A medium-voltage single-line diagram (SLD) is a functional map of the primary power system. It compresses a three-phase installation into one line so that an engineer can understand sources, busbars, switching devices, instrument transformers, feeders and earthing points without reading every secondary circuit. A good SLD review is the first step in switchgear selection, protection design, interlocking and factory testing.

Learning objectives

After studying this guide, you should be able to identify the function of each MV bay, trace every normal and fault-current path, distinguish isolation from interruption, and recognise which details must be confirmed in schematics, data sheets or the operating philosophy.

1. Establish the system boundary

Start with the declared system voltage, frequency, earthing method and short-circuit level. Then mark the physical boundary of the switchboard. Equipment drawn upstream or downstream may belong to the network, transformer, cable system or customer installation rather than to the metal-enclosed assembly itself. This distinction matters because IEC 62271-200 applies to the MV assembly, while cables, transformers and the wider installation have their own requirements.

2. Identify sources, busbars and operating sections

Locate every possible source: utility incomer, transformer secondary, generator, grid-forming converter or tie from another board. Follow each source to the busbar. A horizontal line normally represents a bus section, but the drawing convention must be checked. Sectionalised busbars are separated by a bus-coupler or bus-section bay. A double-bus arrangement requires selection and isolation devices that cannot be understood from the number of horizontal lines alone.

  • Single busbar: simple and compact, but a bus fault or maintenance can affect the complete section.
  • Sectionalised busbar: improves operational flexibility and can limit the impact of a fault if the protection and operating philosophy are coordinated.
  • Bus coupler: connects two live or dead sections under defined synchronism, voltage and interlock conditions.
  • Transfer or double busbar: provides additional routing options but increases switching complexity and the risk of an incorrect sequence.

3. Classify each bay by function

A bay should be named from what it does in the power system, not merely from the device installed in it.

  • Incomer: receives power from an external source and normally contains the main protection, metering and isolation interfaces for that source.
  • Outgoing feeder: supplies a cable, transformer, motor, capacitor bank or another switchboard. The protected object determines the relay functions and switching duty.
  • Bus coupler or section: connects bus sections and must be coordinated with source availability, synchronism-check and interlocking logic.
  • Metering bay: houses voltage transformers or sensors and their isolation, protection and secondary distribution.
  • Bus-riser bay: provides a physical or electrical connection between bus levels or sections without necessarily containing a switching device.
  • Earthing or cable-test function: provides a controlled connection to earth or a test interface and must never be inferred from a generic switch symbol.

4. Trace the primary current path

For each bay, trace the path from busbar to cable terminal. Record the order of the circuit-breaker or switch, disconnector function, current transformers, voltage sensors, surge arresters and earthing switch. Device order affects the protected zone, accessible isolation points, test method and whether a component remains energised in a given state.

Repeat the exercise for fault current. Ask where current returns through the earthing system, which CTs measure it, which relay trips it and which breaker must interrupt it. A drawing that shows the power path but not the measurement and trip boundary is not enough for protection engineering.

5. Distinguish switching functions

A circuit-breaker is selected to make, carry and interrupt declared normal and fault duties. A disconnector establishes an isolating distance but is not automatically capable of interrupting load current. An earthing switch connects the isolated circuit to earth and may have a rated short-circuit making capability. A load-break switch interrupts defined load currents, while a contactor is optimised for frequent operating duty. Similar-looking symbols must not be treated as equivalent equipment.

6. Read CT, VT and protection information

Check CT ratios, cores, accuracy classes, knee-point or transient requirements where applicable, and the destination of each secondary circuit. For VTs, identify primary connection, secondary windings, open-delta circuits, fuses or MCBs and the earthing point. Relay function numbers such as 50/51, 50N/51N, 67, 87 or 27/59 describe functions, not a complete protection scheme. The trip destination, blocking logic, breaker-failure initiation and DC supply must be confirmed in secondary drawings.

7. Convert the SLD into an operating-state model

List the permitted states of the breaker, removable part, disconnector and earthing switch. Then define transitions between states. For withdrawable switchgear, typical positions are service, test and disconnected, but the exact construction is manufacturer-specific. A valid SLD review asks whether closing onto an earthed circuit, racking a closed breaker or accessing a live compartment is prevented by design and verified by test.

Worked example

Consider two transformer incomers feeding two bus sections with a normally open coupler. During normal operation each transformer supplies one section. If one transformer is lost, an automatic transfer scheme may open the failed incomer and close the coupler after voltage, interlock and source-capacity checks. The SLD reveals the possible power route; it does not by itself prove that the surviving transformer can carry both sections or that the protection remains selective. Those questions require load-flow, short-circuit, protection and operating-philosophy studies.

Common review errors

  • Assuming every vertical line is a cable feeder.
  • Confusing a bus coupler with a bus riser.
  • Reading nominal system voltage as the highest voltage for equipment.
  • Ignoring normally open points and alternative sources during fault studies.
  • Assuming an ANSI/IEEE device number defines the CT connection or trip logic.
  • Using the SLD as proof of interlock implementation without checking detailed schematics.

Engineering review checklist

  • Are all sources, normal open points and bus sections identified?
  • Is every bay function unambiguous?
  • Can the normal, transfer and fault-current paths be traced?
  • Are device ratings consistent with voltage, load and short-circuit duties?
  • Are CT/VT locations consistent with protection zones and metering boundaries?
  • Are earthing points and cable-test arrangements shown?
  • Are interlocks and automatic sequences defined in a separate approved philosophy?
  • Do tag names match schematics, layouts, relay files and FAT documents?

Standards basis and further reading

The principal documentation reference is IEC 61082-1 for preparation of electrotechnical documents. Graphical symbols should be selected from the IEC 60617 database. Equipment terminology and assembly boundaries should be coordinated with IEC 62271. ANSI/IEEE function numbers are covered by IEEE C37.2-2022.

Engineering note: This article explains a review method; it does not replace the contracted drawings, current standard editions, manufacturer instructions or project studies.

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