Electrical safety and earthing aspects in the new ...

Budapest University of Technology and Economics

Department of Electric Power Systems

Electrical safety and earthing aspects in the new standards for the a.c. electric power and

electrified traction lines

Dr. Varjú György, professor emeritus

Innorail 2015, Lurdy Conference Center

Budapest, October 12-14. 2015.

Lurdy Conference Center

Content of the presentation:

• Content

Innorail 2015. October 12-14

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1. Relevant standards and specifications.

2. Sources of the potential causing touch voltages.

3. Permissible body current and body voltage.

4. Permissible touch voltage

5. Prospective permissible touch voltage.

6. Conclusions


Recent standards concerned:

1. IEC 60938 ed.2.1:2014, Power installations exceeding 1 kV a.c. – Part 1: Common rules; (Page #: 113)

2. EN 50341-1:2012, Overhead electrical lines exceeding AC 1 kV - Part 1: General requirements - Common specifications; (Page #: 253)

3. EN 50522:2010, Earthing of power installations exceeding 1 kV a.c.; (Page #: 67)

4. EN 50122-1:2011, Railway applications - Fixed installations - Electrical safety, earthing and the return circuit - Part 1: Protective provisions against electric shock; (Page #: 81)

• Relevant specifications

Innorail 2015. Budapest Lurdy Conference Center

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The relevancy of earthing standard to railway installation



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For the purpose of interpreting this standard, an electrical power installation is considered to be one of the following:

a) substation, including substation for railway power supply;

b) electrical installations ……;

The electrical power installation includes, among others, the following equipment:

– rotating electrical machines;

– switchgear;

– transformers and reactors;

– converters;

Scope of EN 50522 (earthing standard)


Further relevant documents:

ITU-T Recommendation K.68: Operator responsibilities in the management of electromagnetic interference by power systems on telecommunication systems, Geneva 2008 NOTE: Power systems involve the electrified traction systems as well.

IEC/TS 60479-1:2005, Effects of current on human beings and livestock – Part 1: General aspects

HD 637 S1:1999, Power installation exceeding 1 kV a.c.

HD 60364-4-41:2007, Low-voltage electrical installation – Part 4-41. Protection for safety – Protection against electric shock (IEC 60364 – 4-41:2005, mod.)



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Voltage effects and consequences:



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Voltage effects Consequences

Danger to people Touch voltage, electric shock

Damage to affected plant Degradation of operational reliability

Electromagnetic Compatibility (EMC)

Degradation of the quality of service

The presentation is focused on the touch voltage only.


Sources of the potential causing touch voltages

• Sources of the potential causing touch voltages


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Voltages due to current carrying earth electrodes



8


Explanations to the voltages shown in the previous figure


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10

Rail potential as a main source of touch voltages in electric traction lines

under normal operation and fault conditions


Legend used in the rail potential figures



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Number of tracks: S1 =1 track

S2 = 2 traks

Messenger wire: A = steel

B = bronz

Reinforcing feeder: 150 = 150 mm2

240 = 240 mm2

Return conductor: N = not applied

V = applied

In the case of double track line: v1 = occupied by train

v2 = clear from train


Maximum value of rail potential vs. train location, train current: 100 A Single track line, 1x25 kV feeding



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Return conductor: N = not applied (blue) V = applied (red)

g= rail to earth leakage


Maximum value of rail potential vs. train location, train current: 100 A Double track line, 1x25 kV feeding



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Return conductor: N = not applied


Maximum value of rail potential vs. train location, train current: 100 A Single track line, 2x25 kV (auto transformer) feeding



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Permissible body current and body voltage

2015.06.17. 15

• Permissible body current and body voltage

Ecordingly to IEC/TS 60479-1


Key terms for touch voltages



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Terms Symbol Stand. # Definitions

body voltage, Ub 50122 product of the current trough the body and the body impedance

permissible touch voltage,

UTp 50522 the voltage across the human body

effective touch voltage

Ute 50122 voltage between conductive parts when touched simultaneously by a person or an animal UT 50522

prospective touch voltage

Utp (=US) 50122 voltage between simultaneously accessible conductive parts when those conductive parts are not being touched UvT 50522

source voltage

US (=Utp) 50122 source voltage

UvTp (=UvT) 50522

voltage difference acting as a source voltage in the touching circuit with a limited value that guarantees the safety of a person when using additional known resistances (for example footwear, standing surface insulating material)


