When the utility fails, what carries the IT load during the seconds before the generators are online?

The UPS and its stored energy (batteries or flywheel)

What does N+1 redundancy mean?

One spare unit beyond the capacity the load needs

Two complete, independent systems that can each carry the full load

One utility feed plus one generator

The load can grow by one rack before upgrades are needed

In a 2N design with dual-corded servers, why is each A and B feed usually kept below about half of its rated capacity?

So either feed can carry the entire load if the other is lost

Because servers only draw power from one cord at a time

Because breakers trip at 50 percent of their rating

Which device moves the building load from utility power to generator power during an outage?

An automatic transfer switch (ATS) or paralleling switchgear

A power distribution unit (PDU)

While a UPS is on maintenance bypass, what is true about the load?

It runs on bypass power and is not protected by the UPS batteries

It runs on battery power until maintenance is complete

It is automatically moved to generator power

Why are standby diesel generators exercised regularly under load, not just started and idled?

To prove they can carry real load and to prevent "wet stacking" from light-load running

To keep the utility company informed

Because diesel fuel must be burned within 30 days

What is a static transfer switch (STS) usually used for in a data center?

Giving single-corded equipment a second source by switching between two feeds in milliseconds

Transferring the building between utility and generator power

Shutting down all power to the data hall in an emergency

Quiz · Power & Electrical

Data Center Power & Electrical Quiz

Ten questions on how power gets from the grid to the rack, and how it stays on when something fails. Each answer comes with a short explanation.

10 questions About 5 minutes Intermediate

Question 1When the utility fails, what carries the IT load during the seconds before the generators are online? The standby generators The UPS and its stored energy (batteries or flywheel) The automatic transfer switch The main switchgear

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Answer: the UPS and its stored energy. Generators need several seconds to start and stabilize. The UPS carries the load from its batteries (or a flywheel) instantly, then hands off once the transfer switch moves the building to generator power. Read how UPS systems work.
Question 2Which UPS topology powers the load from its inverter all the time, so there is no transfer time when the utility fails? Standby (offline) Line-interactive Double conversion (online) Maintenance bypass

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Answer: double conversion (online). A double-conversion UPS converts AC to DC and back to clean AC continuously, so the load never sees a transfer when the input fails. That is why it is the standard for data center critical loads. UPS Systems Fundamentals covers all three topologies.
Question 3What does N+1 redundancy mean? Two complete, independent systems that can each carry the full load One spare unit beyond the capacity the load needs One utility feed plus one generator The load can grow by one rack before upgrades are needed

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Answer: one spare unit beyond what the load needs. If four UPS modules carry the load, N+1 means installing five, so any one module can fail or be serviced. Two complete independent systems is 2N. See the redundancy table.
Question 4In a 2N design with dual-corded servers, why is each A and B feed usually kept below about half of its rated capacity? To lower the electric bill So either feed can carry the entire load if the other is lost Because servers only draw power from one cord at a time Because breakers trip at 50 percent of their rating

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Answer: so either feed can carry everything. Dual-corded equipment normally splits its draw between both feeds. If one feed drops, the other instantly picks up the whole load. A feed that was already loaded past half would then be overloaded. Capacity Planning & Forecasting teaches how to track this.
Question 5Which device moves the building load from utility power to generator power during an outage? A power distribution unit (PDU) A rack PDU An automatic transfer switch (ATS) or paralleling switchgear The UPS static bypass

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Answer: an ATS or paralleling switchgear. The transfer equipment senses the utility failure, starts the generators, and switches the load once generator voltage and frequency are stable. It retransfers to the utility after the utility has been stable for a set time. Emergency/Standby Power Systems Fundamentals walks through the sequence.
Question 6While a UPS is on maintenance bypass, what is true about the load? It runs on bypass power and is not protected by the UPS batteries It runs on battery power until maintenance is complete It is automatically moved to generator power It is fully shut down

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Answer: it is not protected by the batteries. Maintenance bypass routes power around the UPS so technicians can service it. A utility disturbance during that window reaches the load directly, which is why bypass work is scheduled, planned, and done step by step under a written procedure. UPS Operations & Load Testing covers safe bypass operation.
Question 7Under NFPA 70E, the arc flash boundary is the distance at which the incident energy equals: 1.2 cal/cm² 4 cal/cm² 8 cal/cm² 40 cal/cm²

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Answer: 1.2 cal/cm². That is roughly the energy that can cause a second-degree burn on unprotected skin. Anyone inside the arc flash boundary needs arc-rated PPE suited to the hazard. Data Center Arc Flash Safety Training explains boundaries and labels.
Question 8Which OSHA standard is titled "The control of hazardous energy (lockout/tagout)"? 29 CFR 1910.146 29 CFR 1910.147 29 CFR 1910.303 29 CFR 1926.501

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Answer: 29 CFR 1910.147. 1910.146 covers permit-required confined spaces, 1910.303 covers general electrical installation requirements, and 1926.501 is construction fall protection. Electrical work on de-energized parts also has lockout and tagging rules in 1910.333. Read the Data Center Safety Guide.
Question 9Why are standby diesel generators exercised regularly under load, not just started and idled? To keep the utility company informed To recharge the UPS batteries To prove they can carry real load and to prevent "wet stacking" from light-load running Because diesel fuel must be burned within 30 days

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Answer: to prove load capability and prevent wet stacking. Diesels run lightly loaded can accumulate unburned fuel and carbon in the exhaust. NFPA 110 sets monthly exercise and periodic load-test requirements, and many sites add load-bank tests. Generator Operations & Maintenance covers testing.
Question 10What is a static transfer switch (STS) usually used for in a data center? Transferring the building between utility and generator power Giving single-corded equipment a second source by switching between two feeds in milliseconds Shutting down all power to the data hall in an emergency Stepping 480 V down to 208 V

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Answer: protecting single-corded equipment. An STS uses solid-state switches to move a load between two independent sources fast enough that equipment rides through. Emergency shutdown is the job of the EPO system, and stepping voltage down is done by a PDU transformer. Power Distribution Systems Fundamentals covers the hardware.