In this article7 sections
A single line diagram (SLD), also called a one-line diagram, is the electrical drawing that shows how power flows from the supply to each load in a facility, with each three-phase circuit drawn as one line. It starts at the utility incomer or the generator, runs through the transformers, switchgear, motor control centers (MCCs) and distribution boards, and ends at the motors and feeders. Each item carries a tag, a rating and the protection that clears a fault on it. The second and third phases, the control wiring and the physical layout are left off, so the sheet shows topology, ratings and protective devices and little else.
That makes the SLD the first drawing to open when the job is building the electrical equipment register a CMMS is loaded from. It answers three of the register's questions directly, which items exist, what feeds each one, and what each one is rated for. Serial numbers, models, install dates and maintenance plans come from nameplates, vendor data and the site.
Single line diagram, three-line diagram and schematic compared
An electrical drawing set describes the same equipment four ways, and the four are easy to confuse when a document register files them under one discipline code.
| Drawing | How a three-phase circuit is drawn | What it carries | Main use |
|---|---|---|---|
| Single line diagram (one-line) | One line per circuit | Equipment, ratings, protection device numbers, current and voltage transformer (CT and VT) ratios, cable references, normally open points | Operations, switching, studies, the equipment register |
| Three-line diagram | One line per phase, plus neutral and earth | CT and VT connections phase by phase, polarity marks, relay and meter inputs | Protection and metering wiring, commissioning |
| Schematic (elementary) diagram | Control circuits drawn as rungs between two supply rails | Coils, contacts, interlocks, trip and close circuits | Control logic, fault finding |
| Wiring (connection) diagram | Physical terminals and conductors | Terminal numbers, wire numbers, cable cores | Construction and termination |
An SLD cannot show which phase a CT sits on or how its secondary reaches the relay, which is what the three-line diagram is for. The schematic shows the contact chain behind a breaker trip. For the register, the SLD supplies the equipment rows and ratings, and the other drawings supply relay, panel and cable detail.
Larger sites split the SLD into a key one-line for the main buses and transformers and detailed one-lines per switchboard or MCC.
Reading a single line diagram from the source down
Power flows down the sheet on a typical industrial SLD. Read it top to bottom and write down the fed-from item for each piece of equipment as you pass it.
Start at the source. On an industrial site the top of the sheet is usually a utility incomer at the point of connection, with revenue metering and the incoming breaker, and often an on-site or standby generator beside it. The utility's available fault level is frequently written at the incomer, in kA or MVA at the supply voltage.
Read the main transformers. A main transformer steps the supply down to the plant distribution voltage. Its label carries the rating, voltages, vector group (Dyn11 on many IEC drawings), impedance and cooling class. Check how the secondary neutral is earthed, solidly, through a resistor or not at all. A neutral earthing resistor is a maintainable item with its own current and time rating.
Read each bus. A switchgear or switchboard bus is drawn as a heavy horizontal line, with incomers above it and feeders below. The bus label carries the voltage, the continuous current rating and the short-circuit rating. If two buses are joined by a bus-tie breaker, look for the normally open (NO) mark. A main-tie-main lineup with a normally open tie gives each bus a normal source and an alternate.
Follow each feeder. A feeder breaker below the bus goes to a load or to the next level down, a smaller transformer, an MCC, a distribution board or a large motor. Medium-voltage motors usually take a dedicated switchgear feeder with their own protection relay. Low-voltage motors sit in MCC buckets, each with a starter, an overload and a short-circuit device.
End at the loads. Motors, heaters, UPS units, battery chargers, capacitor banks, VFDs and small-power boards sit at the bottom. Small loads may appear only as a numbered feeder with a description, and the load list or the MCC schedule carries their detail.
Mark the alternate paths last. An automatic transfer switch, a pair of interlocked breakers or a generator connection gives a load a second source. Record the normally closed path as the fed-from value and note the alternate beside it. A register that records both as parents shows one motor under two switchboards.
