Reading a single line diagram to scope a switchroom or substation
A single line diagram (SLD) shows an electrical installation as one line per circuit: where the power comes in, how it is transformed, which switchgear it passes through, how it is protected and metered, and where it goes. For a tender it is the fastest route to the scope of a switchroom or substation, because almost everything that drives the price is on it: the number of tiers and panels, the voltage levels, the fault rating, the protection scheme and the incomers and feeders the switchboard has to carry.
What the SLD does not show is also what you need to know before you price. It carries no dimensions, no cable lengths and no room layout, so the SLD gives you the equipment count and the general arrangement gives you the building. Read in the order below, the diagram yields a scope list in under an hour. Reading the SLD against the specification and the equipment list, and returning the counts and ratings with the drawing reference beside each, is work Elora Grid does; what to allow for the parts the drawings leave out stays with your estimator.
What does a single line diagram actually show?
One line for each circuit, regardless of how many conductors it carries, drawn from the source at the top to the loads at the bottom. Along that line sit the symbols that cost money: transformers with their ratio and rating, circuit breakers with their rating, current transformers, relays, meters, busbars with their fault level, and the loads at the end.
The convention is what makes it readable quickly. A busbar is a thick horizontal line. Everything hanging off it is a circuit. A tie between two bus sections means a split board and probably an interlocking scheme. Dashed boxes mark what sits in one panel or one enclosure, which is where the panel count starts.
Where do you start reading?
At the top, with the supply. Who provides it and at what voltage, and through what: a network connection, a dedicated transformer, a generator, or a combination with a changeover. That decides whether you are pricing a low voltage switchroom, a high voltage substation under AS 2067, or both, and whether there is embedded generation to interface with.
Then follow the line down through each transformation and each bus. At every busbar, note three things: the voltage, the fault level in kA, and the number of ways on it, used and spare. Those three numbers choose the switchgear family, the form of construction and the price per tier before you have read a single specification clause.
What on the SLD drives the switchboard price?
The counts and the ratings. Every circuit breaker symbol is a tier or a module; the rating beside it and the fault level of the bus it sits on set the frame size. Every CT and relay symbol is a protection scheme, and the relay type written on the diagram (overcurrent and earth fault, differential, restricted earth fault) sets the relay cost and the testing time. Metering, surge protection, power factor correction and the neutral earthing resistor on a high voltage system each appear as a symbol and each is a line in the estimate.
What the diagram does not say is how the board is built. Form of separation under AS/NZS 61439, ingress protection, access from the front or rear, and whether the client wants withdrawable or fixed pattern breakers live in the specification. Tick them off against the diagram as you go, because a Form 4 board with the same single line as a Form 2 board can cost half as much again.
What is not on the SLD, and where is it?
Dimensions, routes and the room. The diagram tells you there are forty feeders; the general arrangement tells you whether the board fits through the door, how much floor is left for the future tiers the diagram marks as spare, and where the cable trench runs. Cable sizes and lengths, which usually outweigh the switchgear on a substation job, come from the cable schedule and the site layout, and are sized under AS/NZS 3008.1.1 against the route, not against the diagram.
Earthing, lighting, small power, fire detection and the switchroom's own services never appear on a power SLD. On a greenfield substation they are a separate scope that is easy to assume the civil contractor has, and on a brownfield one they are the items most likely to clash with what is already in the room.
How does this become a scope list?
Work down the diagram once with a blank table beside it, one row per symbol, and record what the symbol is, its rating, the bus it sits on and the drawing reference. By the bottom of the page you hold the equipment count: transformers, incomers, feeders, ties, relays, meters, spare ways. Set the specification next to it and add the construction requirements the diagram cannot show, then set the general arrangement next to that and add the room, the cable routes and the services.
The gaps between those three documents are where the clarifications come from. A feeder on the SLD with no load on the GA, or a spare way count the two documents disagree on, is a question to the client before close, or a qualification if there is no time. Elora Grid reads the SLD, the specification and the equipment list together and returns the counts, the ratings and each mismatch cited to its drawing; what to price against the mismatches is your call.
What the single line diagram gives you, and what it does not
| You need | On the SLD? | Where it comes from |
|---|---|---|
| Voltage levels and supply arrangement | Yes | Top of the diagram: source, transformers, changeover |
| Fault level and bus rating | Yes, if annotated | Bus labels; otherwise the specification or a fault study |
| Number of tiers, incomers, feeders and spares | Yes | One breaker symbol per circuit; dashed boxes group panels |
| Protection scheme and relay types | Yes, by symbol and code | Relay and CT symbols; the specification for make and model |
| Metering, surge, power factor correction, NER | Yes, as symbols | Each is its own line in the estimate |
| Form of separation, IP rating, access | No | The switchgear specification (AS/NZS 61439) |
| Board dimensions, room fit, future space | No | The general arrangement and the switchroom layout |
| Cable sizes, lengths and routes | No | The cable schedule and the site layout (AS/NZS 3008.1.1) |
| Earthing, lighting, fire and small power | No | Separate drawings, often a separate scope |
- 01Find the source. Read the top of the diagram: supply voltage, network or generator, any changeover. This sets the standard the job falls under.
- 02Note every busbar. Voltage, fault level, ways used and spare. Three numbers that choose the switchgear family.
- 03Count the symbols. One row per breaker, CT, relay, meter and transformer, with its rating and the drawing reference.
- 04Read the specification against the count. Form of separation, IP rating, access, fixed or withdrawable, relay make. None of it is on the diagram.
- 05Read the general arrangement against both. Room fit, spare floor, cable routes, services already in the room.
- 06List the mismatches. Every disagreement between the three documents is a clarification to send or a qualification to write.
Common questions
What is a single line diagram?
A single line diagram is a simplified drawing of an electrical system that represents each circuit with one line, whatever the number of conductors, and shows the sources, transformers, switchgear, protection, metering and loads along it. It is the standard way to show how power flows through an installation and is the first drawing an estimator reads when scoping a switchroom or substation.
Can you price a switchboard from the single line diagram alone?
You can count it, not price it. The diagram gives the tiers, ratings, fault level and protection, which fix the switchgear family and the panel count. The form of separation, ingress rating, access and breaker type come from the specification, and the dimensions and cable routes come from the general arrangement. Price from all three, or the estimate carries what the other two documents would have told you.
Which Australian standards apply to a switchroom or substation tender?
AS/NZS 3000 for the low voltage installation, AS/NZS 61439 for the switchgear assembly and its form of separation, AS 2067 for substations and high voltage installations above 1 kV, and AS/NZS 3008.1.1 for cable selection. The tender specification names which editions apply and adds the client's own requirements on top; read it beside the diagram rather than instead of it.
Can Elora Grid read the single line diagram for you?
It reads the SLD together with the specification and the equipment list, and returns the equipment count, the ratings and every mismatch between the documents, each cited to the drawing and clause it came from. It also drafts a first general arrangement from the equipment list for your engineers to check. What allowance to carry for the items the drawings leave out, and the price itself, stay with your team.