SCENARIOS // APPRENTICE & INSPECTOR CHALLENGECITY & GUILDS 2391 • 2396 • BS 7671

INTERACTIVE PRACTICAL SIMULATOR

Electrical Diagnostic Challenge

Evaluate measured multi-function tester (MFT) readings, earthing arrangements, and site symptoms. Commit to your diagnosis, receive immediate statutory analysis, and build verified practical competence.

SESSION SCORE:0 / 0Apprentice Level

Inspector LevelEV Charging & Earthing (Section 722)

Case 1: PME Open-PEN Fault at Mode 3 EV Charger

Reported Symptom

Customer reports a tingling shock when touching the vehicle bodywork while unplugging after an overnight charge. A second occupant standing on wet tarmac measured a sting on the door handle. The 7.4 kW Mode 3 wallbox remains energised and shows a green 'ready' LED. No RCD or MCB has tripped.

Plant & Installation Context

Domestic TN-C-S (PME) supply, Ze at origin historically 0.28 Ω. 7.4 kW single-phase Mode 3 tethered wallbox (32 A Type A RCBO, 6 mm² / 6 mm² SWA) feeding a metallic-bodied BEV on the drive. No dedicated TT earth electrode and no open-PEN detection device fitted. Main bonding to gas and water is present. Weather: wet drive, vehicle tyres on tarmac.

📟 CALIBRATED MFT TEST READINGS & MEASUREMENTSGS38 PROBES ATTACHED • LIVE READINGS

ORIGIN ZE TNCS OHM0.31

ORIGIN U L-N V231

ORIGIN U N-TRUE EARTH V79

CHARGE POINT U PE-TRUE EARTH V82

VEHICLE CHASSIS TO TRUE EARTH TOUCH V81

VEHICLE CHASSIS TO REMOTE SPIKE V81

PE CURRENT CLAMP MA AC118

RCBO 1X I DELTA N 30MA MS18 (trips — residual function healthy)

IR L-PE MOHM>299

R2 CPC CONTINUITY OHM0.09

OPEN PEN DEVICE FITTEDNo

EARTH ELECTRODE RA OHMNot installed

Select Your Diagnostic Hypothesis:

Failed 30 mA Type A RCBO on the charge-point circuit — residual current is leaking onto the vehicle chassis.Missing main protective bonding to the incoming metallic water service, leaving the car body as the only return path.Open-circuit PEN on the PME (TN-C-S) supply. Neighbourhood neutral current is returning via the charge-point CPC and the vehicle chassis/tyres, elevating touch voltage above 70 V.High Ze from a corroded TT earth electrode at the wallbox; touch voltage is I_f × R_A on a TT system.

Statutory & Engineering Rationale:

Practical Rectification Procedure:

Apprentice LevelInspection & Testing (2391)

Case 2: High Zs on 32 A Radial — Table 41.3 Exceeded

Reported Symptom

Periodic inspection of a 32 A radial serving a commercial kitchen wall-mounted oven. Live Zs at the appliance terminals is 1.62 Ω. The circuit is protected by a 32 A Type B MCB (BS EN 60898) with no RCD. The oven works, but the inspector has recorded an unsatisfactory result against maximum Zs.

Plant & Installation Context

Three-phase TN-C-S board, 100 A DNO cut-out. Circuit: 32 A Type B single-pole MCB, 6.0 mm² / 2.5 mm² two-core-and-CPC PVC/PVC, 28 m run in surface trunking to a 32 A DP cooker-control switch then 6 mm² tails to the oven. Circuit installed c. 2009. No signs of thermal damage at the appliance. Board is a metal 18-way split-load DB; this MCB is on the non-RCD side.

📟 CALIBRATED MFT TEST READINGS & MEASUREMENTSGS38 PROBES ATTACHED • LIVE READINGS

SUPPLY U0 V230

ORIGIN ZE OHM0.32

PFC KA0.72

R1 LINE OHM0.17

R2 CPC OHM1.13

R1 PLUS R2 OHM1.30

MEASURED ZS OHM1.62

TABLE 41 3 MAX ZS TYPEB 32A OHM1.37

80 PERCENT OF TABLE ZS OHM1.10

IR L-N 500V MOHM>299

IR L-E 500V MOHM>299

IR N-E 500V MOHM>299

POLARITYCorrect

MCB RATING32 A Type B BS EN 60898

VISUAL DB CPC TERMINAL6 mm² CPC ferrule loose on earth bar; 1.5 turns, no tightness

Select Your Diagnostic Hypothesis:

Loose CPC termination on the DB earth bar introducing contact resistance into R2, so measured Zs (1.62 Ω) exceeds Table 41.3 maximum Zs of 1.37 Ω for a 32 A Type B device.DNO Ze has drifted high (fault on the PME electrode); the 1.62 Ω is almost entirely Ze and the installation CPC is satisfactory.2.5 mm² CPC is undersized for a 32 A circuit and cannot satisfy the adiabatic equation, so Zs is bound to fail.The 32 A Type B MCB has an incorrect magnetic trip (should be Type C); replacing it with Type C will raise the permitted Zs and pass the test.

