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DMC3S - Manual - 08 - 2021
—
Residual overcurrent - 50N/51N
Preface
This function has three adjustable thresholds with programmable trip time.
The trip characteristic for the first threshold can be selected with definite or inverse time per IEC
60255-3/BS142.
The trip characteristic for the second and third thresholds 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 can be inhibited when at least one of the two thresholds starts, just as the second
threshold can be inhibited when the third threshold starts.
Operation and settings
The residual overcurrent protection compares the fundamental component of the residual current
(
I
E
) with the set thresholds (
I
E
>,
I
E
>>,
I
E
>>>); violating a threshold starts (START) the threshold itself
and the corresponding timer (
t
E
>,
t
E
>>,
t
E
>>>). If the threshold violation persists, when its trip time
expires, the threshold trips (TRIP), otherwise the threshold is reset.
The first threshold
I
E
> can be set to definite or inverse time with the following characteristic curves:
• IEC 255-3/BS142 type A or SIT, inverse time:
t
= 0.14 ·
t
E
>
inv
/ [(
I
E
/
I
E
>
inv
)
0.02
- 1]
• IEC 255-3/BS142 type B or VIT, very inverse time:
t
= 13.5 ·
t
E
>
inv
/ [(
I
E
/
I
E
>
inv
) - 1]
• IEC 255-3/BS142 type LTI, long time inverse:
t
= 120 ·
t
E
>
inv
/ [(
I
E
/
I
E
>
inv
) - 1]
• IEC 255-3/BS142 type C or EIT, extremely inverse time:
t
= 80 ·
t
E
>
inv
/ [(
I
E
/
I
E
>
inv
)
2
- 1]
Where:
t
:
operating time
I
E
>:
trip threshold
t
E
>
inv
:
operating time regulation
The following applies to all the above inverse time characteristics:
• The minimum trip current is 1.1 times the set threshold (asymptotic reference time).
• The minimum operating time
t
is 0.1 s.
• The characteristics are defined between 1.1 and 20 times the set threshold;
[1]
if threshodl regula-
tion exceeds 2.5
I
n
, the upper limit of the measurement range is 10
I
En
.
For definite time trip characteristics, the upper limit of the measurement range is 10
I
En
.
Each threshold of the protection can be enabled or disabled by setting to
ON
or
OFF
the parameter
IE> Enable
,
IE>> Enable
and/or
IE>>> Enable
in the menu
Set \ Protections \ Residual
overcurrent - 50N/51N \
Threshold IE>
(
Threshold IE>>
,
Threshold IE>>>) \ Parameters
.
The first threshold can be set to definite or inverse time with parameter
IE>Curve
(
DEFINITE,
NIT, VIT, EIT, LIT
) in the menu
Set \ Protections \ Overcurrent - 50N/51N \
Threshold IE> \ Pa-
rameters
.
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
IE
IE>>
IE>>>
tE>
tE>>
tE>>>
IE>
t
Trip characteristic of the three thresholds of the residual overcurrent protection - 50N/51N
TRIP
