Electrical Fundamentals for Technicians

Ohm's law as a field tool, not a formula

V = I × R is worth memorising for one reason: it tells you what a reading should be before you take it. A 5,400 W dryer element on 240 V should draw about 22.5 A and measure roughly 10.7 ohms. If you measure 24 ohms, the element is not simply 'bad' — it is a different element than the machine expects, or you are reading through a series path you did not account for.

The habit worth building is predicting the number first. A measurement you predicted teaches you something whether it matches or not. A measurement you took blind only tells you a value.

• Power: W = V × I — use it to convert a nameplate rating into an expected current draw
• Expected resistance of a resistive load: R = V² / W
• Current is the honest measurement — it tells you what is actually happening, not what could

Series and parallel behaviour

In a series circuit the current is the same everywhere and the voltage divides across the loads in proportion to their resistance. That single fact is the most useful diagnostic idea in appliance work: an open device in a series string drops the entire supply voltage across itself. Measure across a suspect switch or thermostat with the circuit energised and demanding operation — full line voltage across it means it is open; near zero means it is closed and passing.

In parallel, each branch sees full voltage and the currents add. Loads are almost always wired in parallel across line and neutral, while their controls sit in series with them. So the mental model is: a chain of switches in series feeding a load in parallel with everything else.

• Full source voltage across a closed switch = that switch is open, not closed
• Zero voltage across a load that should be running = the load never got fed; look upstream
• Source voltage across a load with no current = the load itself is open

Voltage drop is measured across a component. Current is measured through a conductor. Mixing those up is the most common meter mistake.

Why in-circuit resistance readings lie

An ohmmeter injects its own small voltage and measures the resulting current. If the component you are testing has any parallel path around it — another winding, a bleed resistor, a board trace, a second load — the meter sees the combination, not the part. On a control board that parallel path can be a semiconductor that conducts at meter voltage but not at line voltage, giving a perfect reading on a dead circuit.

Disconnect at least one leg of the component before you trust an ohm reading. And never read resistance on an energised circuit: at best the reading is meaningless, at worst you damage the meter.

• Lift one leg to isolate the device under test
• A reading of exactly 0.0 ohms often means the leads, not the load — zero the meter with the leads shorted first
• Motor windings measure low; a few ohms difference between windings is normal, an open is not

Reading a ladder diagram

A ladder diagram puts line on the left rail and neutral (or the other leg) on the right, with each load on its own horizontal rung. Everything between the left rail and the load is a permission: a switch, a thermostat, a relay contact, a pressure switch. The machine energises a load when every permission in that rung is satisfied.

Diagnose by rung. Pick the load that is not working, find its rung, and walk the permissions left to right with a voltmeter referenced to neutral. The first point where the voltage disappears is the device that is not giving permission. That is a two-minute diagnosis on a machine you have never seen before.

• Reference the meter to neutral and hop along the rung — the voltage disappears at the open device
• Dashed lines usually indicate a mechanical link between contacts operated by the same device
• A coil on one rung with contacts on another is a relay — the contacts carry the label of the coil

Voltage present is not the same as voltage available

A voltmeter reading full supply voltage across an open switch or connector tells you the source is present and the path beyond that point is open — it does not tell you the wiring between the source and your test point is good. A corroded connector, a partially burned contact, or an undersized conductor can still pass full voltage to a high-impedance meter while dropping most of that voltage under real load current. The meter shows what it draws almost no current to show; the appliance needs amps, not microamps.

Voltage drop testing exposes this directly: measure across the suspect component or connection while the circuit is energised and the load is actually running, not at rest. A healthy conductor or contact drops only a small, predictable amount of voltage under load. A degraded one drops far more, and the difference between supply voltage and what actually reaches the load is voltage that turned into heat at the fault, not work at the load.

The distinction changes what you replace. An open circuit needs a broken conductor or a failed switch found and repaired. A high-resistance connection needs that specific connection cleaned, tightened, or replaced — putting a new motor or board on a circuit that is dropping voltage upstream reproduces the same complaint on the new part.

