If you are specifying switchgear for a 110 kV substation, the real question is rarely “which SF6 breaker is best” — it is “which rating set is correct for this network”. Learning how to select an SF6 circuit breaker means matching six interdependent ratings to your system: rated voltage, continuous current, short-circuit breaking capacity, short-time withstand, insulation level, and the environment the breaker will actually live in. Get one of them wrong and the failure mode is not a slightly derated curve — it is a breaker that cannot clear a fault when it matters.
This guide works through the selection sequence in the order a protection engineer actually uses it, using the IEC rating definitions that govern the whole high voltage SF6 circuit breakers family. Every rating below is tied to a specific design input you can read off a system study, so the output is a spec you can hand to a supplier rather than a general wish list.
Selection is not a single decision. It is six decisions that constrain each other, and they must be resolved in order. The table below is the short answer: what each rating protects against, and the rule that sets its minimum value.
| Rating | What it protects against | Typical selection rule |
|---|---|---|
| Rated voltage (U_r) | Sustained system voltage and continuous overvoltage | At least the highest system voltage of the network (U_m) |
| Rated continuous current (I_r) | Conductor, contact and enclosure heating | Maximum load current plus 10–20% margin |
| Rated short-circuit breaking current | Inability to extinguish the arc during a fault | Prospective fault current plus 20–30% margin |
| Rated short-time withstand current (I_th, t) | Thermal and mechanical damage while relays clear the fault | At least the fault current, held for the real backup clearing time |
| Rated peak withstand / making current | Electromagnetic forces on contacts and busbars at first-loop peak | At least 2.5 × the breaking current |
| Insulation level (LI / PF withstand) | Lightning and switching overvoltages | Coordinated with the voltage class and site exposure |
The rest of this article takes each row in turn and shows what actually changes in the specification.
Rated voltage is not the same number as system voltage, and this trips up more first-time buyers than any other rating. A 35 kV network is served by a 40.5 kV breaker. A 110 kV network is served by a 126 kV breaker. The rated voltage must sit above the highest system voltage the network can sustain, because the breaker has to hold off continuous overvoltage — not just the nominal figure on the drawing.
Insulation coordination is designed around the highest system voltage for the equipment class, not around nominal operating voltage. Selecting a breaker rated exactly at nominal leaves no dielectric margin for the overvoltage that follows a load rejection or a single-phase fault elsewhere on the network. The stepped ladder — 12 kV, 24 kV, 40.5 kV, 72.5 kV, 126 kV, 252 kV — exists precisely so that each step gives a defined margin above the system level it serves. If you are still mapping system voltages to breaker classes, the SF6 circuit breaker voltage classes breakdown covers the full range from 12 kV to 800 kV.
Continuous current rating determines how much load the breaker can carry indefinitely without exceeding its temperature limits. The design input is the maximum load current — normal peak, not average — plus a margin. A 10–20% margin is the usual working figure, and it is not padding: it absorbs the load growth between commissioning and the first major upgrade, and it covers the difference between the ambient temperature used in the rating and the temperature inside the switchgear enclosure.
For heavily loaded transformer incomers and tie bays on a 126 kV network, that calculation often lands above 2500 A, which is where a high-current variant becomes the practical choice. A breaker such as the LW36-126 with a 3150A rated current keeps peak demand inside design limits without resorting to parallel breaker arrangements or an early substation upgrade. Note that the current rating and the fault ratings are independent: a higher continuous current does not automatically come with a higher breaking capacity, so confirm both against your study.
The short-circuit breaking current is the core safety rating — the maximum fault current the interrupter can extinguish without damage. The design input is the prospective fault current at the breaker’s location, calculated from source impedance and transformer impedance. Apply a 20–30% margin. If the study gives 40 kA, specify 50 kA; if it gives 32 kA, a 40 kA breaker gives you the margin and matches the standard rating step.
