Generator switchgear is the generator circuit breaker together with the current transformers, relays and enclosure that sit between a generator and the rest of the plant. It’s a separate product from distribution switchgear because the fault it clears behaves differently, and the breakers are type-tested to their own standard for that.
The confusion starts on the supplier side. Search for the term and you may land on a page for turbine generators up to 450 MW, a real Siemens product line, rather than anything about the panel in front of you. The gear for a 500 kW standby set is a different object with a different job.
You may already suspect that “switchgear” isn’t one thing. This article covers what is actually inside the enclosure, and why the generator breaker isn’t interchangeable with the distribution breaker beside it. It also covers where the standard stops, and the numbers a supplier needs before a price means anything.
Key Takeaways
- Generator-fed faults are smaller than grid-fed faults but carry a DC component above 100%, which can delay the current zero for several cycles. A breaker that interrupts at a zero crossing has to wait.
- IEC/IEEE 62271-37-013 covers generators and transformers of 10 MVA and above, between 1 kV and 38 kV. Below 10 MVA it calls the application special and says it is not completely covered.
- Close-and-latch duty is 2.74 times rated current for a generator breaker against 2.6 for a system-fed fault. Out-of-phase duty is 50% against 25%.
- You buy the gear by the section, and a section holds one breaker. Relays, settings and site testing are usually quoted outside the section price.
- Three different products get called generator switchgear: generator circuit-breaker gear, paralleling gear, and the breaker inside a standby set.
What Is Inside a Generator Switchgear Line-Up
A medium voltage line-up is bought by the section. Each section is a steel enclosure holding one breaker and the equipment that supports it. The construction is metal-clad switchgear, covered by ANSI/IEEE C37.20.2. In one manufacturer’s costing paper, a section prices at 50,000to50,000to60,000, which is why a three-section line-up runs past $150,000 with no cable in the price.
| Item | What it does | Always fitted |
|---|---|---|
| Circuit breaker | Makes and breaks load and fault current | Yes |
| Current transformers | Scale current down to a level the relay can read | Yes |
| Voltage transformers | Give the relay a voltage reference | Usually |
| Protective relays | Decide whether to trip, and how fast | Yes |
| Earthing switch | Lets the section be earthed before work | Usually |
| Interlocks | Stop the earthing switch closing onto a live circuit | Yes |
| Cable terminations | Join the section to the cable or busbar | Yes |
| Anti-condensation heater | Keeps moisture off the insulation | Usually |
The enclosure isn’t a box. A proper section is divided into compartments, with the breaker in one, the busbar behind shutters in another and the cable in a third. That arrangement is why a section can be withdrawn and maintained while the busbar stays live. A low voltage distribution board of similar current rating usually has no equivalent arrangement, and the same work needs a shutdown.
The earthing switch deserves its own line. It’s the device that makes a section safe to touch, and it’s interlocked so it cannot close while the circuit is live. On a generator package it is also where the installation is proved safe before any work starts. That’s why the interlock test belongs in the commissioning programme rather than the warranty file.
The breaker is the visible part and rarely the expensive one. The relays are what turn a switch into a protection system. Two relays set correctly do more for a plant than a larger breaker does.
Buying the set and its switchgear together? We size the generator, so our engineers can tell you what the package has to match before it goes to tender. That conversation is cheaper than a requote.
Why a Generator Breaker Is Not a Distribution Breaker
A fault fed from the grid arrives through a transformer. The impedance is low, the bus is short, and the full energy of the network pours into the fault. That’s the case everyone pictures when they say fault current.
A fault fed from the generator itself is smaller. It is also harder to interrupt, and the reason is timing rather than magnitude.
The current is lower, but the zero crossing moves
The generator has a high ratio of reactance to resistance. That ratio governs how quickly the DC component of the fault current decays. In a generator-fed fault the DC component can exceed 100% of the symmetrical value, and the current wave stops crossing zero cleanly. The first zero can slip by several cycles.
Every circuit breaker interrupts at a current zero. No zero means the arc keeps burning. So a generator breaker must withstand a longer arc and absorb more thermal and mechanical stress while it does.
The condition is worst when the generator was running under-excited with a leading power factor just before the fault. A lagging power factor lowers the asymmetry instead. That detail matters because it means the duty is a function of how the plant was operating, not only of the machine’s rating.
Why the DC offset lingers
A transformer-fed fault decays quickly. The energy feeding it comes from a network through a short, low-impedance path, and the offset washes out within a few cycles.
