Mechnical & Electrical Questions

Questions for the EOT

Can you explain the main differences between AC and DC power, and where we see them in the data center?

AC changes direction periodically, while DC flows in one direction. In a data center, AC is used for utility grid distribution and transformers. DC is used to power IT equipment/servers and is stored in UPS battery banks.

what is a transformer and how does it operate

A transformer steps voltage up or down using electromagnetic induction between primary and secondary coils.

Walk me through your process when troubleshooting an electrical fault

First, I gather data from alarms and schematics. Second, I safely isolate the circuit. Third, I use a digital multimeter to test voltage, continuity, and resistance from power source to load to find the fault. Finally, I fix the root cause and verify safe operation.

How do you approach troubleshooting major infrastructure assets like a UPS or a backup generator

I start with the BMS panel to identify active fault codes. For a UPS, I check if it's on static bypass or battery mode. For a generator, I verify fuel levels, battery voltage, and jacket water temperature. For a transformer, I check temperature and voltage balances

Define voltage, current, and resistance

Voltage (V) is electrical pressure; Current (I) is the flow rate of charge; Resistance (R) opposes that flow. For example, In the water analogy: Voltage is water pressure, Current is flow volume, and Resistance is a restriction in the pipe.

What is the difference between three-phase and single-phase power, and why do we use three-phase in data centers

Single-phase power uses one AC waveform and is usually used for smaller loads. Three-phase power uses three waveforms spaced 120 degrees apart, which gives a smoother, more continuous delivery of power. Data centers use three-phase because they have large, continuous electrical loads, it uses less copper, lower loss and voltage drops, and better support

What are the core steps of a LOTO procedure, and how do you determine your PPE?"

Notify, shut down, isolate energy, apply locks/tags, dissipate residual energy, and verify zero energy state with a meter. I determine PPE by checking the Arc Flash equipment label and following set guidelines.

Why is electricity transmitted at high voltage, and how do you safely test a fuse?

By using high voltage, you can transmit the same amount of power using lower current which minimizes power losses
To test a fuse safely: De-energize and Isolate:, remove the fuse, and use a multimeter to check continuity (Ohms). Beep = good (Another way is to check voltage, 0 = good any voltage reading besides that = bad)

What is a VFD and what is its primary function in a data center?

A VFD is Variable Frequency Drives. It adjusts an AC motor's speed by varying its supplied frequency and voltage. They are used on pumps and fan motors to significantly save energy

What is an ATS and how does it function during a utility power failure?

An ATS or Automatic Transfer Switche, transfers the electrical load from a primary source (utility) to a backup source (generator). When utility power drops, the ATS signals the generator to start, monitors its stability, and transfers the load.

What are the four primary components of a double-conversion UPS system?"

The components are the Rectifier (AC to DC / charges batteries), the Battery Bank (stores backup energy), the Inverter (DC back to clean AC for servers), and the Static Bypass Switch (safely routes utility power directly if the UPS fails). (DC bus also which ties everything together)

How does a backup generator work?

an engine burns fuel to spins an alternator. the alternator converts that mechanical rotation into electricity

What is the function of a circuit breaker, and how does it differ from a fuse

A breaker protects equipment by automatically opening/tripping when it detects unsafe current, such as an overload, short circuit. The difference is that breakers can be reset to resume operation, while fuses melt and must be replaced.

Explain the refrigeration cycle and name its 4 main components in order

The cycle moves heat outdoors by changing a refrigerant's physical state.
Compressor: Takes low-pressure gas and compresses it into a high-pressure, high-temperature gas
Condenser: Rejects heat to the outside air/water, causing the high-pressure gas to condense into a high-pressure liquid.
Expansion Valve: Restricts refrigerant flow causing a sharp drop in pressure and temperature, turning it into a low-pressure liquid and vapor mixture.
Evaporator: Absorbs heat from the indoor space, causing the refrigerant to evaporate completely into a low-pressure gas before returning to the compressor to repeat the cycle.

What is the difference between dry bulb and wet bulb temperature

Dry bulb is ambient air temperature measured by a standard thermometer. Wet bulb is the lowest temperature reached by evaporating water into the air. Wet bulb reflects humidity and sets the physical limit for evaporative cooling.

What are the common types of pumps used in data centers, and what do the Pump Affinity Laws state

We use Centrifugal Pumps for water loops. The Affinity Laws state that Flow Q is proportional to motor speed, Head/pressure H is proportional to speed squared, and Power P is proportional to speed cubed
Q∝N
H∝N2
P∝N3

"What is dew point, and why is tracking it more reliable than tracking relative humidity in a data center?"

Dew point is the exact temperature where air saturates and water condenses. Relative humidity changes drastically as temperature changes, but dew point is an absolute measure of moisture, making it reliable for preventing condensation on servers.

How does evaporative cooling work, and when is it most effective

cools air by using heat to turn liquid water into water vapor. As water evaporates, it absorbs latent heat from the surrounding air, so the air leaving the system is cooler—but also more humid. It is highly efficient but only works well in hotless humid environments (sweat example)

How do chillers, cooling towers, and heat exchangers work together in a facility

chiller removes heat from the building or IT equipment
cooling tower rejects that heat outdoors
heat exchangers transfer heat between separate fluid loops without mixing them

What is differential pressure, and why is it monitored in a server room

It is the difference in static pressure between two spaces. helps verify that the cooling system is delivering air where it is needed. Also tells when the filter needs to be changed

What is the role of a BMS/BAS system, and how do you interact with it as a technician

Building Management System (BMS) or Building Automation System (BAS) is a centralized platform that monitors and controls HVAC, power, and generators. Technicians use it to check live data, acknowledge active alarms, adjust setpoints, and verify operational sequences.