• Measure across a suspect connection or component under real load, not with the circuit open or idling
• A healthy connection drops a small, consistent amount of voltage under load — compare against the documented expected drop where available
• Voltage present at rest proves a source; it does not prove the path can deliver current without excessive loss

If a customer reports a fault that comes and goes with load — worse when the compressor kicks in, worse under a heavy wash load — think voltage drop under load before you think about the load itself.

Ghost voltage and why current is the truth-teller

A high-impedance digital meter can display a real-looking voltage on a wire that is not actually connected to a live source — induced or capacitively coupled voltage from a nearby energised conductor can read several volts, or even close to line voltage, on an open circuit. This is not a meter fault; it is the meter doing exactly what it is built to do, which is draw almost no current, and a floating conductor near other wiring will pick up a voltage with essentially no ability to do work.

The test that resolves the ambiguity is loading the circuit: an analogue meter, a solenoid tester, or simply measuring current instead of voltage will collapse a ghost reading to near zero, because a real source can sustain current under load and an induced voltage cannot. When a reading looks plausible but the fault behaviour does not match — a switch reads open but the load has no other explanation, or a wire that should be dead shows voltage — load-test before you condemn anything.

Current measurement is the more reliable diagnostic in general, not just for ghost voltage. A motor, heater, or compressor draws a current that reflects what it is actually doing electrically and mechanically. Voltage tells you a source exists; current tells you work is happening, and the magnitude of that current is where locked-rotor conditions, partial winding shorts, and mechanical binding actually show themselves.

• A reading that appears live on a circuit that should be open is not automatically proof of a wiring fault — load-test before condemning
• Current under load confirms real delivery; voltage at rest only confirms a source is upstream
• Compressor and motor current draw reveals mechanical and winding problems that a voltage reading at the terminals cannot

Resistance is not impedance, and RMS is not always RMS

An ohmmeter measures DC resistance with the circuit unpowered — useful for windings, heaters, and simple resistive loads, but incomplete for anything inductive under real operating conditions. A motor winding's opposition to current changes with frequency and with the magnetic field building in the core, which is impedance, not the resistance an ohmmeter reports. A winding can measure a plausible resistance and still draw an abnormal running current because of a partial short that only shows itself as reactance changes under power — this is why a resistance check alone clears a motor that current measurement under load would condemn, or the reverse.

True-RMS matters here too: variable-frequency drives, PWM-controlled loads, and many modern control boards produce and consume non-sinusoidal current and voltage waveforms. An averaging meter calibrated to read correctly on a clean sine wave will misreport these by a meaningful margin, sometimes enough to make a real fault look normal or a normal reading look faulty. On any inverter-driven or PWM-controlled circuit, confirm the meter is true-RMS before trusting the number.

Neutral and bonding faults compound all of this. A poor neutral connection on a switched-neutral or split-load circuit can put unexpected voltage on a conductor that should be at or near zero, feed voltage backward through a load that appears off, and produce symptoms — flickering, partial operation, phantom energisation — that look electronic before they are traced to a mechanical connection issue.

• Use resistance checks for de-energised windings and heaters; do not treat a good ohms reading as proof of correct operation under load
• Confirm true-RMS capability before trusting a reading on inverter, PWM, or drive-controlled circuits
• A poor neutral or bonding connection can energise conductors that should read zero — trace the connection, not just the symptom

When resistance and current-under-load disagree about a motor's health, trust the current measurement — it reflects the winding's actual behaviour while doing the job, which resistance alone cannot capture.

Failure modes and what confirms them

Test procedures

Manufacturer differences

Safety and professional boundaries

• Never treat a voltage reading on an apparently de-energised conductor as automatically a ghost voltage — confirm with a loaded measurement before any contact
• Perform voltage drop testing under load with a meter and leads rated for the CAT category and voltage present, keeping clear of adjacent live terminals
• When a neutral or bonding fault is suspected, treat any exposed metal that could be energised by it as potentially live until proven otherwise

Expected readings and what they mean

Field scenarios

Knowledge check