Site documentation and the single-line diagram

A live facility is understood through its documents. Nobody can see a cable buried in a duct bank or a breaker's position inside a closed switchboard; the drawings, schedules and records say where everything is and how it connects. This lesson covers the documents a data centre technician uses most, how to read a single-line diagram, and how documents are kept true to the building. It is about reading and maintaining documents, not designing systems: the engineering of the power system belongs to the engineers, and to the DCE school, especially DCE-310 and DCE-340.

The documents of a facility

Stonebridge's operations documents fall into a few families:

• Drawings: architectural floor plans, electrical single-line diagrams and riser diagrams, mechanical flow diagrams for the chilled water and coolant systems, fire protection and security layouts, and cable pathway drawings. Each has a number, a revision and a title block.
• Schedules and lists: panel schedules listing every breaker and what it feeds, equipment lists with ratings, valve lists, point lists for the BMS and EPMS, and the alarm database (lesson 4).
• As-built records: drawings and schedules updated after construction or changes to show what was actually installed, which often differs from the design.
• Equipment manuals and data sheets: the makers' instructions for operation and maintenance.
• Procedures: MOPs, SOPs and EOPs (lesson 10).
• The DCIM model (lesson 5), which holds much of the same information in database form and links it to live data.

These overlap, and the overlaps are where errors hide: a breaker that the panel schedule says feeds A-31-03, the drawing says feeds A-31-04 and DCIM says feeds both. One of the desk's quiet jobs is to notice such disagreements and get them resolved.

The single-line diagram

The single-line diagram (SLD), also called a one-line diagram, shows a power system's connections using one line for each three-phase circuit, with symbols for the equipment. It is the most important drawing in a data centre, because it is how everyone, from the electrician opening a breaker to the shift lead assessing an outage, understands which path power takes and what depends on what. The EPMS one-line overview on the desk wall is a live version of it.

Reading an SLD starts at the top, where power enters, and follows the lines downward:

• Sources: utility incoming lines, generators, and sometimes solar or battery systems, at the top or the edges.
• Transformers: two overlapping circles, with the voltages on each side.
• Buses: thick horizontal lines in switchgear and switchboards, to which several circuits connect.
• Breakers and switches: small boxes or symbols on each line, shown open or closed in their normal state, with their names and ratings.
• Transfer switches: symbols connecting two sources to one load.
• UPS systems, PDUs and RPPs: boxes with their names and ratings.
• Loads: at the bottom: floor PDUs, mechanical panels, and ultimately racks.

Each piece of equipment carries the same name on the SLD as on its label and in DCIM, which is why consistent naming (DCT-130, DCT-220) matters so much. A normally open breaker, such as SUB-1's tie breaker between the two medium-voltage buses (DCT-220 lesson 2), is drawn open; anyone reading the drawing can see at a glance which paths are normally live and which are standby.

Reading the SLD for a task

Technicians use the SLD for three common questions. Upstream: what feeds this equipment, and what would have to fail or be switched for it to lose power? Downstream: what depends on this equipment, and what would lose power if it were opened? Redundancy: is this load fed by two independent paths all the way up, or do the paths join somewhere? The DCIM connection trace (lesson 5) answers the same questions from its database, and comparing the two is a good check on both.

The third question matters most in planning work. Two paths that look separate at the rack may meet higher up: two PDUs fed from the same UPS, or two UPS systems fed from the same switchboard. Such a shared point is a single point of failure, and an SLD is the clearest way to find one. Stonebridge's design keeps the A and B sides separate from the rack up to the unit substations, and the SLD shows it.

# Trace upstream from a rack feed through a simplified SLD and check that A and B are independent
feeds = {  # item: what feeds it
    "A-31-03 A": "PDU-A2-05 br 1-17", "A-31-03 B": "PDU-B2-05 br 2-17",
    "PDU-A2-05 br 1-17": "PDU-A2-05", "PDU-B2-05 br 2-17": "PDU-B2-05",
    "PDU-A2-05": "UPS-A2", "PDU-B2-05": "UPS-B2",
    "UPS-A2": "SWBD-A2", "UPS-B2": "SWBD-B2",
    "SWBD-A2": "USS-A1", "SWBD-B2": "USS-A2",
    "USS-A1": "MV bus 1", "USS-A2": "MV bus 2", "MV bus 1": "SUB-1", "MV bus 2": "SUB-1",
}
def upstream(item):
    path = [item]
    while path[-1] in feeds:
        path.append(feeds[path[-1]])
    return path
a, b = upstream("A-31-03 A"), upstream("A-31-03 B")
print("A path:", " <- ".join(a))
print("B path:", " <- ".join(b))
shared = [x for x in a if x in b]
print("First shared item:", shared[0] if shared else "none")

A path: A-31-03 A <- PDU-A2-05 br 1-17 <- PDU-A2-05 <- UPS-A2 <- SWBD-A2 <- USS-A1 <- MV bus 1 <- SUB-1
B path: A-31-03 B <- PDU-B2-05 br 2-17 <- PDU-B2-05 <- UPS-B2 <- SWBD-B2 <- USS-A2 <- MV bus 2 <- SUB-1
First shared item: SUB-1

The two paths are separate all the way to the substation, SUB-1, which has two utility lines and two transformers of its own (DCT-220 lesson 2). Only at the campus level do they meet, which is why the campus also has generators.