A generator-fed fault decays slowly, and the reason sits inside the machine. The windings store magnetic energy, and the time constant that governs its release is set by the X/R ratio, the machine’s reactance divided by its resistance. A machine with a high ratio releases that energy over a longer period, so the offset is still on the waveform during the cycles when the breaker is trying to open.
Put plainly, the current in a generator circuit does not settle on the timescale a distribution breaker was designed around.
The ratings that change
The percentages below are of the symmetrical peak, which is a different measure from the DC component described above.
| Duty | System-fed fault | Generator-fed fault |
|---|---|---|
| Peak asymmetry | 75% | 130% |
| Close and latch | 2.6 x rated current | 2.74 x rated current |
| Out-of-phase breaking | 25% x rated current | 50% x rated current |
Read the table as three separate questions. Can the breaker take the peak, and close onto a fault that is already there and hold it? Can it open while the generator is out of step with the network?
On all three, the generator-fed column asks for more. Every value in it is a duty a distribution breaker is never asked to perform, because a distribution breaker never has a generator on the other side of it. The figures come from an IEEE conference presentation that sets the two sets of duties side by side.
The relay sees the same problem from its side. A current transformer has to reproduce a wave with a large offset without saturating, or the relay receives a smaller and more distorted copy of the current than actually flowed. A relay fed that signal either waits too long or trips against the wrong threshold. Choosing the CT is part of the switchgear design, not an accessory to it.
In practice the failure is quiet. A distribution breaker applied to a generator either trips on the offset it was never built for, or holds through a fault it should have cleared. The first registers as nuisance trips that get logged and reset. The second registers once, at the equipment the breaker was there to protect.
None of this lives in the generator control panel. The control panel trips on engine and alternator faults. The switchgear relay protects the network from the generator and the generator from the network. The two sets of settings have to be coordinated, or one will trip before the other can.
The 10 MVA Line in the Standard
IEC/IEEE 62271-37-013 is the standard written for this product. It covers generator circuit-breakers, usually shortened to GCBs, on systems above 1 kV and up to 38 kV, at 50 or 60 Hz, mounted between the generator and the transformer terminals. Its scope names generators and transformers rated 10 MVA and above.
The sentence after that scope is the one to remember. Below 10 MVA the standard treats the circuit as a special application and states that it isn’t completely covered.
That isn’t a prohibition. It’s a warning that the rulebook thins out exactly where a lot of mid-size projects sit. A 4 MVA set at 11 kV is a real product with a real breaker in front of it, and the generator-specific standard doesn’t fully cover that breaker.
Above 10 MVA the position is simpler and stronger. The standard gives you a certificate to ask for, a list of duties the breaker must have been proved against, and something concrete to compare across three bidders. Below the line you are asking a supplier to demonstrate judgement, which is a harder thing to compare.
Ezra Mwangi took delivery of a 9 MVA set at 11 kV for a tea factory in Kenya in 2023. The breaker came with a type-test certificate. The relay settings came on a memory stick, unsigned, and nobody had asked what the test programme would cover. The delayed-zero case was never demonstrated, so commissioning stalled for six weeks while a witnessed test was arranged at the supplier’s works.
The practical response is three questions on the enquiry: state the MVA figure rather than the kW, ask which standard the breaker is type-tested to, and ask whether the delayed-zero condition was in the test programme. If the last two answers are vague, you’re being quoted distribution gear with a generator behind it.
Put it in writing. Suppliers respond differently to “special application under 62271-37-013” than to “medium voltage switchgear” sitting in a line item, and the first wording tends to reach an engineer rather than a catalogue.
Three Products Called Generator Switchgear
| Product | Job | Typical scale |
|---|---|---|
| Generator circuit-breaker switchgear | Clears faults between the generator and the step-up transformer | Medium voltage, 10 MVA and up |
| Paralleling switchgear | Ties several sets to one bus, synchronises and shares load | Low to medium voltage, two sets and up |
| The breaker on a standby set | Connects one set to the building | Low voltage, one set |
The first is this article’s subject. The second is as much a control and metering package as a switching one, and the connection side is covered in paralleling bus and switchgear. The third is often a moulded-case breaker inside the set’s own enclosure, or a small panel on a wall.
Settle the scale question first. Two sets at 400 V sharing a load need paralleling switchgear and no MV switchgear at all. One machine at 12 MVA needs generator circuit-breaker gear and probably nothing from the paralleling column.
