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44
DMC3S - Manual - 08 - 2021
— Overcurrent - 50/51
Preface
This function has four adjustable thresholds with programmable trip time.
The trip characteristics for thresholds I >, I >> and I >>>> can be selected with definite or inverse time
per IEC 60255-3/BS142
The trip characteristic for threshold I >>> is definite time.
A constant reset time can be set for each threshold, to reduce the time for eliminating intermittent
faults (while being no less than the protection start time).
Each protection threshold can be enabled or disabled.
The first threshold I > can be inhibited when at least one of the three thresholds starts, just as the
second threshold can be inhibited when the third threshold starts and the third when the fourth
starts.
Operation and settings
The overcurrent protection compares the fundamental component of each of the three phase cur-
rents ( I
L1
, I
L2
, I
L3
) with the set thresholds; if current I
L2
is not available (input C2 dedicated to the
measurement of I
SQL
or I
E
) the value of I
L2
is forced to zero.
Violating a threshold of at least one of the three phase currents starts (START) the threshold in ques-
tion along with the related timer. If the threshold violation persists, when its trip time expires, the
threshold trips (TRIP), otherwise the threshold is reset.
The trip characteristic for the thresholds I >, I >> e I >>>> can be selected with definite or inverse time
per the following characteristic curves (symbology for the first threshold):
• IEC 255-3/BS142 type A or SIT, inverse time: t = 0.14 · t >
inv
/ [( I / I >
inv
)
0.02
- 1]
• IEC 255-3/BS142 type B or VIT, very inverse time: t = 13.5 · t >
inv
/ [( I / I >
inv
) - 1]
• IEC 255-3/BS142 type LTI, long time inverse: t = 120 · t
2
>
inv
/ [( I / I >
inv
) - 1]
• IEC 255-3/BS142 type C or EIT, extremely inverse time: t = 80 · t >
inv
/ [( I / I >
inv
)
2
- 1]
Where:
t : operating time
I >: trip threshold
t >
inv
: operating time regulation
The trip characteristic for threshold I >>> is definite time.
The following applies to all inverse time characteristics:
• The minimum trip current is 1.1 times the set threshold (asymptotic reference time).
• The characteristics are defined between 1.1 and 20 times the set threshold;
[1]
if threshold regula-
tion exceeds 2.5 I
n
, the upper limit of the measurement range is 40 I
n
.
• The minimum operating time t is 0.1 s.
• For definite time trip characteristics, the upper limit of the measurement range is 40 I
n
.
Each threshold of the protection can be enabled or disabled by setting to
ON
or
OFF
the parameter
I> Enable
,
I>> Enable
and/or
I>>> Enable
in the menu Set \ Protections \ Overcurrent -
50/51 \ Threshold I> ( Threshold I>> , Threshold I>>> , Threshold I>>>>) \ Parameters .
Note 1 For input values greater than 20 times the threshold, the operating time is limited to the value corresponding to 20 times the threshold
I
I>> I>>> I>>>>
t>
t>>
t>>>
t>>>>
I>
t
Trip characteristic of the thresholds of the overcurrent protection - 50/51
TRIP