Origin of electric risk
Electic risk take origin from:
· Direct contact with normally live parts
· Indirect contact with casually live parts
· Indirect action as a consequence of electric arc.
Working on not live parts
During work on not live installations the electric risk takes origin form:
· Unsuccessful shunt
· A wrong move during operation
· Unsuccessful determination of all the possible feeding points
· Induced tension by parallelism
· Induced tension by atmospheric discharges
Work on live installations
During work on installations the electric risk takes origin from:
· Short circuit between near live parts for casual interposition of tools and metallic materials (electric arc)
· Interruption of consistent loads (electric arc)
· Unsuccessful use or inadequacy of protection means
· Non-compliance with security space
· Coming live of metallic parts due to an insulation breakdown.
To avoid electric risks it is necessary to:
· To switch off the circuit
· To prevent if possible switch on
· To test voltage absence
· Earthing and short circuiting
· To reach equipotentiality conditions
· To use insulated tools and protection means
· To screen and insulate the nearest live parts
Earthing and short circuiting – general warnings
· Before use check the good conditions of all the devices
· Devices must be suitable for supporting the short circuiting current of the system in which they are used
· It is not possible to reuse devices which have one supported the max short circuiting load
· Only short circuiting devices, such as phase clamps and phase wires are suitable for supporting short circuit. Ground wires have only to disperse residual current
· It is forbidden to use bare wires for earthing and short circuiting devices
· Short circuiting wires length must be almost 1,2 times the distance among phases
· In case of short circuiting current over 20 kA x 0,5 s it is necessary to have a screw lock.
How to determine wires sections
To determine the min. wires section for earthing and short circuiting devices it is necessary referring to the most crucial example of short circuiting. In this particular situation, the initial alternated current of short circuit (I) is equal to permanent current of short circuit (Is) and equal to the breakdown alternated current (Ia).
I=Is=Ia
From diagrams in TAB 1, given value and time of short circuiting is it possible to determine section S of the wires.
When finally temperature of 350° and 500° are defined (normally used), wire section must be calculated as follows:
.png)
S= section of the conductor (mm2)
I= Short circuiting current (Ampere)
t= time of short circuiting (s)
T= Ending temperature of the conductor (350° or 500° C)
K= material constant (13 for copper)
Determination of grounding cables
A= section of the conductor (mm2)
I= Max value of the fault current Ig that can interest the grounding cable (Ampere)
t= time of fault elimination (s)
K= material constant that consider the material of the conductors and the initial and ending temperature (181 to 20° C or 176 to 30° C) – CEI 11-17 (See Tab. 1 pag. III
Picture 1 Permissible current charge on copper cables of short circuiting for the use on single-phase and three-phase systems.
Initial temperature of the cable: 20° C
Ending temperature of the cable: 500° C
|
Equivalent section on copper for conductor and/or short circuiting bar
|
Minim equivalent section on copper for grounding cable
|
|
mm2
|
mm2
|
|
16
|
16
|
|
25
|
16
|
|
35
|
16
|
|
50
|
25
|
|
70
|
35
|
|
95
|
35
|
|
≥120
|
50
|
Tab. 1
CEI EN 50124-1 09-2001 STANDARD
Application: Railway, Tramway, Trolley-bus line, Underground railway
Insulating coordination
- I° part: base requirement – Air Distances and Grounding Distances for all the electric and electronic equipments
Test voltage for the check of the Grounding Distance
|
Distance
(mm)
|
Ui
(kV)
|
Uac
(kV)
|
Udc
(kV)
|
|
0,01
|
0,33
|
0,23
|
0,33
|
|
0,04
|
0,52
|
0,37
|
0,52
|
|
0,1
|
0,81
|
0,5
|
0,7
|
|
0,5
|
1,55
|
0,84
|
1,19
|
|
1,5
|
2,56
|
1,39
|
1,97
|
|
2
|
3,1
|
1,69
|
2,39
|
|
2,5
|
3,6
|
1,96
|
2,77
|
|
3
|
4,06
|
2,21
|
3,13
|
|
3,5
|
4,51
|
2,45
|
3,47
|
|
4,5
|
5,33
|
2,9
|
4,1
|
|
5,5
|
6,09
|
3,32
|
4,69
|
|
8
|
7,82
|
4,26
|
6,02
|
|
11
|
9,95
|
5,4
|
7,63
|
|
14
|
12,2
|
6,61
|
9,35
|
|
18
|
15,1
|
8,17
|
11,6
|
|
22
|
17,8
|
9,68
|
13,7
|
|
25
|
19,9
|
10,8
|
15,3
|
|
32
|
24,5
|
13,3
|
18,8
|
|
40
|
29,5
|
16
|
22,7
|
|
60
|
41,6
|
22,6
|
31,9
|
|
90
|
58,5
|
31,7
|
44,9
|
|
120
|
74,6
|
40,5
|
57,2
|
|
160
|
95
|
51,5
|
72,9
|
|
260
|
143
|
77,6
|
110
|
|
310
|
166
|
90
|
127
|
|
370
|
193
|
104
|
148
|
|
480
|
240
|
130
|
184
|
|
600
|
289
|
157
|
222
|
NOTES:
(1)
Ui :is the amplitude of voltage of the impulse test 1,2/50
Uac:is the effective amplitude of the voltage of the test at net frequency
Udc:is the value of the test at d.c. voltage
(2) It is admitted the interpolation between the values near the Tab.