Name: The Foundation of Iron — Week 1 — Electricity and Power
Author: Amine RAITI

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THE FOUNDATION OF IRON · COURSE MATERIAL · WEEK 1

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ELECTRICITY
AND POWER
Week 1 of 26 · Block 1 — Fundamentals
20h theory · 15h practice

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◆ WEEKLY LEARNING OBJECTIVES

1. Understand voltage, current, resistance and their relationship (Ohm's law)
2. Distinguish active, reactive and apparent power, convert kW to kVA
3. Read and interpret an electrical equipment nameplate
4. Measure voltage and current with a multimeter safely
5. Calculate the admissible electrical load of a server rack

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NOTE FOR THE INSTRUCTOR

The theory section of this material is a structured course outline — key concepts, formulas, worked examples — meant to serve as a guiding thread that the instructor develops and illustrates orally in their own style. The practical exercises and their solutions are written in full and stand on their own.

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Amine RAITI · Infrastructure Architect & SRE
Public document · CC BY-NC-SA 4.0 · AI Powered by Amine
Opération Dindon

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COURSE OUTLINE · 20H · PART 1/2

THEORY GUIDING THREAD

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1.1 · Voltage, current, resistance2h

— Define: voltage U (V), current I (A), resistance R (Ω)
— Hydraulic analogy to develop: voltage=pressure, current=flow, resistance=pipe diameter
— Point of attention: stress the conventional direction of current

1.2 · Ohm's law2h

— Formula: U = R × I
— 3 numerical examples to work through on the board (varying the unknown: U, then R, then I)
— Quick oral exercise: "12V circuit, R=4Ω, what is the current?" (answer: 3A)

1.3 · Electrical power, DC vs AC3h

— Formula: P = U × I (Watts)
— Difference between direct and alternating current
— Concept of phase shift in AC (intuitive, no advanced trigonometry at this stage)

1.4 · Active, reactive, apparent power4h

— P (W) = useful work · Q (VAR) = reactive energy · S (VA) = apparent power
— Key formula: S² = P² + Q²
— Power factor cos φ = P/S — why it matters

EXAMPLE TO DEVELOP ON THE BOARD

A 10 kVA UPS with a power factor cos φ = 0.9 delivers a useful active power of: P = S × cos φ = 10 × 0.9 = 9 kW.

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COURSE OUTLINE · 20H · PART 2/2

THEORY GUIDING THREAD — CONTINUED

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1.5 · kVA vs kW in a datacentre — why the distinction matters3h

— PDU/UPS sized in kVA (what they must physically be able to supply)
— Actual server consumption in kW (active power)
— Modern PFC power supplies → cos φ close to 1 → kVA ≈ kW in recent practice
— Point of attention: never assume cos φ = 1 without checking the datasheet

1.6 · Reading a nameplate3h

— Step-by-step method: rated voltage → rated current → power in kVA → frequency
— Guided exercise on a real PDU nameplate (to project)

1.7 · Electrical safety in a datacentre3h

— Risks: electric shock, electrocution, arc flash
— Basic rules: never work alone on a live rack, PPE, simplified lockout procedure
— Experience to share: a typical incident and the lesson learned

Key definitions to fix before the practical exercises

Voltage (U): electrical potential difference, in Volts (V).
Current (I): rate of electrical charge flow, in Amperes (A).
Resistance (R): opposition to current flow, in Ohms (Ω).
Active power (P): actually useful electrical work, in Watts (W).
Apparent power (S): vector combination of P and Q, in Volt-Amperes (VA).

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EXERCISE 1 · MULTIMETER MEASUREMENTS · 5H

Equipment: digital multimeter (1 per pair), adjustable low-voltage lab power supply (0-15V), various resistors (100Ω, 220Ω, 1kΩ, 4.7kΩ with visible colour code), test leads, breadboard, electrical safety sheet signed at the start of the session.

(30 min) Introduction to the multimeter: function selector, ranges, measurement ports, safety rules (never measure current in voltage mode, check the range before connecting).

(45 min) DC voltage measurement: measure the lab supply's output voltage set successively to 5V, 9V, 12V; record the readings.

(1h) Resistance measurement: measure 5 different resistors with the multimeter, compare against the theoretical value read from the colour code, calculate the % deviation.

(1h30) Series circuit: build a power supply + resistor circuit on a breadboard, measure the current by inserting the multimeter in series, measure the voltage across the resistor.