Common single line diagram symbols and device numbers
IEC 60617 covers graphical symbols on IEC-based projects, and IEEE 315 (also issued as ANSI Y32.2) covers them on North American ones. Relays on both carry the device function numbers of IEEE C37.2. The project legend sheet overrides all of them, so read it before the drawing.
| Item | How it is drawn | What to read beside it |
|---|---|---|
| Circuit breaker | IEC, a switch contact with a cross at the fixed contact. ANSI, a square marked 52 for medium voltage, a short arc between two terminals for low voltage | Frame and trip rating, breaking capacity in kA |
| Disconnector (isolator) | IEC, a switch contact with a bar across the fixed contact. ANSI, device 89 | Rated current |
| Contactor | IEC, a switch contact with a semicircle at the fixed contact | Starter type and size |
| Fuse | A small rectangle on the line, or an S-shaped curve on older ANSI sheets | Fuse rating in A |
| Transformer | Two overlapping circles (IEC) or two coil symbols (ANSI) | kVA or MVA, voltages, vector group, impedance |
| Current transformer | A small ring or loop on the conductor | Ratio, such as 600/5 |
| Motor or generator | A circle with M or G | Tag, kW, hp or kVA, voltage |
| Earth (ground) | Three horizontal lines of decreasing length | Earthing method, resistor rating |
Device numbers seen on most industrial SLDs are listed below.
| Device number | Function in plain words | Usually found on |
|---|---|---|
| 52 | AC circuit breaker | Each breaker, medium and low voltage |
| 50 / 51 | Instantaneous and time-delayed overcurrent | Incomers, feeders, transformer primaries |
| 50N, 51N, 51G | Neutral or ground overcurrent, the earth-fault elements | Feeders, transformer neutrals |
| 87 with a suffix | Differential protection, 87T transformer, 87B bus, 87M motor, 87G generator | Large transformers, main buses, large motors, generators |
| 86 | Lockout relay, keeps the breaker tripped until it is reset | Behind differential and other serious trips |
| 27 / 59 | Undervoltage and overvoltage | Incomers, buses, motor feeders |
| 81 | Under or over frequency | Generator and utility interfaces |
| 49 | Thermal overload of a machine or transformer | Motors, transformers |
| 25 | Synchronism check | Points where two sources can be paralleled |
A slash joins functions inside one relay, so 50/51 is one device. A numerical relay with six numbers beside it is still one relay and one maintainable item. Lowercase a and b after 52 (52a, 52b) mark breaker auxiliary contacts, which belong on the schematic. The IEEE C37.2 device number reference covers the rest.
The data each item carries
Each abbreviation on an SLD label is a register field.
- Transformers. Tag, kVA or MVA for each cooling stage (ONAN/ONAF on IEC drawings, OA/FA on older North American ones), primary and secondary voltage, vector group, impedance, tap range and neutral earthing.
- Switchgear and switchboards. Tag, voltage, bus continuous current, short-circuit rating in kA for a stated duration, and the sections, incomers and feeders.
- Breakers. Device or cubicle number, frame rating, trip rating, breaking capacity, drawout or fixed, and the relay or trip unit functions that open it. North American low-voltage breakers are often labelled 800AF/600AT, an 800 A frame carrying a 600 A trip. Keep both values, because the frame decides the spare part and the trip decides the protection.
- MCCs. Tag, voltage, horizontal bus rating, short-circuit rating, and the bucket list with starter type (direct-on-line, reversing, star-delta, soft starter or VFD), starter size and motor rating.
- Motors. Tag, kW or hp, voltage, sometimes full load current, and the driven equipment.
- Cables. Cable tag, conductor size and core count, written along the feeder line. Length and route sit on the cable schedule.
- Protection and metering. Device numbers, CT and VT ratios, and the meters on each incomer and feeder.
- Fault levels. The prospective short-circuit current at each bus, in kA or MVA.
Fault levels and protection settings are outputs of short-circuit and protection studies. The register records them as drawn, with the study revision, and revising them is work for the engineer responsible for the study.