Panel schedules and labels

A panel schedule lists every breaker in a panelboard, PDU or RPP: its number, rating, number of poles, and what it feeds. It is usually posted inside the panel door and kept in the document system and DCIM. When Jamal traced breaker 2-17 for the electrician in lesson 5, the panel schedule, DCIM and the label on the rack receptacle all had to agree. When they do not, the floor is checked and the records corrected by ticket.

Labels on equipment are part of the documentation. The rule from DCT-130 applies across the site: every breaker, receptacle, cable, valve, sensor and panel has a label that matches the drawings and DCIM, printed, durable and legible. A handwritten label is a temporary fix that must be replaced. A missing label is a P4 ticket; a label that is wrong is more serious, because people trust it.

Mechanical and liquid cooling drawings

Mechanical systems have their own diagrams: chilled water and coolant flow diagrams show pumps, valves, heat exchangers, sensors and pipe sizes, with flow directions and normal valve positions; they are the cooling equivalent of the SLD. DCT-230 used them to trace Hall D's loops from the dry coolers through the CDUs to the racks. Valve lists give every valve's tag, location, normal position and purpose. As with electrical drawings, the tags on the drawing, the label on the valve and the point names in the BMS must agree. The valve left partly open in the DCT-230 capstone was found quickly because its tag led straight to its place on the flow diagram.

Keeping documents true

Documents drift from reality whenever the facility changes and the records do not. The main defences are the ones built into this course: every change ends with record updates as part of its close-out (lesson 9); every MOP is walked down against the drawings (lesson 10); DCIM is audited monthly (lesson 5); and anyone who finds a mismatch raises a ticket. Stonebridge also has a red-line process: during work, the person in charge marks any difference between the drawing and the as-installed condition in red on a copy, and that marked copy goes to the document owner, who updates the controlled drawing and issues a new revision.

Revisions are tracked in each drawing's title block: the revision letter or number, date, description and who approved it. A technician using a drawing checks that it is the current revision from the document system, not a copy from a folder. Out-of-date drawings in the field are a known cause of mistakes; Stonebridge marks printed drawings with the date printed and 'check revision before use'.

# Reconcile the as-built panel schedule with DCIM for PDU-B2-05 (extract)
schedule = {  # breaker: (rating A, feeds)   from the as-built panel schedule
    "2-15": (30, "A-31-01 B"), "2-16": (30, "A-31-02 B"), "2-17": (30, "A-31-03 B"),
    "2-18": (30, "A-31-04 B"), "2-19": (60, "A-31-05 B"), "2-20": (30, "spare"),
}
dcim = {
    "2-15": (30, "A-31-01 B"), "2-16": (30, "A-31-02 B"), "2-17": (30, "A-31-03 B"),
    "2-18": (30, "A-31-05 B"), "2-19": (60, "A-31-05 B"), "2-21": (30, "A-31-06 B"),
}
for br in sorted(set(schedule) | set(dcim)):
    s, d = schedule.get(br), dcim.get(br)
    if s == d: continue
    if s is None: print(f"breaker {br}: in DCIM ({d[1]}) but not on the schedule")
    elif d is None: print(f"breaker {br}: on the schedule ({s[1]}) but not in DCIM")
    else: print(f"breaker {br}: schedule {s[0]} A to {s[1]}, DCIM {d[0]} A to {d[1]}")

breaker 2-18: schedule 30 A to A-31-04 B, DCIM 30 A to A-31-05 B
breaker 2-20: on the schedule (spare) but not in DCIM
breaker 2-21: in DCIM (A-31-06 B) but not on the schedule

Three mismatches, three tickets. Breaker 2-18 feeds A-31-04 on the schedule but A-31-05 in DCIM; since 2-19 also feeds A-31-05 in both, DCIM is probably wrong, but the floor must be checked. Breaker 2-20 is a spare on the schedule but missing from DCIM, which means DCIM's capacity view undercounts free breakers. Breaker 2-21 is in DCIM but not on the schedule: either the schedule is out of date or the DCIM record is for a breaker that does not exist. Roz checked all three on her next round.

Documents during an incident

When something fails, the documents are what the team works from. The shift lead opens the SLD or flow diagram, marks the failed equipment, and works out the current state: which paths are live, what is at N, what depends on the failed part. The relevant EOP gives the steps. A team that has to find the right drawing during an incident, or discovers that it is out of date, loses minutes it may not have. That is why Stonebridge keeps the current SLDs and main flow diagrams printed and laminated in the operations room and at each electrical room door, as well as in the document system, and checks their revisions every quarter.