15 kV Switch Types in Medium-Voltage Switchgear
Medium-Voltage Load-Interrupter Switches
Think of a medium-voltage load-interrupter switch as a heavy-duty shutoff valve on a city water main. The valve controls normal flow and lets workers to isolate part of the system. In the same way, a switch connects, disconnects or isolates the normal flow of medium-voltage electricity.
An arc forms when energized contacts separate. The switch uses an arc-control system and quick-make, quick-break mechanism to interrupt its rated load current. Stored-energy mechanisms make the switching action fast and consistent.
A load-interrupter switch isn’t designed to clear high-level fault current by itself. That’s done by properly rated power fuses or a relay-controlled circuit breaker.
15 kV switches for medium voltage power distribution systems control power, isolate equipment into manageable parts.
Understanding the 15 kV Switchgear Equipment Class
The term 15 kV switchgear describes an equipment class, not necessarily a system operating at exactly 15,000 volts. This equipment is applied on nominal systems such as 4.16 kV, 12.47 kV and 13.8 kV.
Voltage is only one part of the specification. You’ll also need to consider continuous current, load-interrupting current, fault-closing capability, maximum fault current and basic impulse level, or BIL. BIL indicates the ability of the equipment’s insulation system to withstand short-duration voltage surges.
Equipment ratings need to match system voltage, available fault current, grounding method, connected load and intended operating sequence. These requirements should be set through system studies and an accurate one-line diagram.
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15 kV Fused Switches and Fault Protection
A 15 kV fused switch combines a load-interrupter switch with medium-voltage fuses. The switch controls normal power flow and isolates the circuit, while the fuses protect against overcurrents and short circuits.
It’s a simpler alternative to using a circuit breaker with protective relays and instrument transformers. The fuses need to be properly sized for the transformer’s normal current, startup inrush, overload limits and available fault current. An undersized fuse might open when the transformer starts, while an oversized fuse might not provide enough protection.
Medium-Voltage Disconnect and Isolation Switches
A medium-voltage disconnect switch provides an isolation point between an electrical source and the equipment being serviced. Visible-open construction lets personnel confirm the physical switch position as part of an established safety procedure.
You shouldn’t assume every disconnect can interrupt current. A load-break disconnect can open its rated load current. A non-load-break disconnect must only be operated after another device has removed the load.
Safety note: visible isolation doesn’t replace lockout/tagout, absence-of-voltage testing, protective grounding or other required electrical-safety practices.
Main and Tie Switches in Metal-Enclosed Switchgear
In 15 kV metal-enclosed switchgear, the main switch controls power to the lineup from a utility, generator or upstream. The main may be fused or unfused, depending on where fault protection is.
A tie switch connects or separates two bus sections. In main-tie-main switchgear, two main switches normally supply separate sections while the tie remains open. If one source becomes unavailable, an approved transfer sequence lets the other source serve both sections.
Medium-Voltage Feeder Switches
A medium-voltage feeder switch controls an outgoing circuit supplying a transformer, motor, separate building or another switchgear lineup. Multiple feeders let you isolate one circuit without de-energizing every load connected to the bus.
Feeder switches may be fused or protected elsewhere in the system. Selection should consider the connected load, cable characteristics, switching duty, available fault current and coordination with upstream and downstream devices.
Transformer-Primary Switch Applications
A transformer-primary switch is installed on the medium-voltage side of a transformer. It allows you to energize, de-energize and isolate that transformer without opening the entire upstream system.
Fused load-interrupter switches are widely used in this position. The switch performs normal transformer switching and isolation, while the fuses respond to qualifying overcurrent and short-circuit conditions.
Coordination between the primary fuses and secondary main breaker is important. When system conditions permit, a downstream breaker should clear a secondary fault before the primary fuse operates.
Fused Switch Versus a 15 kV Circuit Breaker
A fused switch is practical when you need load switching, isolation and straightforward fault protection. A 15 kV circuit breaker, however, can interrupt both normal current and fault current within its ratings.
Protective relays monitor current, voltage and other electrical conditions, then command the breaker to trip when they identify a qualifying fault. Circuit breakers can provide adjustable protection, frequent switching, remote operation, event recording, communications or advanced transformer, generator and bus protection.
Your decision shouldn’t be based on initial price. Consider switching frequency, coordination requirements, maintenance resources, automation needs and outage consequences.
Specifying Engineered 15 kV Switchgear
When specifying engineered 15 kV switchgear, begin with an one-line diagram. Define the nominal voltage, grounding method, continuous load, available fault current and equipment served by every feeder.
Identify the required main, tie and feeder arrangement; fused or unfused positions; manual or motor operation; interlocking; metering; control power; communications; cable-entry direction and future expansion provisions.
Indoor and outdoor installations can have different enclosure, heating and environmental requirements. Cable-bending space, access to fuses and terminations, and compartment arrangement affect installation and long-term maintainability.
BCS Switchgear can coordinate main, tie, feeder and transformer-primary sections for permanent or temporary power applications. By treating the lineup as part of the complete distribution system, you’ll receive equipment designed around your loads, installation requirements and operating sequence.
| Term | Definition |
|---|---|
| 15 kV class | An equipment voltage and insulation class commonly applied to systems such as 4.16 kV, 12.47 kV and 13.8 kV. |
| BIL | Basic impulse level, indicating the insulation’s ability to withstand short-duration voltage surges. |
| Continuous-current rating | The amount of current equipment can carry continuously under its specified operating conditions. |
| Fault-closing rating | The switch’s ability to close safely onto a circuit carrying a specified level of fault current. |
| Feeder switch | A switch that controls an outgoing medium-voltage circuit supplying a transformer, motor, building or other equipment. |
| Interlock | A mechanical, electrical or key-based device that helps prevent an unsafe operating sequence. |
| Load-interrupter switch | A switch designed to open and close a circuit carrying normal load current within its specified rating. |
| Main switch | The switch that controls incoming power to a bus or medium-voltage switchgear lineup. |
| Power fuse | A medium-voltage fuse designed to interrupt qualifying overcurrent or short-circuit current. |
| Tie switch | A switch used to connect or separate two bus sections for load transfer, sectionalizing or maintenance flexibility. |
| Transformer-primary switch | A switch installed on the medium-voltage side of a transformer to provide switching and circuit isolation. |
15kV Switch for Power Distribution FAQ
What does 15 kV class mean?
What does a 15 kV load-interrupter switch do?
Can a 15 kV load-interrupter switch clear a short circuit?
What is the difference between a fused switch and an unfused switch?
Why are fused switches used on transformer primaries?
What does a tie switch do in 15 kV switchgear?
When should a circuit breaker be used instead of a fused switch?
Can 15 kV switches be operated remotely?
BCS Switchgear is an industry leader in new and obsolete electrical control and distribution equipment. Since 1997, BCS has been servicing customers to extend electrical life of low and medium voltage electrical power equipment.
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