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Circuit breakers and isolators

A circuit breaker makes, carries and breaks the current of its circuit, the fault current included. An isolator isolates a part of the circuit, and an earth switch connects the isolated part to the earth mat. The design specifies these devices per bay of the high-voltage (HV) switchyard and per panel of the medium-voltage (MV) switchgear, with ratings read from the voltage level.

Read first: Bays and their equipment · Bus schemes

1The ratings of a switching device

A switching device is specified by the voltage it withstands, the current it carries without overheating and the fault current it carries and interrupts. The design gives each HV device these ratings from its voltage level:

the highest voltage for equipment $U_{m}$ , which the ANSI / IEEE set calls the rated maximum voltage
the rated lightning impulse withstand voltage, printed as the basic lightning impulse insulation level (BIL) in kVp
the rated short-time withstand current and its duration, which the design calls the design fault level
the rated normal current, one for the main bus and one for the bays

The design fault level is the short-circuit current the HV switchgear is rated to carry for its duration and to interrupt. The short-circuit study checks the calculated currents against it. The bill of materials prints the same voltage and fault ratings on a bay's breaker, isolators and instrument transformers.

The design takes one rated normal current for the main bus and one for the bays of a level. A switching substation steps its design fault level up to the next standard rating, because the other HV side feeds the fault through the interconnecting transformers (ICTs).

2Ratings per level in the IEC + CEA / CBIP set

The CEA Construction Regulations 2022 set the highest system voltage and the lightning impulse withstand per level (Reg. 45(1), Table 9, and Reg. 50(1), Table 14 for 33 kV). They set a minimum short-time withstand per level at 66 kV and above (Reg. 43(1), Table 6). The design's ratings of a new pooling project are these:

Level	$U_{m}$	BIL	Design fault level	Main bus	Bays
33 kV	36 kV	170 kVp	25 kA, 3 s	1250 A	800 A
66 kV	72.5 kV	325 kVp	31.5 kA, 1 s	1250 A	800 A
132 kV	145 kV	650 kVp	40 kA, 1 s	2000 A	1250 A
220 kV	245 kV	1050 kVp	50 kA, 1 s	3150 A	1600 A
400 kV	420 kV	1425 kVp	63 kA, 1 s	4000 A	3150 A

At 400 kV the equipment also carries a switching impulse withstand level (SIWL) of 1050 kVp. A 33 kV HV level is the HV side of a 33/11 kV substation, and its fault level lasts 3 s as the MV switchgear's does. The user can enter the design fault level of the project, and the short-circuit study then checks the calculated currents against that value.

3Ratings per level in the ANSI / IEEE set

The ANSI / IEEE set takes the rated maximum voltage and the BIL of its levels from IEEE C37.04-2018 Table 6. Its design fault level is one of the rated short-circuit currents that Tables 12 and 16 list at the level's rated maximum voltage. The bus and bay currents are continuous currents listed on the line of that short-circuit current.

Level	Rated maximum	BIL	Design fault level	Main bus	Bays	Rated short-circuit currents listed
69 kV	72.5 kV	350 kVp	31.5 kA, 2 s	2000 A	1200 A	20, 31.5, 40, 50 and 63 kA
115 kV	123 kV	550 kVp	40 kA, 1 s	2000 A	2000 A	31.5, 40, 50 and 63 kA
138 kV	145 kV	650 kVp	40 kA, 1 s	2000 A	2000 A	31.5, 40, 50, 63 and 80 kA
161 kV	170 kV	750 kVp	40 kA, 1 s	2000 A	2000 A	31.5, 40, 50, 63, 80 and 100 kA
230 kV	245 kV	900 kVp	50 kA, 1 s	3000 A	2000 A	31.5, 40, 50, 63 and 80 kA
345 kV	362 kV	1300 kVp	50 kA, 1 s	3000 A	2000 A	40, 50 and 63 kA

At 345 kV the equipment carries a basic switching impulse insulation level (BSL) of 825 kVp. The rated short-time duration is the maximum permissible tripping delay: 2 s below 100 kV and 1 s at 100 kV and above (C37.04-2018 Tables 12 and 16).