From the single line diagram to the equipment register
A CMMS register takes two things from the SLD, the list of maintainable items and the hierarchy that places them. The hierarchy follows the power path, from site to substation to switchboard, then cubicle or bucket, then the item. SAP PM functional locations and Maximo locations both map onto those levels. The P&ID to CMMS article covers the split between a location and an equipment record, which applies here unchanged.
| SLD item | Register treatment | Fields the SLD supplies | Fields from elsewhere |
|---|---|---|---|
| Transformer | Equipment row | Tag, kVA or MVA, voltages, vector group, impedance, cooling class, fed-from | Manufacturer, serial, oil volume, install date |
| Switchgear or switchboard | Parent row or location | Tag, voltage, bus rating, short-circuit rating, sections | Manufacturer, enclosure rating, room |
| Breaker or cubicle | Child row under its switchboard | Cubicle number, frame and trip, breaking capacity, drawout or fixed, load fed | Serial, trip unit model, settings record |
| Protection relay | Child row, or an attribute of its breaker | Device numbers, CT and VT ratios | Model, firmware, setting revision, last test |
| MCC | Parent row or location | Tag, voltage, bus rating, short-circuit rating | Manufacturer, section layout, room |
| MCC bucket or starter | Child row under its MCC | Bucket position, starter type and size, load tag | Overload setting, contactor model |
| Motor | Equipment row | Tag, kW or hp, voltage, fed-from bucket, driven equipment | Frame size, speed, bearings, serial |
| Generator, UPS, VFD, capacitor bank | Equipment rows | Tag, rating, voltage, fed-from | Model, battery data, maintenance plan |
| Cable | Attribute of its feeder, or its own record | Cable tag, size, cores | Route, length, install date |
The fed-from field is the column the SLD is best placed to supply. It names the upstream item by tag, so a planner reading a motor row finds the bucket and the MCC that supply it without opening the drawing. Spare buckets, equipped and unused, get rows marked spare, so a later modification can claim one without a new position number.
Equipment class comes from the owner's taxonomy, and ISO 14224 is a common basis for it. Tags follow the owner's numbering, which may be a project series (M-301 for the motor driving pump P-301), a device number joined to a bus reference (52-MV-01) or an IEC 81346 reference designation (=E1+Q1-F1). Hold each tag as printed, with the drawing number and revision it came from.
Stale single line diagrams and as-built drift
NFPA 70E (Article 205.2) requires a single line diagram, where one is provided, to be kept legible and current. On an operating plant the switchboard changes first and the drawing follows, sometimes years later. The gaps found while building a register tend to fall into five patterns.
As-built drift. Typical drift is a transformer replaced with a different rating, a trip unit swapped for a larger one, a motor uprated during a turnaround or a new load landed in a spare bucket. Compare the SLD with nameplates and the latest MCC schedule, and treat the as-built drawing as a claim to check.
Revision gaps. A site with a key one-line and twenty detailed sheets rarely holds them all at one revision. A tie breaker added on a detailed sheet and missing from the key sheet produces two hierarchies for one bus. With the drawing number and revision on each row, a conflict traces back to its sheet.
Missing ratings. Values written as TBA, by vendor or hold, typical-of notes that give one rating to a row of identical feeders, and blank bucket sizes are common on drawings issued before vendor data arrived. Leave the field empty with the reason recorded, so the gap stays visible.
Disagreement between documents. The MCC schedule, the load list, the cable schedule and the relay setting records each repeat part of the SLD. If two disagree on a rating, record both with their sources and leave the row open for the site to confirm.
Brownfield marks. Revision clouds, existing and new labels, and dashed future equipment are normal on modification drawings. Future equipment gets a register row once it is installed, and removed equipment keeps its row with a removed status so its history stays attached.
Checks before the register is loaded
Run these on the draft register before it goes into the CMMS.
- Each row has one fed-from item, and that item is itself a row.
- Each motor on the SLD appears on the MCC schedule and the load list with the same rating and voltage.
- Each bus with a normally open tie or a transfer switch records a normal source and an alternate.
Tagsight returns an SLD as an Excel workbook listing the transformers, breakers, motors, panels and feeders on the drawing, each with its rating, source, destination and protection device.