Permissible touch (body) voltage accordingly to IEC/TS 60479-1



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Expression for the permissible touch voltage:

Duration s

Body current mA

0,05

0,1

0,2

0,5

1

2

5

10

900

750

600

200

80

60

51

50

Body factor

BF

1.0 hand to hand or hand to feet

0.75 hand to feet 0.50 both hand to feet

Heart current

factor

HF

1.0 left hand to feet 0.8 right hand to feet; 0.4 hand to hand

0,04 foot to foot


Time/current zones of effects of a.c. currents

(current path: left hand to feet, 15 Hz to 100 Hz)



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Left to the c2 curve the probability of ventricular fibrillation is 5 %


Description of time/current zones:



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Current boundaries Physiological effects

Perception Up to 0,5 mA: curve a

No ‘startled’ reaction

Reaction 0,5 mA up to curve b

Involuntary muscular contractions

Let-go Curve b and above, about 10

mA

Strong involuntary muscular contractions

Ventricular fibrillation C1

C2

C3

Cardiac arrest, breathing arrest may occur

Probability of ventricular fibrillation about 5 %

Probability of ventricular fibrillation about 50 %

The safety limits are based on the ventricular fibrillation (c2 curve)




20

Total body impedances ZT for path hand to hand For large surface areas of contact in dry conditions, a.c. 50/60 Hz


• Permissible touch voltage


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Permissible touch voltage (Body voltage)


Permissible touch (body) voltage accordingly to earthing standard (EN 50522)



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Expression:

Conditions: Probability factor of ZT: 50 %

Curve IB=f(tt): c2

Heart current factor: HF=1

Body factor: BF=1

NOTE: For duration of current flow considerably longer than 10 s a value of 80 V may be used as permissible touch voltage UTp


Touch voltage and EPR relation:



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Permissible EPR based on the EPR ≤ 2UTp limit



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Fault duration tf

[s]

Permissible touch voltage UTp

[V]

Pemissible EPR EPR ≤ 2UTp limit

[V]

0.05 716 1432

0.10 654 1308

0.20 537 1074

0.50 220 440

1.00 117 234

2.00 96 192

5.00 86 172

10.002)

85 170

10.000<t 80 160

Assumption: In case of F=2, the voltage occurring on the body impedance does not exceed the permissible voltage UvT


Permissible touch (body) voltage accordingly to railway standard (EN 50122)



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Expression:

Conditions:


Body voltage

Considered in EN

50522 50122

Body impedance %

Curve IB=f(tt): c1

Total body impedance: Zb(75) 75 % of the total body impedance of Zb(100)




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Time duration Permissible body voltage

Ub,max

Vrms Condition s

Short

term

0,02 370

0,05 360

0,1 345

0,2 295

0,3 230

0,4 150

0,5 120

0,6 100

<0,7

Lon

g term

0,7 90

0,8 85

0,9 80

1 75

300 65

>300 60

Maximum permissible body voltages Ub, max in a.c. traction systems



• Prospective permissible touch voltage


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Prospective permissible touch voltage

Earthing EN 50522: UvTp Railway EN 50122: Utp = US


Consideration of the additional resistances



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The formulae to determine prospective permissible touch voltage becomes:

1. As the sum of the voltage drops:

2. As the sum of the body voltage and voltage drops on the additional resistances:

Note: Terms are given in the next slide


Circuit scheme for the consideration of the additional resistances in the earthing standard (EN 50522)



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Source voltage (Prospective permissible touch voltage)

Permissible (body) touch voltage

Additional resistance

RF1 For example resistance of the footwear RF2 Resistance to earth of the standing point


Examples for the prospective permissible touch voltage UvTp for different additional resistances, RF

(Earthing standard EN 50522)



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Curve number

Additional resistance (RF) options

Remarks Footwear RF1

Soil resistance ρ Ω.m

Standing point RF2 Ω

Total

RF Ω

(1) − < 500 − 0 Identical with the UTp curve

(2) 500 750−1100 750−1100 Approximately identical with 2UTp

(3) 1000 500 750 1750

Increases with the value of RF (4) 1000 1000 1500 2500

(5) 1000 2000 3000 4000

NOTES: 1. RF1 = 1 000 Ω represents an average value for old and wet shoes.