System studies usually report symmetrical (AC component) fault current, while the breaker’s rating is defined against the symmetrical breaking current at contact separation. Near a generator, the DC component decays slowly and the asymmetrical current can be substantially higher than the symmetrical figure at the moment the contacts part. If your fault point is electrically close to generation, check the percentage DC component and confirm the breaker’s rating is defined for that duty — this is a conversation to have with the supplier using your study output, not an assumption to make.
Breaking capacity and making capacity are not the same test. Closing onto a fault is mechanically harder on the contacts than opening one, because the full first-loop peak current flows through contacts that are still seating. The rated peak withstand and making current are therefore specified at roughly 2.5 times the breaking current for a 50 Hz system, and this value drives the mechanical design of the contact assembly and the busbar support spacing. A 40kA short-circuit breaking capacity rating on a 126 kV outdoor breaker is the reference point for substations specified around a 40 kA fault level, and it comes with the matched peak and withstand values that the rating implies.
This is the rating buyers most often leave blank, and it is the one that decides whether the breaker survives a fault that its own protection does not clear. The rated short-time withstand current is the current the breaker can carry while remaining closed — thermally stable and mechanically intact — for a stated duration, conventionally written as I_th for t seconds.
The selection rule is straightforward: the withstand current must be at least the prospective fault current, and the duration must cover the actual clearing time of the backup protection, not the primary protection. If the primary relay clears in 100 ms but the upstream backup takes 2.5 s, a breaker rated 40 kA/1 s is being asked to do something it was never tested for. A rating such as the 40kA/3s short-time withstand on the LW36-126 exists for exactly this scenario: it lets the breaker stay closed and stable while a backup protection sequence completes.
Insulation level is expressed as two numbers: the rated lightning impulse withstand voltage (LI) and the rated power-frequency withstand voltage (PF), each stated to earth and across the open contact gap. These are type-test values that prove the dielectric strength of the interrupter and the external insulation.
The values step up sharply with voltage class. In this product family, a 40.5 kV breaker is proven at 95 kV power-frequency withstand, a 72.5 kV breaker at 350 kV lightning impulse to earth and 350 + 60 kV across the fracture, and a 252 kV breaker at 1050 kV lightning impulse to earth with 1050 + 206 kV across the open gap. All of these figures sit inside the framework of IEC 62271-100 rating definitions, which is also the standard that defines every current rating discussed above. If a supplier quotes an insulation level without naming the standard it was tested to, ask for the type-test report — the type and routine tests behind a rating are what make the number meaningful.
Once the electrical ratings are fixed, the mechanical specification narrows quickly. Three decisions remain: how the breaker is operated, how many poles it has, and how it is mounted.
At the top of the range, these decisions converge: a 252 kV, 4000 A breaker with a 50 kA rated short-circuit breaking current and a 125 kA peak withstand is a split-type outdoor unit by necessity, as the LW-252 252 kV split-type breaker illustrates. Nothing about that specification is optional at that voltage.
Every rating discussed so far is defined at reference conditions. The site will not provide them. Four environmental factors change the specification, and all four should be resolved before the order is placed.
| Site condition | Effect on the breaker | What to specify |
|---|---|---|
| Altitude above 1000 m | Lower air density reduces external insulation strength | Reinforced external insulation or a higher insulation class |
| High ambient temperature | Continuous current rating must be derated | Apply the manufacturer’s thermal derating curve, or step up the current rating |
| Heavy pollution | Risk of pollution flashover on external insulation | Increased creepage distance — for example a porcelain-column design |
| Low ambient temperature | SF6 liquefies, reducing pressure and interrupting capability | Anti-condensation heating plus density monitoring with alarm and block thresholds |
The low-temperature case deserves a specific look because it interacts with the gas system rather than the electrical design. SF6 begins to liquefy at around −30 °C at typical filling pressures, and liquefaction lowers the pressure inside the interrupter — which degrades both insulation and arc-quenching performance. Breakers intended for cold regions are therefore specified with heating, and with a graduated pressure monitoring scheme rather than a single gauge. In the LW8-40.5 family, for example, the rated gas pressure is 0.5 MPa, the alarm threshold is 0.47 MPa, and the minimum functional pressure is 0.15 MPa. Those three numbers are the ones an operator acts on, so they belong in the selection spec, not just the manual.
