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Electrical clearances

An electrical clearance is a least distance in air that the switchyard keeps around its live parts. The IEC + CEA / CBIP set takes four clearances per voltage level, checks the yard's geometry against them, and increases them for a site above 1000 m.

1The clearances of a level

The design holds the clearances of each level in its level table, for sites up to 1000 m. The yard's layout takes its phase spacing, equipment level and bus levels from the same table. The design then checks the distances of the yard against the clearances:

phase-to-earth clearance: the distance from the outer phase to the edge of its bay
phase-to-phase clearance: the phase spacing, the height of the main bus above the equipment, and the height of a strung jumper above a main bus at one-and-a-half breaker
sectional clearance: the gap between the nearest phases of the two main buses, for maintenance with one bus live (not on a single bus or a GIS yard)
ground clearance: the equipment level, the height of the live parts above the ground

An outdoor collector bus takes the same checks for its phase spacing and its equipment level. The clearances of the design's levels are in the table.

Level	Phase to earth	Phase to phase	Sectional	Ground	Phase spacing of the yard
33 kV	0.32 m	0.32 m	2.8 m	3.7 m	1.5 m
66 kV	0.63 m	0.63 m	3.0 m	4.0 m	2.2 m
132 kV	1.3 m	1.3 m	4.0 m	4.6 m	3.0 m
220 kV	2.1 m	2.1 m	5.0 m	5.5 m	4.5 m
400 kV	3.5 m	4.0 m	6.5 m	8.0 m	7.0 m
765 kV	4.9 m	7.6 m	10.3 m	12.4 m	15.0 m
33 kV outdoor collector bus	0.32 m	0.32 m	2.8 m	3.7 m	1.5 m
11 kV outdoor collector bus	0.178 m	0.229 m	2.6 m	2.75 m	1.0 m

2Regulations and standards

The CEA Safety Regulations 2023 require safety clearances as per the relevant standards (Reg. 46(2)(i)). Their text gives no table of phase-to-earth or phase-to-phase clearances for AC, so the values are the design's. The checklist cites the safety working clearance of Schedule V for the sectional and ground clearances (Reg. 46(2)(iii)).

IEC 61936-1:2021 gives the minimum clearances of live parts by the insulation level: range I in §5.4.2 (Table 2) and range II in §5.4.3 (Table 3). The checklist reports the phase-to-earth check under it. The ranges are those of the insulation levels (Insulation levels, The ratings of a level).

3Correction for altitude

The clearances of the level table hold for sites up to 1000 m. Above it, the design multiplies the electrical distances by the atmospheric correction factor $K_{a}$ of IEC 60071-2.

K_{a} = e^{m H /8150}

(1)

IEC 60071-2 §4.2.2

where
$K_{a}$	atmospheric correction factor for altitude
$m$	exponent of the correction, which the design takes as 1
$H$	height of the site above the 1000 m the clearances hold for, 0 below it	m

The phase-to-earth and phase-to-phase clearances are multiplied by $K_{a}$ . The sectional and ground clearances keep their working-zone part and grow only by the added phase-to-earth distance $d_{pe} (K_{a} - 1)$ , where $d_{pe}$ is the phase-to-earth clearance up to 1000 m. The yard's geometry does not grow with altitude in this set, so a check that passes at a low site can fail at a high one.

Figure 1 shows the corrected clearances of the 220 kV level from sea level to 4000 m. Notice the ground clearance rising towards the yard's equipment level of 5.9 m. From 2421 m the ground clearance is above it, and the yard's ground clearance check is a Fail.

Site altitude2000 m · Ground: 5.8 m · Sectional: 5.3 m · Phase to earth and phase to phase: 2.4 m

Figure 1.The phase-to-earth, sectional and ground clearances of the 220 kV level against the site altitude, with the yard's equipment level. The design service calculated the values by Eq. (1) and the correction of its yard rules.

Example 1Clearances of the default 220 kV project at a site of 2000 m

Given the default 220 kV project, placed at a site altitude of 2000 m. The level's clearances up to 1000 m are 2.1 m phase to earth, 2.1 m phase to phase, 5.0 m sectional and 5.5 m ground.

Altitude factor by Eq. (1), for the 1000 m above 1000 m:

K_{a} = e^{1 \times 1000 /8150} = 1.1305

Phase-to-earth and phase-to-phase clearances:

d_{pe} = 2.1 \times 1.1305 = 2.37 m, d_{pp} = 2.1 \times 1.1305 = 2.37 m

Sectional and ground clearances, each raised by $d_{pe} (K_{a} - 1)$ = 0.27 m:

d_{sec} = 5.0 + 2.1 \times 0.1305 = 5.27 m, d_{g} = 5.5 + 2.1 \times 0.1305 = 5.77 m

Checks of the yard against the corrected clearances:

Check	Clearance	Provided	Status
Phase spacing, phase to phase	2.37 m	4.50 m	Pass
Outer phase to the bay edge, phase to earth	2.37 m	4.50 m	Pass
Equipment level, ground	5.77 m	5.90 m	Pass
Main bus above the equipment, phase to phase	2.37 m	3.10 m	Pass
Gap between the main buses, sectional	5.27 m	9.00 m	Pass

Result At 2000 m the phase-to-earth and phase-to-phase clearances rise by the factor 1.1305, and the sectional and ground clearances by the added distance of 0.27 m. The yard passes the checks above. The check nearest to its limit is the equipment level against the ground clearance.

Sources

[1]CEA Safety Regulations 2023, Central Electricity Authority (Measures relating to Safety and Electric Supply) Regulations, 2023.

[2]IEC 61936-1:2021, Power installations exceeding 1 kV AC.

[3]IEC 60071-1:2019, Insulation co-ordination. §4.2.2.

CitationSubstationLayout Reference, Electrical clearances, revision 1, 8 Nov 2026, https://substation.solarlayout.app/library/reference/electrical-clearances

Electrical clearances

1The clearances of a levelCopy link to The clearances of a level

2Regulations and standardsCopy link to Regulations and standards

3Correction for altitudeCopy link to Correction for altitude

1The clearances of a level

2Regulations and standards

3Correction for altitude