Most of the equipments in a substation are provided with sufficient insulation from breaking down. There is a need for assurance that the breakdown or flashover will not occur to the operating personnel and some safe distance is to be maintained. Clearances are broadly categorized as below.
a) Phase to earth clearance: - Equipment phase to earth clearance depends on the type of insulation material used. While equipment bushings take care of external clearances, the insulating material inside the equipment like oil, SF6, and vacuum take care of earth clearance internally.
b) Phase to phase clearance: - It is the clearance between two conductors charged electrically. Sufficient phase-to-phase clearance has to be provided in air to prevent flash over & breakdown of air insulation. This clearance is one of the factors in deciding bay width in substations.
c) Section clearance: - This clearance is required from point of safety to operating personnel. It is distance between two sections of a substation that enables a person to work on one section of a substation, in a safe manner, that the phase to earth clearance is maintained between the live point and the approach of the working personnel with sufficient margin.
d) Ground clearance: - It is a distance between ground level and bottom of any insulator in an outdoor substation.
Standard Safe Clearances
|
Voltage (KV) |
33 KV |
66
KV |
110
KV |
220
KV |
400
KV |
|
BIL
(KVp) |
170 |
325 |
550 |
1050 |
1425 |
|
Ph-Earth
Clearance (Cm) |
32 |
63 |
115 |
240 |
350 |
|
Ph-Ph
Clearance (Cm) |
40 |
75 |
135 |
210 |
410 |
|
Ph-Ground
Clearance (Mtr) |
3.7 |
4.0 |
4.6 |
5.5 |
8.0 |
|
Section Clearance (Mtr) |
2.8 |
3.0 |
3.5 |
4.3 |
6.5 |
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