Higher values of footwear resistance may be used where appropriate. 2. RF2 = 1.5×ρ in Ω, where ρ is the specific resistivity of the soil


Examples for the prospective permissible touch voltage UvTp = f (tf)

for different additional resistances, RF (Earthing standard EN 50522)



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Circuit scheme for the consideration of the additional resistances in the railway standard (EN 50122)



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Earth

Standing surface

Additional resistance for shoes

Total body impedance Body voltage

Additional resistance for standing surface

Effective touch voltage

Source voltage = Prospective touch voltage

Body current


Permissible prospective touch voltage for a.c. railways for different additional resistances



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Time duration

s

Body voltage

Ub,max

Effective 1)

Touch volt..

Ute,max

50122 D2 táb. Ra = 1500 Ω

Utp,max

HD 637 S1:1999

a=2

2× Ute,max

HD 637 S1:1999

a=3,3

3,3× Ute,max

0,02 370 865 940 1880 3100

0,05 360 835 905 1810 2990

0,1 345 785 850 1700 2800

0,2 295 645 695 1390 2290

0,3 230 480 520 1040 1720

0,4 150 295 320 640 1060

0,5 120 220 235 470 776

0,6 100 180 190 380 630

<0,7 155 165 330 545

0,7 90 90

0,8 85 85

0,9 80 80

1 75 75

300 65 65

>300 60 60

NOTE: 1) An additional resistance of 1 000 Ω for old wet shoes is included in the values


Permissible voltages for safety in telecom circuits specified by ITU-T in Recommendation K.68



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Curve IB = f(tf) : c2

Current path: hand to feet or hand to hand;

Body impedance ZT: 50 % probability (750 Ω);

Source impedance represented by the telecom circuit: 180 Ω;

Footwear resistance: 3000 Ω;

Additional resistance of the standing point: 0 Ω;

Conditions in the application of IEC/TS 60479-1:


Permissible induced voltages in telecom circuits specified by ITU-T for people safety in Recommendation K.68



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NOTE: These values are approximately the ones shown curve 3 in the earthing standard.


Comparison of the permissible voltages given in different specifications



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Fault duration

t [s]

Permissible touch voltage Stress voltage

Földelési

EN 50522

2UTp

ITU-T

K.68

Vasúti

EN 50122

Ute,max

ITU-T

K.68

[Veff]

Földelési

EN 50522

[Veff]

t 0,02 865

1200

t 0,05 1432 835

t 0,10 1308 2000 785

t 0,20 1074 1500 645 1030

t 0,30

440 1000

480

t 0,35 295 780

t 0,50 650 220 650

t 1,0 234 430

75

430

t 2,0 192 150

300

t 3,0 172

250

t 5,0

60

200

250 t 10,0 170 (t=300) 65 150

t > 10,0 160 (t>300) 60 60


Conclusions:

• Conclusions


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1. The permissible touch voltage is classified into two categories: Permissible touch voltage, UTp Prospective permissible touch voltage; UvTp; source UvTp

2. The value of the permissible touch voltage UTp = f(tf) is highly affected by the assumptions made when applying the technical specifications of IEC/TS 60479-1

3. Prospective permissible touch voltage UvTp, i.e. voltage difference acting as a source voltage in the touching circuit with a limited value that guarantees the safety of a person when using additional known resistances (for example footwear, standing surface insulating material). Its magnitude is highly affected by the value of the additional resistance relevant to actual environmental conditions.


2015.06.17. 38

• Thanks

?

Thanks for your attention!

Dr. Varjú György,

[email protected]

mailto:[email protected]



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