Free tool
Follow the power, box by box
A data center power train runs from the utility intake and a standby generator, through a transfer switch, an LV switchboard, a UPS and a PDU, to dual supplies at the rack. Follow that one path end to end and most power interview questions become answerable, because every box on it is a place where work stops, a document exists, and somebody signs.
What happens when the utility fails?
Nothing visible, if the train is doing its job. The UPS carries the load immediately, because it was already in the path conditioning power rather than waiting on standby. The generator starts, comes up to a stable voltage and frequency, and only then does the transfer switch move the load across, which is why the ride-through has to last longer than the start plus the stability check rather than just the start. When utility returns the sequence runs in reverse, usually with a deliberate delay so a flickering supply cannot cause repeated transfers. Being able to narrate that order, and to say which step your site would abort on, is a stronger answer than naming the equipment.
N: Exactly the capacity the load needs, and no more. Any single failure or any maintenance takes capacity away.
N+1: One spare unit beyond need. A single component can fail or be serviced without losing capacity, but the path may still be single.
2N: A fully duplicated system, two independent paths. Costly, and the level people most often claim without having.
Definitions are the standard capacity conventions, not a tier claim. Tier language belongs to Uptime and is covered in the tiers resource.
Scope. Every scenario here is fictional and generalized, set on the fictional Example Site EX1. This is career and interview preparation at awareness level, not a live operating procedure, engineering advice or any form of authorization. On a real site, follow your employer's procedures, your authorized training and local requirements, and if the two ever disagree, the site wins.
Where on the train does this belong?
Power questions are rarely about equipment. They are about where on the path a symptom, a document or a decision belongs, and whether you can walk from there in both directions. Place each of these on the train, then do the harder half out loud: say what feeds that stage, what it feeds, and what the load sees at the moment that stage fails. A reader who can do that has stopped memorising a diagram and started using one.
Things somebody could say to you on the fictional Example Site EX1. For each one, name the stage of the train it belongs to. Tracing beats naming: the answer an interviewer remembers is the one that walks the path in order.
-
01 Utility is lost. The critical load does not see a break at the instant it happens.
Choose the stage
Answer
UPS · UPS module
The ride-through is already in the path, conditioning power, which is why it carries the load the moment the supply goes rather than starting when it does. Everything downstream stays quiet because of that, not because the generator was fast.
Not GEN · Standby generator. The set is still starting at that instant. It takes the load later, after a stability check and a transfer, and the gap in between is exactly what the ride-through exists to cover.
-
02 The set starts, but the load does not move across until voltage and frequency have been stable for the agreed interval.
Choose the stage
Answer
ATS · Transfer switch
The transfer switch decides which source feeds the load, and it waits for stability rather than for the engine. That interval is why a ride-through has to be sized to start plus stability, not to start alone, and quoting the start time on its own is the answer that gets followed up.
-
03 During a planned source change the load sees a break of a fraction of a second. Nothing protected notices; one unprotected mechanical panel drops out.
Choose the stage
Answer
ATS · Transfer switch
An open transition breaks before it makes, so there is a dead interval by design; a closed transition overlaps the sources briefly and avoids it. Knowing which one your site has is the difference between predicting that panel and being surprised by it.
Not UPS · UPS module. Anything fed through the ride-through does not see the break, which is precisely why the outage looks selective. The break was created by the transfer, not by the UPS.
-
04 Somebody asks which board each of the two cords in a cabinet lands on, and follows both upstream until the paths either diverge or meet.
Choose the stage
Answer
RACK · Rack PSU
Two cords are only two paths if they stay separate the whole way up. The point where they meet is the real single point of failure, and it is read off the single line rather than asserted from the floor. This is the question that separates tracing a diagram from having seen one.
-
05 Before anything is energised, the protection settings have to be checked against the approved schedule at its current revision.
Choose the stage
Answer
SWB · LV switchboard
The switchboard is where distribution to the floor begins and where protection is set. It is a common place for a hold point, because energising it is irreversible in practice: after that, checks happen on live plant or they do not happen.
-
06 A capacity conversation starts with per-cabinet current readings taken where the metering actually lives.
Choose the stage
Answer
PDU · PDU
Distribution to the row or the rack is usually where per-rack metering sits, so it is where a capacity question gets an answer instead of an estimate. If you cannot say where the reading came from, you are quoting a design figure and calling it a measurement.
-
07 The question is how long the site has before the load is at risk, and what happens during that time.
Choose the stage
Answer
UTIL · Utility intake
Losing the intake is the failure that everything downstream exists to answer, which is why interviewers open here. The answer is not a number, it is the order: ride-through carries, set starts, stability is confirmed, transfer happens, and cooling restarts in its designed order.
-
08 Fuel, starting reliability, and the time from a start signal to a stable voltage and frequency are what is being asked about.
Choose the stage
Answer
GEN · Standby generator
Standby generation is not instant, and saying so is most of the answer. There is a start, a stability check, then a transfer, and the seconds in between belong to the ride-through rather than to the engine.
Now say it out loud
Clicking the right option is the easy half.
In the room you will not be given the options. Cover them, and answer these three out loud, in full sentences, as if somebody had just asked. If a sentence stalls, that is the part to go back and read again.
- 01 Narrate the loss of utility from the first instant to the load being carried by the generator, naming every stage in order and what it is waiting for.
- 02 Explain what open and closed transition mean, and name one thing on a site that would behave differently under each.
- 03 Say how you would check whether a rack's two supplies are genuinely two paths, and what you would ask for to prove it.
Why does dual cording matter at the rack?
Because two cords only buy you something if they come from two paths. Racks are routinely fitted with dual supplies that both trace back to the same upstream board, which looks redundant on the floor and is not redundant at all in a failure. The useful habit is to follow each cord upstream until the paths either diverge or meet, and the useful interview move is to ask which board each side lands on rather than to assert that the rack is resilient. It shows you think in paths instead of in equipment, which is the difference between someone who has read a diagram and someone who has traced one on site.