t >>>
I >
t
TRIP
General operation time characteristic for the phase overcurrent elements - 50/51