400 V lines are mostly carried on wood poles with four (or
sometimes five) wires in a vertical array. The bottom wire
is an earth wire which screens the fields produced by the wires
above it, making electric fields from 400 V lines very low.
The maximum field shown here is produced when the ground clearance
is the minimum allowed – 5.5 m.
Typical fields are lower than the maximum field because the
clearance is usually higher. This graph is for 8 m clearance.
Sometimes, the separate conductors are insulated and twisted
together, called “aerial bundled conductors” (abc). Then the
fields are even lower.
| 400 V open wire: |
400 V abc: |
 |
 |
This table gives some actual field values for the same conditions.
| |
|
|
|
|
electric field
in V m-1 at distance from centreline |
|
maximum under line |
10 m |
25 m |
50 m |
100 m |
|
400 V |
wood pole |
vertical array
50 mm2 |
maximum |
clearance 5.5 m
single circuit
|
1 |
1 |
0 |
0 |
0 |
|
typical |
clearance 8 m
single circuit
|
0 |
0 |
0 |
0 |
0 |
Note:
1. All fields calculated at 1 m above ground level.
2. All electric fields are calculated for the nominal voltage.
In practice, voltages (and hence fields) may rise by a few percent.
3. All electric fields calculated here are unperturbed
values.
4. All fields are given to the same resolution for simplicity
of presentation (1 V/m) but are not accurate to better than
a few percent.
5. Calculations ignore zero-sequence voltages. 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.
6. Supplies to single houses could be carried on different
arrangements of conductors which could produce different fields.
