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An inaccurate switchgear quotation almost never starts at the manufacturer’s pricing desk — it starts with an incomplete datasheet or an ambiguous single-line diagram (SLD). When a buyer sends a one-line sketch without short-circuit ratings, or a switchgear specification sheet that leaves out breaking capacity, every supplier has to fill in the blanks. Filling in blanks means adding contingency, and contingency shows up as a higher price or a change order three months later.
Chuanli Electric designs, manufactures, and type-tests medium voltage (MV) switchgear from 12 kV to 40.5 kV for grid, industrial, and infrastructure projects — including China State Grid programmes and major transmission works such as the Three Gorges and West–East Power Transmission projects. In this guide, our engineering team walks you through exactly how to read an MV switchgear datasheet, how to prepare a single-line diagram (SLD) for a substation, and how to turn both into an RFQ that returns an accurate, comparable quote the first time.
A manufacturer can only quote what it can clearly define, and two documents carry almost all of that definition:
Leave out a parameter and the supplier has to assume worst-case conditions. Leave out the topology and the switchgear gets priced with extra contingency. Both of those push your quote up, and neither is the supplier’s fault. Get both documents right and pricing becomes a calculation rather than a guess.
A medium voltage switchgear specification reads like a wall of numbers, but every number protects you in a specific way. Here is what to look for, in the order an engineer checks it.
The first line of any datasheet sets the rated voltage (Ur) — commonly 12 kV, 24 kV, 36 kV, or 40.5 kV for MV systems — together with the rated normal current (typically 630 A to 4000 A) and the system frequency (50 Hz or 60 Hz).
Check that the rated voltage matches your highest system voltage, not just the nominal value, and that the current rating covers your calculated load plus a sensible growth margin. A cabinet rated 1250 A will not be quoted at the same price as one rated 3150 A — busbar, breaker, and current-transformer sizing all scale with current.
This is the parameter that most often moves a price. A complete datasheet should state three related values:
If your specification gives a breaking capacity but no short-time withstand, the manufacturer cannot confirm that the busbar and the disjoncteurs à vide intérieurs are properly coordinated. Always supply both, because the difference between 20 kA and 31.5 kA is not a rounding error — it changes the breaker class, the busbar cross-section, and the price.
Insulation is specified as a power-frequency withstand voltage et un lightning impulse withstand voltage (for example 75 kV, 95 kV, 125 kV, or 185 kV, depending on voltage class). Enclosure protection is given as an IP rating (IP3X, IP4X, IP54…), and arc-tested equipment carries an Classification IAC such as “IAC A FLR 20 kA 1 s”.
These values depend on your environment — a dusty indoor mining site behaves nothing like an exposed coastal installation. If arc protection matters for your project, it is worth understanding how internal arc fault suppression technology is designed into a medium voltage assembly, because a claim on the datasheet must be backed by a type test.
A credible medium voltage switchgear specification names the standards it is designed to, and — crucially — the type tests that prove it. For MV metal-enclosed switchgear, look for IEC 62271-200; for circuit breakers, IEC 62271-100; and for the low-voltage assemblies used alongside, IEC 61439. Chinese projects often add GB/T equivalents, and ISO 9001 is a common baseline requirement.
Ask for the type-test report, not just the standard number. A datasheet that claims IEC 62271-200 without a reference to an arc test or a temperature-rise test is marketing, not engineering. And if terminology is part of your confusion, our explanation of switchboard vs switchgear clarifies how the two differ before you write the specification.
Finally, the datasheet should describe the mechanical side: main busbar material and rating (usually copper, sized to the short-time current), earthing busbar, cable entry arrangement, and — if you want a smarter installation — metering, protection relays, and SCADA/communication interfaces. This is the point where a standard medium and high voltage switchgear cabinet becomes a project-specific build, so the more detail you supply, the more the supplier can standardise instead of customise.
| Datasheet field | What it defines | Why it changes the quote |
|---|---|---|
| Rated voltage (Ur) | System voltage class | Drives insulation and switchgear class |
| Rated normal current | Continuous load capability | Sizes breaker, busbar, and CTs |
| Short-circuit breaking current (kA) | Fault interruption duty | Selects the circuit breaker and its cost |
| Short-time withstand current (kA/s) | Busbar and structure duty | Sets busbar cross-section and bracing |
| Insulation level (LIWV) | Dielectric strength | Affects bushings and phase spacing |
| IP / IAC rating | Environmental and arc protection | Changes enclosure type and testing |
| Standards and type tests | Compliance evidence | Determines whether the product can be certified |
Our engineers review medium voltage switchgear datasheets and specification sheets every week. Share yours and we will flag the parameters that affect price and lead time — free, and with no obligation.
