See also the companion tables for electric
fields, underground
fields and the simpler summary.
| |
|
|
|
|
magnetic
field in µT at distance from centreline |
|
maximum under
line |
10 m |
25 m |
50 m |
100 m |
|
400 kV
and
275 kV |
largest lines |
L6
quad bundles
0.305 m
zebra
|
maximum |
clearance 7.6 m
phasing U
load 4.7/4.7 kA
|
108.422 |
95.780 |
38.422 |
11.697 |
3.096 |
|
typical |
clearance 13 m
phasing T
load 0.4/0.6 kA
|
5.783 |
5.247 |
2.194 |
0.578 |
0.119 |
|
smaller lines |
L2
twin bundles
0.305 m
zebra
|
maximum |
clearance 7.6 m
phasing U
load 2.4/2.4 kA
|
54.142 |
46.300 |
16.283 |
4.865 |
1.278 |
|
typical |
clearance 13 m
phasing T
load 0.4/0.6 kA
|
4.971 |
4.158 |
1.557 |
0.400 |
0.084 |
|
typical design used
for new lines |
L12
twin bundles
0.5 m
araucaria
|
maximum |
clearance 7.6 m
phasing U
load 3.5/3.5 kA
|
81.942 |
72.818 |
22.103 |
8.148 |
2.145 |
|
typical |
clearance 13 m
phasing T
load 0.4/0.6 kA
|
5.604 |
4.938 |
1.979 |
0.514 |
0.106 |
|
132 kV
and
66 kV |
largest lines |
L7
twin bundles
0.305 m
lynx
|
maximum |
clearance 7 m
phasing U
load 1.4/1.4 kA
|
30.445 |
20.532 |
5.553 |
1.528 |
0.392 |
|
typical |
clearance 10 m
phasing U
load 0.13/0.13
|
1.848 |
1.359 |
0.468 |
0.138 |
0.036 |
|
smaller lines |
L132
single conductors
0.4 sq in
|
maximum |
clearance 7 m
phasing U
load 1.2/1.2 kA
|
24.585 |
17.217 |
4.587 |
1.247 |
0.318 |
|
typical |
clearance 10 m
phasing U
load 0.13/0.13 kA
|
1.731 |
1.317 |
0.451 |
0.132 |
0.034 |
|
smallest wood-pole
design |
trident
150 m span
single conductors
lynx
|
maximum |
clearance 7 m
single circuit
load 0.7 kA
|
12.347 |
12.347 |
0.738 |
0.192 |
0.048 |
|
typical |
clearance 10 m
single circuit
load 0.1 kA
|
1.764 |
0.385 |
0.099 |
0.027 |
0.007 |
|
33 kV |
larger lines on
steel pylons |
D
single conductors
r=9.8 mm
|
maximum |
clearance 5.5 m
phasing U
load 1/1 kA
|
25.686 |
10.742 |
2.274 |
0.594 |
0.150 |
|
typical |
clearance 8 m
phasing U
load 0.1 kA
|
1.556 |
0.822 |
0.214 |
0.058 |
0.015 |
|
smaller lines on
wood poles |
wood pole
horizontal array
+-2 m
|
maximum |
clearance 5.5 m
single circuit
load .5 kA
|
14.748 |
2.961 |
0.541 |
0.138 |
0.035 |
|
typical |
clearance 8 m
single circuit
load 0.1 kA
|
1.325 |
0.471 |
0.103 |
0.027 |
0.007 |
|
11 kV |
larger lines on
steel pylons |
SL11
single conductors
|
maximum |
clearance 5.5 m
phasing U
load 0.5/0.5 kA
|
9.456 |
7.664 |
1.490 |
0.364 |
0.090 |
|
typical |
clearance 8 m
phasing U
load 0.1 kA
|
1.004 |
0.942 |
0.274 |
0.071 |
0.018 |
|
smaller lines on
wood poles |
wood pole
horizontal array
+-1.15 m
|
maximum |
clearance 5.5 m
single circuit
load 0.2 kA
|
3.744 |
0.668 |
0.124 |
0.032 |
0.008 |
|
typical |
clearance 8 m
single circuit
load 0.05 kA
|
0.399 |
0.134 |
0.030 |
0.008 |
0.002 |
|
400 V |
wood pole |
vertical array
50 mm2
|
maximum |
clearance 5.5 m
single circuit
load 0.2 kA
net 0.01 kA@90
|
1.227 |
0.277 |
0.088 |
0.041 |
0.020 |
|
typical |
clearance 8 m
single circuit
load 0.05 kA
net 0.005 kA@90
|
0.215 |
0.099 |
0.041 |
0.020 |
0.010 |
Notes to table:
1.
Calculations performed on computer programme EM2D by
John Swanson January 2005.
2.
Typical phases are T=transposed for 275 kV and 400 kV,
U=untransposed for 132 kV and below (phases apply to double-circuit
lines only)
3.
For magnetic fields, the phasing which gives the highest
field is usually U, but with the exception that close to the
centreline at low clearances T can give higher field. The table
uses whichever gives the higher field.
4.
The “maximum field under the line” is the largest field,
which is not necessarily on the route centreline; it is often
under one of the conductor bundles for a two-circuit line and
can even be outside the conductors eg for a 400 V vertical array.
5.
All fields calculated at 1 m above ground level.
6.
All fields are given to the same resolution for simplicity
of presentation (1 nT ) but are not accurate to better than
a few percent.
7.
Calculations ignore zero-sequence current. This means
values at larger distances are probably underestimates, but
this is unlikely to amount to more than a few percent except
for 400 V lines. For lines with transposed phasing, typical
loads are entered with an unbalance between the two circuits
to approximate realistic conditions.
8.
Generally speaking, the lower the voltage, the more
different variants of design there are. The “maximum” calculation
is presented here in good faith but may not take account of
every last variant in operation. To meet specific local circumstances,
lines at 132 kV and below may occasionally have been constructed
with higher ratings. Within the legal framework of the Regulations,
different DNOs may have different policies regarding minimum
clearances.
9.
All calculations are for three-phase lines, single-phase
lines at low voltages have not been investigated.
Notes on specific voltages:
1.
For 400 kV, L12 lines are included as these are likely
to be used for new construction. Phasing U gives the maximum
field under the line and is included for that reason but phasing
T would be used in practice so is also included.
2.
275 kV lines use identical constructions to 400 kV therefore
for magnetic fields the fields are basically the same. (This
assumes 275 kV lines are built to the same minimum clearance,
7.6 m, as 400 kV lines, rather than the permitted 7.3 m.)
3.
132 kV lines have, in the past, been constructed on
275/400 kV towers with a view to future uprating. This is unlikely
to happen again so is not included here. Similarly, lines constructed
to any voltage may be operated at a lower voltage.
4.
400 V lines: calculations are for open-phase designs.
ABC (aerial bundled conductors) produce lower fields but are
not susceptible to easy calculations.
Version 3 12 January 2005
