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Overhead Lines

Tables of calculated electric fields produced by overhead lines in operation in the UK

See also the companion table for magnetic fields, underground fields and the simpler summary

					electric field in V m^-1 at distance from centreline
					maximum under line	10 m	25 m	50 m	100 m
400 kV	largest lines	L6 quad bundles 0.305 m zebra	maximum	clearance 7.6 m phasing U	11400	10130	774	446	156
	largest lines	L6 quad bundles 0.305 m zebra	typical	clearance 13 m phasing T	4244	4206	962	106	32
	smaller lines	L2 twin bundles 0.305 m zebra	maximum	clearance 7.6 m phasing U	9897	6618	283	357	118
	smaller lines	L2 twin bundles 0.305 m zebra	typical	clearance 13 m phasing T	3128	2964	469	56	27
	typical design used for new lines	L12 twin bundles 0.5 m araucaria	maximum	clearance 7.6 m phasing U	10642	8410	669	404	136
			maximum	clearance 7.6 m phasing T	9384	7933	737	151	30
			typical	clearance 13 m phasing T	3704	3595	732	98	26
275 kV	largest lines	L6 quad bundles 0.305 m zebra	maximum	clearance 7.6 m phasing U	7838	6964	532	307	107
	largest lines	L6 quad bundles 0.305 m zebra	typical	clearance 13 m phasing T	2918	2892	661	73	22
	smaller lines	L2 twin bundles 0.305 m zebra	maximum	clearance 7.6 m phasing U	6804	4550	195	245	81
	smaller lines	L2 twin bundles 0.305 m zebra	typical	clearance 13 m phasing T	2151	2038	322	39	19
	typical design used for new lines	L12 twin bundles 0.5 m araucaria	maximum	clearance 7.6 m phasing U	7316	5782	460	278	94
	typical design used for new lines	L12 twin bundles 0.5 m araucaria	typical	clearance 13 m phasing T	2547	2472	503	67	18
132 kV	largest lines	L7 twin bundles 0.305 m lynx	maximum	clearance 7 m phasing U	3615	913	182	81	23
	largest lines	L7 twin bundles 0.305 m lynx	typical	clearance 10 m phasing U	2372	890	103	72	23
	smaller lines	L132 single conductors 0.4 sq in	maximum	clearance 7 m phasing U	2628	697	154	66	19
	smaller lines	L132 single conductors 0.4 sq in	typical	clearance 10 m phasing U	1780	689	86	59	18
	smallest wood-pole design	trident 150 m span single conductors lynx	maximum	clearance 7 m single circuit	1174	588	73	11	2
	smallest wood-pole design	trident 150 m span single conductors lynx	typical	clearance 10 m single circuit	583	458	89	15	3
66 kV	largest lines	L7 twin bundles 0.305 m lynx	maximum	clearance 7 m phasing U	1808	456	91	40	12
66 kV	smaller lines	L132 single conductors 0.4 sq in	typical	clearance 10 m phasing U	890	345	43	30	9
33 kV	larger lines on steel pylons	D single conductors r=9.8 mm	maximum	clearance 5.5 m phasing U	897	46	30	10	3
	larger lines on steel pylons	D single conductors r=9.8 mm	typical	clearance 8 m phasing U	543	81	21	9	3
	smaller lines on wood poles	wood pole horizontal array +-2 m	maximum	clearance 5.5 m single circuit	505	165	16	2	0
	smaller lines on wood poles	wood pole horizontal array +-2 m	typical	clearance 8 m single circuit	241	147	21	3	0
11 kV	larger lines on steel pylons	SL11 single conductors	maximum	clearance 5.5 m phasing U	215	109	13	5	1
	larger lines on steel pylons	SL11 single conductors	typical	clearance 8 m phasing U	122	82	8	4	1
	smaller lines on wood poles	wood pole horizontal array +-1.15 m	maximum	clearance 5.5 m single circuit	108	32	3	0	0
	smaller lines on wood poles	wood pole horizontal array +-1.15 m	typical	clearance 8 m single circuit	50	29	4	1	0
400 V	wood pole	vertical array 50 mm²	maximum	clearance 5.5 m single circuit	1	1	0	0	0
400 V	wood pole	vertical array 50 mm²	typical	clearance 8 m single circuit	0	0	0	0	0

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 electric fields, U always gives the highest maximum field and the highest field at large distances, so is used for that reason, but at intermediate distances from the line, electric fields are complex, and either T or U can be higher.

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 and for nominal voltage

6. 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.

7. Calculations ignore zero-sequence voltage. 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.

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. In efforts to reduce aerodynamic problems, a small number of 400 kV lines with quad bundles have had expanded bundles fitted, eg 500 mm horizontally. This produces slightly higher electric fields but is not included in these tables.

2. 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.

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

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