MSCDN – MP2A Applications Guide
2.2 Setting Example
Resistor Thermal Characteristics
CURRENT IN AMPS
TIME IN SECS
12 Continuous
16 20
19 9
20 8
21 7
22 6
23 5
25 4
28 3
34 2
46 1
125 Maximum
CT Characteristics
Ratio 20/1
Alarm & Trip Requirements
Alarm level
105 %
Trip level (k)
110 %
Now I
B
= 12/20 = 0.6 amps
And I
θ
= k x I
B
= 1.1 x I
B
= 1.1 x 0.6 = 0.66 Amps
At an applied current of I = 16/20 = 0.8 amps, the resistor maximum withstand time is t = 20 seconds.
Using a safety margin of 50%, then
⎭
⎬
⎫
⎩
⎨
⎧
−
×
=
×
2
2
2
66
.
0
8
.
0
8
.
0
ln
20
5
.
0
τ
s
Thus
sec
76
.
8
sec
2044
.
0
64
.
0
ln
10
=
⎟
⎠
⎞
⎜
⎝
⎛
=
τ
∴
τ
= 9 seconds will be used to satisfy the 50% safety margin.
©2010 Siemens Protection Devices Limited
Chapter 5 Page 5 of 7
Содержание 7SG23 MSCDN - MP2A
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