Operations & commissioning

The electrical sequence of operation, and how to write one that survives commissioning

Every switchboard schedule says what the plant is. The sequence of operation says what it does — event by event, device by device. It is the document the integrated systems test is written from, and the one most often produced last, by hand, from a drawing that has since moved on.

What the document is for

Three different readers need it, and they need the same facts for different reasons.

The four things each case has to state

A case that leaves any of these out cannot be tested, which means it cannot be argued about before the test either.

1. The cause. One event, stated plainly: utility A lost, transformer TR-1 faulted, Q-A1 opened for maintenance, EPO pressed. Not two at once unless the case is deliberately a double contingency, in which case say so.
2. The state of every device afterwards. Not only the ones that move. A sheet that lists five breakers out of sixty is silent about the other fifty-five, and silence is where disagreements live. Four states cover it: ON, OFF, FAULT, ISOLATION.
3. The time the plant takes to settle. A generator reaches rated speed and voltage in roughly ten seconds; a UPS carries the load meanwhile. A case judged at t = 0 and a case judged at t = 900 s are different cases. State the settle time.
4. The redundancy left afterwards. The load being carried is the minimum bar, not the result. What matters commercially is whether the facility is still concurrently maintainable with one source gone — or whether it is now one event from going dark.

A case, written out

A 2N facility, 400 kW of IT load on two cords, two utilities, two generator sets and a normally open tie. The case is the loss of utility A.

Written like that, the case is testable. The commissioning agent can witness Q-G1 closing, time it, and read the UPS log for the battery discharge. Nothing is left to interpretation on the day.

The case list: what to include

A list of forty cases that all test the same path is worse than a list of twelve that each test a different one. Build the list from the plant, not from habit.

• Loss of each incoming supply, separately.
• Loss of each generator set while it is carrying load, and failure of one set to start.
• Each UPS to internal and to external maintenance bypass, and back.
• Each transformer and each main breaker out for maintenance — one at a time, which is the whole meaning of concurrent maintainability.
• Each distribution board isolated at its incomer.
• Loss of a cooling source: one chiller, one pump, one room unit — and the chilled-water pumps on the uninterruptible supply, if continuous cooling is claimed.
• EPO, and restoration after it.
• The double contingencies the brief actually names, labelled as such.

Four ways the document goes wrong

1. It is written from the drawing by hand, once. The single line is then revised four times before issue. Nothing in the process forces the sheet to follow.
2. Device designations do not match the drawing. The sheet says “generator breaker”; the drawing says Q-G1; the panel label says something else again. On site this costs hours.
3. No settle time. The reader cannot tell whether “UPS on battery” is the expected intermediate state or the final one.
4. The residual state is asserted, not derived. “Tier III maintained” appears under every case, including the ones where it is not true.

How to check a sheet you have been handed

Take three cases from the middle of the list and trace each one on the single line yourself, naming every device. If the devices in the sheet are not the devices on the drawing, or if two cases that start differently somehow end identically, the sheet was written from an idea of the plant rather than from the plant.