A set that stays low voltage and still has to reach 11 kV does it through a generator step-up transformer instead, and that arrangement carries its own protection question. Acquiring all three because the enquiry said “generator switchgear” is how a site ends up with equipment nobody there is trained to maintain.
The confusion also costs money in the other direction. Ask for “generator switchgear” and receive a price for a paralleling board, and you’ve bought synchronising you may not need, and protection you do need and didn’t get.
One more thing that is not switchgear at all: the automatic transfer switch. ATS versus manual transfer is a separate decision, and the two are often quoted in the same breath.
What a Serious Switchgear Quote Contains
A switchgear enquiry asks different questions from a generator enquiry, and the quote that comes back should answer them in a different shape. A quote that gives only a current rating and a price hasn’t answered either.
Here’s what a usable one contains.
- A breaker type designation, not the word “vacuum” on its own. A type number can be looked up and its test record found.
- The standard the breaker is type-tested to, with the certificate reference. This is the line that tells you whether anyone has tested it against a generator-fed fault.
- The rated values as a set: rated voltage, rated current, short-circuit breaking current, peak withstand and close-and-latch. One number without the others isn’t a rating.
- Current transformer ratios, class and burden, because the relay is only as good as the CT feeding it.
- The relay model, and who sets it, at works or on site. Settings left to site are settings left to whoever is there that week.
- Enclosure details: ingress protection, whether it is arc-resistant, and the dimensions and weight, which decide the plinth and the door width.
- The test regime, including which tests are routine at works and which will be witnessed, set out as hold and witness points.
- Exclusions. Cable, civils, the earthing grid, relay settings and site testing are frequently outside the price, and finding that out after award is the expensive way.
Two figures have to be carried across from the generator enquiry: the voltage class and the generator rating. Neither is negotiable at this stage, because the insulation and clearances are fixed when the section is built.
Switchgear sizing starts with the fault level, which is the number that moves the price most. If you have no study, the generator’s sub-transient reactance is enough for a supplier to derive it. Give the number rather than the adjective.
Hakan Yilmaz put “suitable for the generator” in the enquiry for a 6.6 kV package at a desalination plant in Turkey in 2024. The first quote came back with the price held open until the fault level was confirmed, and no delivery date. He resent it with the reactance figure, the earthing arrangement and the relay model he wanted. The second came back firm, with a date, the earthing switch included and the CT class named.
Frequently Asked Questions
What is generator switchgear?
Generator switchgear is the switching, measuring and protective equipment between a generator and the rest of the electrical system. It contains at least one circuit breaker, the current and voltage transformers that feed its relays, an earthing switch and the enclosure. Its job is to disconnect the generator safely, including when the fault current is the generator’s own.
Is generator switchgear the same as a switchboard?
No. A switchboard is a low voltage assembly for distributing power, built and tested in North America to UL 891. Low voltage metal-enclosed switchgear with drawout breakers is a different assembly under UL 1558, and that standard addresses the harder cases: higher available fault current, short-time withstand, multiple sources and generator-backed systems. The difference is construction and testing, not size alone.
Does a small standby generator need generator switchgear?
No. A single standby set needs a breaker and a way to transfer the load, which is usually an automatic transfer switch. “Generator switchgear” in the sense used here means medium voltage gear type-tested for a generator.
Breakers in that class exist well below 10 MVA, but the standard that fully covers them only starts there. Buying it for a 200 kW set would be a costly misreading of the term.
Why is generator switchgear more expensive than a distribution board?
The breaker is rated for higher asymmetry and close-and-latch duty. It’s type-tested rather than assembled. The relays have to be set and coordinated with the rest of the system, and the enclosure has to carry the insulation level of its voltage class. None of that scales with the price of the board beside it.
Specify the Fault, Not the Amps
The mistake to avoid is choosing generator switchgear by current rating alone. Two breakers can carry the same amps and be built for different faults.
Ask what the breaker was type-tested to interrupt, at what asymmetry, and whether the delayed-zero case was in the programme. Then check the MVA figure against the standard’s scope, because below 10 MVA you’re relying on the supplier’s judgement rather than a published rule.
The order matters as much as the equipment. Commissioning and acceptance testing is where the relay settings, the interlocks and the earthing switch get proved. A package that arrives without a test schedule has skipped the only stage that would have caught a mismatch.
If you are scoping a generator switchgear package now, send us the single-line diagram and the generator data sheet. We will tell you which questions a quote has left unanswered, which figures are missing, and which specification items to add before you send it.
