400 V/230 V

thumbnail of 400 V separate phases wood pole   thumbnail of 400 V aerial bundled conductor

400 V and 230 V refer to the same lines - 400 V is the designation when it is three phase, 230 V when it is single phase (more on single phase and three phase).  These voltages used to be 415 V and 240 V but were reduced to harmonise with Europe.

400 V lines are mostly carried on wood poles with four (or sometimes five) wires in a vertical array, but sometimes the conductors are twisted together in a single "aerial bundled conductor" (abc).

Magnetic field

At 400 V, the magnetic field depends as much on the net current as on the load current.

The maximum field shown here is produced when the ground clearance is the minimum allowed – 5.5 m. The maximum loads assumed are 200 A load current and 10 A net current but higher values might exceptionally be found.

 graph of maximum field 400 V

Typical fields are lower than the maximum field because the clearance is usually higher and the loads are usually lower. This graph is for 8 m clearance, load current 50 A and net current 5 A.

graph of typical field 400 V 

Sometimes, the separate conductors are insulated and twisted together, called “aerial bundled conductors” (abc). Then the fields are even lower.  The load currents make no contribution to the magnetic field, which now comes solely from the net current:

graph of typical field 400 V abc 

Please note the fields depend on the current that is assumed.  That is, of course, true for all power lines, but is particularly important for 400 V lines, where the currents can vary greatly.

This table gives some actual field values for the same conditions.

 magnetic field in µT at distance from centreline
maximum under line10 m25 m50 m100 m
400 Vwood polevertical array 50 mm2maximumclearance 5.5 m
single circuit
load 0.2 kA
net 0.01 kA@90°
1.2270.2770.0880.0410.020
typicalclearance 8 m
single circuit
load 0.05 kA
net 0.005 kA@90°
0.2150.0990.0410.0200.010
abctypicalclearance 8 m
single circuit
net 0.005 kA
0.1430.0820.0390.0200.010

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. Supplies to single houses could be carried on different arrangements of conductors which could produce different fields.

Electric field

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.

graph of maximum field 400 V 

Typical fields are lower than the maximum field because the clearance is usually higher.  This graph is for 8 m clearance.

graph of typical field 400 V 

Sometimes, the separate conductors are insulated and twisted together, called “aerial bundled conductors” (abc).  Then the fields are even lower.

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.

Underground cables

400 V underground cables are usually a single cable: the three cores are twisted round each other in a single outer sheath. Because the cores are so close together and twisted, the fields they produce directly are very small.  Instead, the field comes from any net current in the sheath.  This is very variable and cannot be predicted accurately.  The average is about 4 A.  more on net currents.

The following graph shows typical fields from the net current in a 400 V cable.

graph of typical field 400 V underground cable 

Underground cables do not produce any external electric fields.

This table gives some actual field values for the same conditions.

    

magnetic field in µT at distance from centreline

0 m

5 m

10 m

20 m

400 V

single cable

0.5 m depth

typical

0.50

0.14

0.07

0.04

Notes

1. All fields calculated at 1 m above ground level

2. All fields are given to the same resolution for simplicity of presentation (0.01 µT = 10 nT) but are not accurate to better than a few percent.

3. These calculations are for a single, isolated cable.  This is rare in practice.  Other nearby cables would modify the field.