A breaker's rated closing and latching current is a peak current, a multiple of its rated short-circuit current $I_{sc}$ :

I_{close} = 2.6 I_{sc} at 60 Hz, I_{close} = 2.5 I_{sc} at 50 Hz

(1)

IEEE C37.04-2018 §5.6.2.3

where
$I_{sc}$	rated short-circuit current of the breaker, r.m.s. symmetrical	kA
$I_{close}$	rated closing and latching current, peak	kA

The factors hold for a DC time constant up to the rated 45 ms (C37.04-2018 §5.6.2.3). The design does not choose the breaker's interrupting time. The short-circuit study takes the rated interrupting times C37.04-2018 lists for the rated maximum voltage, and the larger duty where two are listed: 3 or 5 cycles at 69 kV, 3 cycles at 115 kV, and 2 or 3 cycles from 138 kV to 345 kV.

4HV circuit breakers

The HV breaker of an air-insulated switchyard (AIS) is an SF6 circuit breaker, three-pole and spring operated. It is rated at the level's voltage, its design fault level and the bay current. The number of breakers follows the bus scheme: one per circuit and one for the coupler or bus section, or three per full diameter of a one-and-a-half breaker scheme (Bus schemes, Bus schemes of the IEC + CEA / CBIP set).

The bill of materials carries one spare interrupter pole per breaker type. In a yard of gas-insulated switchgear (GIS), the breakers are part of the GIS bay modules (Gas-insulated switchgear (GIS)). The single-line diagram (SLD) names the breaker −Q0 in the IEC + CEA / CBIP set, and 52 in the ANSI / IEEE set after IEEE C37.2-1996.

5Isolators and earth switches

The design's HV isolator is a three-phase, motor-operated, horizontal centre-break (HCB) isolator, at the bay's current and the level's ratings. The bus isolators, which connect a bay to each bus, have no earth switch. The line isolator and the transformer isolator carry one earth switch each (Bays and their equipment, Line bay).

The IEC + CEA / CBIP set designates the bus isolators −Q1 and −Q2, the line isolator −Q9 and its earth switch −Q8. The ANSI / IEEE set calls the isolator a disconnect switch and the earth switch a grounding switch. Its SLD prints 89 for the disconnect switches, and an 89 with its grounding switch draws no separate grounding device. A ring bus has no main buses, so its disconnect switches stand on both sides of the breakers. In a diameter of a one-and-a-half breaker scheme, a disconnect switch also stands on both sides of a breaker.

6MV switchgear

The MV switchgear connects the transformers' low-voltage windings to the plant's collector feeders. The design offers indoor switchgear or an outdoor MV AIS yard. Indoor switchgear is a line of metal-clad panels in the control building, each with a withdrawable vacuum circuit breaker (VCB). An outdoor bay carries a VCB, two isolators, a current transformer and a surge arrester.

The MV bus is sectionalised, one section per transformer low-voltage winding, and the bus-section breakers are normally open. Each section has its incomer, its plant feeders, the reactive power and harmonic filter feeders and a bus voltage transformer panel. Sections 1 and 2 also feed the auxiliary transformers. The plant feeders and the spare feeder panels for future circuits are each spread evenly over the sections, the first sections taking any remainder. An indoor bus section takes a pair of panels, the breaker and a riser.

The panels share the MV level's ratings. The incomer's rated current is the smallest current of the standard series 630, 800, 1250, 1600, 2000, 2500, 3150, 4000, 5000 and 6300 A at or above the full-load current of one low-voltage winding. The bus of each section has at least the incomer's rating, because one transformer's full current can flow along the section to its incomer. The plant feeders carry a fixed rating per MV level.