Putting the sequence together makes the arithmetic clear. Take an outdoor 110 kV substation with the following design inputs from the system study: highest system voltage 123 kV, maximum load current 2400 A, prospective fault current 32 kA, backup protection clearing time 2.5 s, and an overhead line connection in a lightning-prone area.
| Design input | Required value | Selected rating |
|---|---|---|
| Highest system voltage 123 kV | Rated voltage ≥ 123 kV | 126 kV |
| Maximum load current 2400 A | Continuous current ≥ 2640 A (10% margin) | 3150 A |
| Prospective fault current 32 kA | Breaking capacity ≥ 38.4 kA (20% margin) | 40 kA |
| Backup clearing time 2.5 s | Short-time withstand for ≥ 2.5 s at 32 kA | 40 kA / 3 s |
| Overhead line, lightning exposure | Insulation level coordinated to the 126 kV class | Class-rated LI and PF withstand |
| Outdoor switchyard | Weather-exposed installation | 3-pole outdoor, spring mechanism |
Each of those outputs maps directly onto an available configuration: the 3150 A continuous rating, the 40 kA breaking capacity and the 40 kA/3 s withstand are the three variants discussed in Steps 2, 3 and 4 above, all within the same LW36-126 platform. That is the practical value of resolving the ratings in order — the result is a coherent specification rather than a collection of maximums, which is also how you avoid paying for a 50 kA breaker on a 32 kA network.
Selecting an SF6 circuit breaker comes down to resolving six ratings in sequence against six design inputs you already have: highest system voltage, maximum load current, prospective fault current, backup clearing time, insulation exposure, and site conditions. Work through them in order, apply the standard margins, and the specification writes itself. The two figures that most often go missing are the short-time withstand duration and the environmental derating — both are cheap to specify up front and expensive to discover after commissioning.
If you have a system study in hand and want the rating set confirmed against a specific model, the LW36-126 range covers the 126 kV class from 3150 A continuous current through 40 kA breaking and 40 kA/3 s withstand, with 40.5 kV and 72.5 kV options available for the lower voltage steps. Send us your fault level, load current and site conditions, and we will map them to a configuration and provide the type-test documentation behind each rating.
Breaking current is what the breaker can interrupt — the arc is extinguished and the circuit opens. Short-time withstand current is what the breaker can carry while staying closed for a defined duration. Both are tested, and both must exceed the prospective fault current, but they answer different questions.
Apply 20–30% above the calculated prospective fault current, then round up to the nearest standard rating step. A 32 kA study result leads to a 40 kA breaker; a 40 kA result leads to 50 kA.
Yes — that is the correct match. Rated voltage is set against the highest system voltage of the equipment class, not the nominal operating voltage, so the breaker class sits one step above the nominal network level.
Above 1000 m, reduced air density lowers the external insulation strength of the breaker. Either apply the manufacturer’s altitude correction or specify a reinforced external insulation design; the internal SF6 insulation is unaffected.
Making capacity is the peak current the breaker can withstand while closing onto an existing fault. It is higher than the breaking current — roughly 2.5 times it at 50 Hz — and it drives the mechanical strength of the contact assembly and busbar supports.
IEC 62271-100 defines the ratings and type tests for high-voltage alternating-current circuit breakers, including rated voltage, continuous current, breaking capacity, short-time withstand and insulation level. IEEE C37 series standards cover the equivalent requirements for North American projects.
It depends on the design. A typical 40.5 kV outdoor breaker uses a 0.5 MPa rated pressure with an alarm threshold around 0.47 MPa and a minimum functional pressure of 0.15 MPa. Always specify the alarm and block thresholds alongside the rated pressure, because those are the values operators act on.
