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Earthing

Earthing

Calculati

Calculati

ons

ons

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Ze

Ze

R

1

R

2

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Z

Z

e

R

1

R

2

Z

s

Total Impedance

Total Impedance

Zs = Ze + (R

Zs = Ze + (R

1

1

+R

+R

2

2

)

)

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Where:

Ze is the external earth fault

loop impedance

()

Typical values based on a
100A service are:

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TN-S system

0.8

0.8





TN-C-S system

0.35

0.35





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R1

R1

The resistance of the final

The resistance of the final

circuit phase conductor

circuit phase conductor

R2

R2

The resistance of the final

The resistance of the final

circuit protective conductor

circuit protective conductor

Unit of measurement, the

Unit of measurement, the





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Indirect shock calculations

Indirect shock calculations

A circuit is wired in pvc/pvc 70C sheath with

copper

conductors having phase and cpc sizes of

2.5/1.0mm

2

respectively.

The length of circuit is 30 metres and

protection

is by a 20A BS 88 cartridge fuse.

If the value of Ze = 0.7 and disconnection

is to occur

within 0.4 seconds check for indirect shock

protection.

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Length of final circuit 30 metres

2.5/1.0mm

2.5/1.0mm

2

2

70

70





C sheath

C sheath

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Cross-sectional area

(mm

2)

Resistance/metre

or (R1+R2)/metre

m/m

)

)

Phase

conduct

or

Protecti

ve
conduct

or

Copper

Aluminiu

m

1
1

1.5
1.5
2.5
2.5
2.5
2.5

4

-

1

-

1.5

-

1

1.5
2.5

-

18.10
36.20
12.10

24.2
7.41

25.51
19.51
14.82

4.61

TABLE 1A

TABLE 1A

(Page 107 Guidance Notes 3)

(Page 107 Guidance Notes 3)

Values of resistance for copper and aluminium

Values of resistance for copper and aluminium

conductors and of R1 + R2 at 20ºC in m

conductors and of R1 + R2 at 20ºC in m





/metre

/metre

25.5

1m

25.5

1m





per

met

re

per

met

re

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Tot

al R

1 +

R2

=

76

5.3

m



at 2

0C

els

ius

30 metres

30 metres

25.51m

25.51m





per

per

metre

metre

Total R1 + R2 = 25.51 m x 30 metres = 765.3 m

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Ambient temperature

Ambient temperature

Heat generated by

Heat generated by

current flow

current flow

Heat produced by

Heat produced by

other circuit conductors

other circuit conductors

Electrical resistance

Electrical resistance

increases with

increases with

temperature rise

temperature rise

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20

20





C

C

70

70





C

C

R1 + R2 @ 20C

Multiplied by 1.2 gives

R1 + R2 @ 70C

Increase of resistance with temperature

Increase of resistance with temperature

Remember! Resistance
increases with
temperature rise

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Conductor

Conductor

installation

installation

Conductor

Conductor

insulation

insulation

70

70





C

C

pvc

pvc

85

85





C

C

rubber

rubber

90

90





C

C

thermosetting

thermosetting

Not incorporated in a

Not incorporated in a

cable and not bunched

cable and not bunched

- notes 1,3

- notes 1,3

1.04

1.04

1.04

1.04

1.04

1.04

Incorporated in a cable

Incorporated in a cable

or bunched - note 2, 3

or bunched - note 2, 3

1.2

1.2

1.26

1.26

1.28

1.28

Table 1C

Table 1C

(Page 109 Guidance Notes 3)

(Page 109 Guidance Notes 3)

Conductor temperature factors

Conductor temperature factors

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Value of R1 + R2 calculated at

Value of R1 + R2 calculated at

20

20





C

C

To allow for temperature rise

To allow for temperature rise

multiply

multiply

by

by

1.2

1.2

Therefore 765.3 m

Therefore 765.3 m





x 1.2 =

x 1.2 =

918.36 m

918.36 m





Convert to ohms (

Convert to ohms (





) = 918.36

) = 918.36

m

m





/1000

/1000

0.918

0.918





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Calculating Zs

Calculating Zs

Corrected value of R1+R2

Corrected value of R1+R2

=

=

0.918

0.918





Zs = Ze + (R1 + R2) = 0.7 + 0.918

Zs = Ze + (R1 + R2) = 0.7 + 0.918

=

=

1.618

1.618





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Comparing Results

Comparing Results

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The calculated value of

The calculated value of

total

total

impedance Zs must not be

impedance Zs must not be

greater than tabulated

greater than tabulated

values in

values in

BS 7671

BS 7671

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Maximum permitted values

Maximum permitted values

of Zs

of Zs

41b1 - maximum values of Z

41b1 - maximum values of Z

s

s

for fuses

for fuses

where disconnection is to be within 0.4

where disconnection is to be within 0.4

seconds

seconds

41b2 - maximum values of Z

41b2 - maximum values of Z

s

s

for circuit

for circuit

breakers

breakers

41d - maximum values of Z

41d - maximum values of Z

s

s

for fuses

for fuses

where disconnection is to be within 5.0

where disconnection is to be within 5.0

seconds

seconds

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Where to find maximum values of Zs

Where to find maximum values of Zs

Tables 41b1 -

Tables 41b1 -

page 41

page 41

Tables 41b2 -

Tables 41b2 -

page 42

page 42

Tables 41d -

Tables 41d -

page 44

page 44

B

S

B

S

7

6

7

1

7

6

7

1

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Table 41 B1

Table 41 B1

(a) General purpose (gG) fuses to BS 88 Parts 2 and 6

(a) General purpose (gG) fuses to BS 88 Parts 2 and 6

Rating 6

Rating 6

10

10

16

16

20

20

25

25

32

32

(amperes

(amperes

)

)

Zs (ohms) 8.89 5.33 2.82 1.85 1.50 1.09

Zs (ohms) 8.89 5.33 2.82 1.85 1.50 1.09

Extract only - Not full table

Extract only - Not full table

Values of Zs for 0.4 seconds disconnection

Values of Zs for 0.4 seconds disconnection

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Calculating Zs

Calculating Zs

Corrected value of R1+R2 =

Corrected value of R1+R2 =

0.918

0.918





Zs = Ze + (R1 + R2) = 0.7 + 0.918 =

Zs = Ze + (R1 + R2) = 0.7 + 0.918 =

1.618

1.618





To allow disconnection to be achieved

To allow disconnection to be achieved

within 0.4 seconds the calculated value

within 0.4 seconds the calculated value

must be within the tabulated value

must be within the tabulated value

Tabulated value

Tabulated value

1.85

1.85





Therefore circuit complies

Therefore circuit complies

Greater Than

Greater Than

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Disconnection

Disconnection

times

times

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Rooms containing a bath tub or shower

Rooms containing a bath tub or shower

0.4 seconds

0.4 seconds

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Fixed equipment within the main

Fixed equipment within the main

equipotential zone

equipotential zone

5.0 seconds

5.0 seconds

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Fixed equipment outside the main equipotential zone

Fixed equipment outside the main equipotential zone

0.4 Seconds

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Socket outlet circuits feeding equipment

located within the main earthed

equipotential zone

0.4 Seconds

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Socket outlet circuits feeding portable

equipment intended to be used outside

of the main earthed equipotential zone

Rcd Protected