(1h) Verifying Ohm's law: from the previous measurements, calculate R = U/I and compare to the value shown by the multimeter in resistance mode; calculate the deviation.

(15 min) Write-up: fill in the provided measurement table, calculate the theory/practice deviations.

SOLUTION — EXERCISE 1

Expected measurement table: voltage readings should fall within ±2% of the supply setting (5V, 9V, 12V). Measured resistances should fall within the tolerance indicated by the colour code (typically ±5% for a gold 4th band).

Expected calculation for step 5: if the circuit uses a 220Ω resistor powered at 9V, the theoretical expected current is I = U/R = 9/220 ≈ 0.041 A (41 mA). The deviation between the calculated resistance (R = measured U / measured I) and the resistance shown directly by the multimeter should remain below 5%.

Sources of error to mention: internal resistance of the multimeter in ammeter mode, resistor tolerance itself, contact quality on the breadboard, drift of the lab supply if unregulated.

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EXERCISE 2 · SERVER RACK LOAD CALCULATION · 5H

Equipment: datasheets for 5 servers (provided in annex — rated power in W, current in A, 230V supply voltage), calculator, datasheet for a typical PDU (16A/230V single-phase capacity, roughly 3.68 kVA).

(30 min) Collective reading of the 5 datasheets, identifying the rated power of each server.

(1h) Calculating the total active power required for a rack of 10 servers (2 units of each model), in Watts then in kW.

(1h) Applying a power factor of 0.95 (given in the brief) to obtain the apparent power in kVA: S = P / cos φ.

(1h30) Comparison with the provided PDU's capacity (3.68 kVA): can the rack be powered by a single PDU? If not, how many PDUs are needed?

(1h) Applying a 20% safety margin to the calculated load and re-checking the required capacity.

SOLUTION — EXERCISE 2

Example dataset (to adapt to the actually distributed datasheets): 5 server models with rated powers of 350W, 450W, 550W, 650W and 800W. For 2 units of each (10 servers): total power = 2 × (350+450+550+650+800) = 2 × 2800 = 5600 W = 5.6 kW.

Conversion to kVA: S = P / cos φ = 5.6 / 0.95 ≈ 5.89 kVA.

Comparison with the PDU (3.68 kVA): a single 3.68 kVA PDU is insufficient. At least 2 PDUs are required (5.89 / 3.68 ≈ 1.6, rounded up to 2 for capacity, which also conveniently provides redundancy).

With a 20% safety margin: load to cover = 5.89 × 1.2 ≈ 7.07 kVA, confirming the need for 2 PDUs and prompting a check of balanced load distribution between them.

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EXERCISE 3 · SIMPLE POWER SUPPLY SIZING · 5H

Equipment: provided case study (paper layout of 3 racks, list of 25 servers spread across the 3 racks with datasheets), calculator. Site data: three-phase 400V/63A incoming supply.

(1h) Case study review: reading the layout and equipment list, identifying total power per rack.

(1h30) Determining the number and capacity of UPS units needed for the 3 racks, with a 20% safety margin.

(1h) Checking compatibility with the site's available electrical supply (three-phase 400V/63A) — is the calculated total power compatible?

(1h30) Writing up a final sizing sheet with a summary table and justification for each choice.

SOLUTION — EXERCISE 3

Method for calculating available three-phase capacity: P = U × I × √3 × cos φ = 400 × 63 × 1.732 × 0.95 ≈ 41.4 kVA available on the site's incoming supply.

Example distribution (to adapt to the provided dataset): if each rack requires about 6 kVA after the safety margin (consistent with Exercise 2), 3 racks represent about 18 kVA — comfortably within the 41.4 kVA available, leaving a healthy margin for future expansion.

Expected sizing sheet: a table with one row per rack (active power, apparent power, required PDUs), a total row, and an explicit conclusion on compatibility with the site's incoming supply and the remaining margin for future growth.

◆ SUMMARY SHEET — WEEK 1 SELF-ASSESSMENT

1. I can state Ohm's law and apply it to calculate U, R or I.

2. I can explain the difference between active, reactive and apparent power.

3. I can calculate an apparent power (kVA) from an active power (kW) and a power factor.

4. I can read a UPS or PDU nameplate and extract the key information.

5. I can use a multimeter safely to measure voltage, current and resistance.

6. I can calculate the total electrical load of a server rack from datasheets.

7. I can apply a 20% safety margin to an electrical sizing calculation.

8. I know the basic safety rules for working on a live rack.