Selecting medium-voltage switchgear is mostly a matter of fixing five numbers before you look at any catalogue. Get those right and the choice narrows itself. Get one wrong and you either pay for capability you will never use, or discover at commissioning that the equipment cannot be connected. This guide works through the five, then the choices that follow from them, so the specification you send out is one suppliers can answer without guessing.
The Five Numbers That Decide the Selection
Everything else is detail. Settle these first, in this order:
- Rated voltage – the system voltage at the point of connection, and the highest voltage the equipment will see. The insulation level follows from this, not from the nominal figure.
- Rated current – the continuous current the busbars and incoming circuit must carry, derived from the load list with diversity applied, not from a round number.
- Short-circuit withstand – the prospective fault current at the point of connection, and the duration it must be withstood for. This is the number that most often changes the selection.
- Busbar arrangement – single busbar, sectionalised, or a double or duplicate arrangement, which is a reliability decision as much as a technical one.
- Enclosure and installation – indoor or outdoor, the ingress protection required, and whether the equipment sits in a switchroom, a compact substation or a dedicated enclosure.
If you can state those five, a supplier can tell you what they propose and why. If you cannot, the quotation you get back is a guess dressed up as a price.
Insulation Medium: Air, Gas or Solid
The choice of insulation medium is where the layout and the footprint get decided:
| Air-insulated | Gas-insulated | Solid-insulated | |
|---|---|---|---|
| Footprint | Largest | Smallest | Small |
| Typical use | Switchrooms with space | Tight sites, high ratings | Compact and outdoor units |
| Maintenance access | Straightforward | Requires specialist handling | Limited, by design |
| Best fit | Standard indoor installations | Where space is the constraint | Prefabricated and pad-mounted arrangements |
Footprint, not preference, usually decides this. A gas-insulated switchboard inside a compact substation buys space you do not have; the same switchboard in a roomy switchroom adds cost and specialist maintenance for no benefit. The medium-voltage switchgear range we build covers the arrangements that suit prefabricated units and switchrooms alike.
Fixed or Withdrawable
This is about how maintenance is done, not about how well the equipment works. Fixed units are simpler and cheaper; withdrawable units let you take a circuit out of service and replace it quickly, which matters where downtime is expensive or where spare circuits are part of the plan. If your site can tolerate a planned outage to work on a fixed unit, fixed is usually the better value. If it cannot, withdrawable pays for itself the first time you need it.
Protection and Control
The switchgear is only as useful as the protection and control it carries. Decide early whether you need local indication only, or whether the unit has to report back to a supervisory system. That decision fixes the relays, the current transformers, the wiring to the control compartment, and whether the equipment needs communication interfaces and the associated testing. It is much cheaper to settle this at the drawing stage than to retrofit it.
Routine Tests and What They Tell You
Routine testing happens on the finished assembly and is where the specification is either met or not. Expect power-frequency withstand tests on the main circuits, measurement of resistance or impedance on the main circuit, and a functional test of the mechanical operation, interlocks and protection. The reports travel with the equipment. The full sequence we follow is on the testing page.
Common Selection Mistakes
Three come up often enough to be worth naming. The first is sizing the short-circuit rating from a rule of thumb rather than the actual fault level at the point of connection, which is either unsafe or needlessly expensive. The second is choosing an arrangement for the initial load only, with no allowance for the expansion the site plan already shows. The third is treating protection and control as an afterthought, then discovering the communication requirement was in the specification all along. All three are settled on the drawing, at the point where they are still cheap to fix.
Standard and Scope
The applicable standard and its clause set are agreed against your project specification before production begins. Where a project names IEC 62271-200 for the switchgear assembly, or IEC 62271-202 for a prefabricated enclosure, we work to those clauses and issue the routine test report with the equipment. If your specification is silent on the standard, that is settled in writing first rather than assumed. How we handle this is described on the quality control page.
Frequently Asked Questions
What is the first thing to decide when selecting MV switchgear?
The five ratings: rated voltage, rated current, short-circuit withstand, busbar arrangement, and enclosure with installation. Everything else, including the insulation medium and whether units are fixed or withdrawable, follows from those.
Should I choose air-insulated or gas-insulated switchgear?
It is usually decided by footprint rather than preference. Gas-insulated equipment buys space in tight sites and prefabricated units; air-insulated equipment is simpler to maintain and better value where a switchroom has room. The short-circuit rating can also push the decision toward gas.
How do I know what short-circuit rating I need?
From the prospective fault current at the point of connection, and the duration it must be withstood for. That comes from the system study or the utility, not from a rule of thumb. Sizing this wrongly is the most expensive common mistake.
Does the switchgear need a communication interface?
Only if the unit has to report to a supervisory system. Deciding this early fixes the relays, current transformers and wiring, and it is far cheaper to settle at the drawing stage than to retrofit after the equipment is built.
Where do the routine test values come from?
From the standard the project names and the specification for the unit. Power-frequency withstand, main-circuit resistance or impedance, and functional testing of operation, interlocks and protection are agreed before production and the reports are issued with the equipment.
Get a Drawing and a Price
Send the single-line diagram, the load list and the site conditions – altitude, ambient range, indoor or outdoor, enclosure rating. We come back with a proposed arrangement, the standard set we would work to, and a quotation. Email info@xsdfftransformer.com or call +8615867897761.