If the datasheet describes what the equipment must withstand, the single-line diagram describes how it is arranged. An SLD is the drawing a manufacturer reads to count breakers, size busbars, and build the bill of materials — which makes it the single most useful page in your enquiry.
When the SLD is for a substation rather than a single assembly, add the grid interface: the incoming line from the utility, the main transformer, the MV busbar with its feeder bays, and the outgoing distribution circuits. Where space and installation time are tight, this is exactly the scope that compact substations compress into one pre-assembled unit — but the SLD still has to define the bays, because that is what gets quoted.
Standardise on IEC 60617 symbols and a consistent one-line convention. A one-line diagram (the SLD) shows three phases as a single line and is the normal basis for a quotation; a three-line diagram shows each phase separately and is usually a detailed design deliverable, not a procurement document. If you cannot draw to standard yet, a clear hand sketch with labelled ratings is still far better than an empty template.
Every symbol on the SLD becomes a line item. Two incomers, six feeders, one bus-tie, four CTs, and a set of relays translate directly into panels, breakers, and accessories — which is why a missing symbol becomes a missing line in the quote, and later a change order.
Upload your single-line diagram together with your load data, and our team will return a panel-by-panel quotation instead of a lump-sum figure you have to reverse-engineer.
Now combine both inputs. A complete RFQ for MV switchgear contains the datasheet parameters, the single-line diagram, and a short commercial brief. Use this checklist:
When you answer all five, you remove almost every contingency a supplier would otherwise add — and your quotes become directly comparable, line for line.
Send us your datasheet and single-line diagram together. As a manufacturer of switchgear from 12 kV to 40.5 kV with IEC 62271 type-tested designs, we will come back with a compliant, comparable quotation — and tell you if anything in your specification is likely to increase cost.
At minimum: rated voltage, rated normal current, frequency, rated short-circuit breaking current, rated short-time withstand current and duration, insulation level (power-frequency and lightning impulse withstand), IP rating, internal arc classification, and the standards and type tests the equipment complies with. A datasheet missing any of these forces the supplier to assume worst-case conditions.
The datasheet defines what the switchgear must withstand — its electrical and mechanical ratings. The single-line diagram defines how the circuits are arranged — how many bays, breakers, transformers, and how they connect. You need both: the datasheet sets the equipment class, and the SLD sets the quantity and configuration.
Five things: the technical parameters (voltage, current, short-circuit data, insulation, ratings), the SLD showing the configuration, the installation environment, the commercial terms (quantity, delivery, certification, schedule), and clarity on scope — whether the transformer, protection relays, SCADA interfaces, and installation support are included.
Yes, if you want an accurate and comparable one. Without an SLD, a manufacturer cannot count bays, breakers, or instrument transformers, so the price is quoted with contingency or left open. A simple one-line sketch with labelled ratings is enough to start; it does not need to be a finished design drawing.
A one-line diagram (also called a single-line diagram, or SLD) represents all three phases with a single line and is the standard basis for a switchgear quotation. A three-line diagram shows each phase separately and is normally a detailed design or construction deliverable rather than a procurement document.
For MV metal-enclosed switchgear, IEC 62271-200 is the key standard; for circuit breakers, it is IEC 62271-100; and for associated low-voltage assemblies, IEC 61439. Many projects also require GB/T equivalents in China or national standards elsewhere, and ISO 9001 is a common quality baseline. Always confirm the type-test report, not just the standard number.
State both the rated short-circuit breaking current (what the breaker interrupts) and the rated short-time withstand current with its duration (what the busbar and structure carry under fault), plus the peak withstand current. These values come from your system short-circuit study — do not guess them, because under-specifying risks a change order and over-specifying adds avoidable cost.
Continue with these guides from Chuanli Electric and our technical library:
About the author: this guide was prepared by the Chuanli Electric engineering team, which designs and manufactures medium voltage switchgear (12 kV–40.5 kV), ring main units, vacuum circuit breakers, and compact substations to IEC 62271 and IEC 61439, and supplies grid, industrial, and infrastructure projects worldwide.