MV level	$U_{m}$	BIL	Design fault level	Feeder breaker	Indoor switchgear up to
33 kV	36 kV	170 kVp	31.5 kA, 3 s	1250 A	4000 A
11 kV	12 kV	75 kVp	25 kA, 3 s	630 A	4000 A
34.5 kV	38 kV	200 kVp	40 kA, 2 s	1200 A	4000 A

An incomer current above the indoor switchgear's top rating raises a warning. The warning advises outdoor switchgear, a lower transformer rating or more transformers. In the ANSI / IEEE set the MV breakers take the C37.04-2018 class of their system. Indoor switchgear feeds the collector cables and takes the class for cable systems, with currents 1200, 2000, 3000 and 4000 A at its rated maximum voltage. An outdoor MV yard takes the class for line systems of Table 12, with currents 1200, 2000 and 3000 A.

Example 1Switching devices of the default 220 kV project

Given the default project: a 220 kV double main bus with 1 line bay and 2 ICTs of 160 MVA, and a 33 kV bus in 2 sections with indoor switchgear.

HV ratings of the level: $U_{m}$ 245 kV, BIL 1050 kVp, 50 kA for 1 s, bays 1600 A, main bus 3150 A.

HV devices, from the bill of materials:

Device	Quantity
SF6 circuit breaker, three-pole	4
Isolator without earth switch (bus isolators)	10
Isolator with one earth switch (line and ICT isolators)	3
Spare interrupter pole	1

MV incomer by the full-load current of one low-voltage winding, of rating $S$ at the MV voltage $U$ :

I_{L V} = \frac{S}{3 U} = \frac{160 \times 1000}{3 \times 33} = 2799 A

The smallest standard current at or above it is 3150 A, and the bus of each section takes the same 3150 A.

Result: the 220 kV yard has 4 breakers, 10 bus isolators and 3 isolators with earth switches. The 33 kV switchgear has 2 incomers of 3150 A, 17 feeder panels of 1250 A and 1 bus-section panel set, rated 31.5 kA for 3 s.

Example 2Breakers of a 230 kV project in the ANSI / IEEE set

Given a new 230 kV pooling project of the ANSI / IEEE set at 60 Hz: a one-and-a-half breaker scheme, 2 transformers of 168 MVA and a 34.5 kV collector bus.

HV ratings: rated maximum voltage 245 kV, BIL 900 kVp. At that voltage, C37.04-2018 Table 16 lists the rated short-circuit currents 31.5, 40, 50, 63 and 80 kA, and the design takes 50 kA for 1 s. The bays take 2000 A and the main bus 3000 A.

HV devices: 5 SF6 circuit breakers, 12 horizontal-break disconnect switches on both sides of the breakers and at the bus voltage transformers, and 3 disconnect switches with a grounding switch.

MV incomer: one winding carries 2811 A at 34.5 kV. The smallest cable-system current at or above it is 3000 A, rated 40 kA for 2 s at 38 kV.

Result: the breakers are rated 50 kA at 245 kV, and the MV incomers 3000 A.

7What the design leaves to the specification

The design sets the ratings that follow from the voltage level, the fault level and the circuit currents. It does not choose a make or the breaker's interrupting time. The ANSI / IEEE short-circuit study covers the interrupting time by taking the larger duty of the listed times. The makes of each item are kept in the Library and print beside the bill of materials.

Sources

[1]CEA Construction Regulations 2022, Central Electricity Authority (Technical Standards for Construction of Electrical Plants and Electric Lines) Regulations, 2022.

[2]IEEE C37.04-2018, IEEE Standard for Ratings and Requirements for AC High-Voltage Circuit Breakers. §5.6.2.3.

[3]IEEE C37.2-1996, Standard Electrical Power System Device Function Numbers.

CitationSubstationLayout Reference, Circuit breakers and isolators, revision 2, 8 Nov 2026, https://substation.solarlayout.app/library/reference/circuit-breakers-and-isolators

Circuit breakers and isolators

1The ratings of a switching deviceCopy link to The ratings of a switching device

2Ratings per level in the IEC + CEA / CBIP setCopy link to Ratings per level in the IEC + CEA / CBIP set

3Ratings per level in the ANSI / IEEE setCopy link to Ratings per level in the ANSI / IEEE set

4HV circuit breakersCopy link to HV circuit breakers

5Isolators and earth switchesCopy link to Isolators and earth switches

6MV switchgearCopy link to MV switchgear