33 kV lines are either carried on small lattice steel pylons
or on wood poles. There are many different variants of lines
design but they all produce roughly similar fields.
The maximum field shown here is produced by a lattice pylon design
when the ground clearance is the minimum allowed – 5.5 m – and
the loads are the highest allowed – not usually above 1 kA in
each circuit but there may be exceptions. The field also depends
on the phasing. 33 kV lines usually have Untransposed (U)
phasing.
Typical fields are lower than the maximum field because the clearance
is usually higher and the loads are usually lower. The two curves
shown here are for typical loads, the normal U phasing, and two
different line designs: a lattice steel pylon (the higher field),
and a wood-pole design (the lower field).
| 33 kV double-circuit line: |
33 single-circuit wood pole: |
 |
 |
This table gives some actual field values for the same conditions.
| |
|
|
|
|
magnetic
field in µT at distance from centreline |
|
maximum under line |
10 m |
25 m |
50 m |
100 m |
|
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 |
Note:
1. All fields calculated at 1 m above ground level.
2. All fields are given to the same resolution for simplicity
of presentation (1 nT = 0.001 µT) but are not accurate to better
than a few percent.
3. 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 and less closer
to the line.
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.
5. Sometimes, a 33 kV circuit could be carried on a line
designed for a higher voltage. Then the magnetic fields could
be larger than shown here.
6. Calculated fields agree well with measured fields. more