Statutory & Engineering Rationale:

Practical Rectification Procedure:

Spark LevelMotor Control & Industrial (2396 / BS EN 60204-1)

Case 3: Star-Delta Changeover Instantaneous Trip

Reported Symptom

An 18.5 kW, 400 V, 32 A FLC squirrel-cage pump motor starts in star and runs up smoothly. At the star-to-delta transition the 40 A MCCB trips instantaneously (magnetic), every time. Thermal overload never operates. After reset the same sequence repeats. The motor had been reliable for eight years until a panel overhaul last week, when the star-delta timer was replaced.

Plant & Installation Context

Star-delta starter in a Form 4 Type 2 MCC: KM1 line contactor, KM2 star, KM3 delta, mechanical interlock between KM2 and KM3, electrical NC interlocking, electronic on-delay timer (set 8 s). Motor: 18.5 kW, 400 V Δ / 690 V Y, 32.0 A, IE3. Incoming 40 A MCCB (Ir = 36 A, Ii = 10 Ir). Panel overhaul replaced the pneumatic star-delta timer with a DIN-rail electronic timer; auxiliary wiring was disturbed. No VFD, no soft-starter.

📟 CALIBRATED MFT TEST READINGS & MEASUREMENTSGS38 PROBES ATTACHED • LIVE READINGS

MOTOR IR 500V MOHM185 (windings to earth)

WINDING RESISTANCE OHM AVG0.42 (balanced ±2 %)

STAR RUN CURRENT A11.8 (≈ FLC/√3, healthy)

LINE VOLTAGE V400 / 401 / 399

TIMER STAR DWELL S8.0

OPEN TRANSITION DEAD TIME MS0 (KM3 coil energises as KM2 drops — overlap)

KM2 KM3 MECHANICAL INTERLOCKPresent but slack; both armatures can seat ~2 mm overlap

KM2 NC AUX IN KM3 COILWelded closed (continuity 0.1 Ω with KM2 picked up)

PEAK LINE CURRENT AT TRANSITION A>2 800 (clamp peak-hold, MCCB instantaneous)

OVERLOAD RELAY SETTING A32 (correct, never trips)

MCCB II SETTING10 × Ir (instantaneous)

Select Your Diagnostic Hypothesis:

Thermal overload is set too low for IE3 inrush, so the motor trips on the delta inrush spike.Star and delta contactors are closing together. Failed KM2 NC electrical interlock plus zero open-transition time on the new timer produce a momentary winding short-circuit at changeover, and the MCCB instantaneous element clears it.Single-phasing on L2: the motor starts in star on two phases then stalls at changeover, drawing LRA until the MCCB trips.Motor is connected in star on a 400 V Δ nameplate, so delta closing applies √3 overvoltage and the MCCB sees an earth fault.

Statutory & Engineering Rationale:

Practical Rectification Procedure:

Inspector LevelRCD Selection & Renewables (Reg 531.3.3 / Section 712)

Case 4: Type AC RCD Blinded by PV DC Leakage

Reported Symptom

Six-monthly RCD test of a 30 mA device protecting a kitchen ring fails the 1 × I_Δn test at both 0° and 180° (no trip within 300 ms). The 5 × I_Δn test is delayed (>40 ms). The test button still trips. Occupants report the RCD has never tripped in service. A 4 kW transformerless string inverter was added on the same TT consumer unit two years ago. The ring itself has no history of faults.

Plant & Installation Context

TT domestic installation, Ra = 48 Ω, 100 mA S-type time-delayed RCD at origin and 30 mA Type AC RCCB (BS EN 61008) as the main 30 mA device for sockets (Reg 411.3.3). 4 kW G98 transformerless PV inverter on a dedicated Type B 16 A MCB, sharing the same Type AC 30 mA RCCB as the socket circuits (installer used the existing split-load board). Inverter running at 3.1 kW, clear midday. Kitchen ring: 2.5 mm², IR healthy.

📟 CALIBRATED MFT TEST READINGS & MEASUREMENTSGS38 PROBES ATTACHED • LIVE READINGS

RCD TYPE AND I DELTA NType AC RCCB, 30 mA, BS EN 61008

RCD 1X I DELTA N 0DEG MSNo trip (>300)

RCD 1X I DELTA N 180DEG MSNo trip (>300)

RCD 5X I DELTA N 0DEG MS52 (limit 40 ms — FAIL)

RCD RAMP I DELTA MANo trip to 50 mA (instrument max)

RCD TEST BUTTONTrips

PE CLAMP DC MA8.6 (inverter on), 0.2 (inverter isolated)

PE CLAMP AC MA3.1

IR RING L-E 500V MOHM>299

RA OHM48

INVERTER TYPETransformerless string, G98

REPEAT 1X I DELTA N INVERTER ISOLATED MS21 (PASS)

Select Your Diagnostic Hypothesis:

The Type AC RCCB has a failed polarized trip relay; the test button works because it is a mechanical bypass of the toroid.Neutral-earth reversal on the kitchen ring is returning current outside the toroid, so 1 × I_Δn cannot produce a residual.Socket circuit leakage from damp underfloor heating is already occupying most of the 30 mA, so the MFT residual cannot reach the trip threshold.Smooth DC leakage >6 mA from the transformerless PV inverter is saturating the Type AC toroid core, blinding it to AC residual. The 1 × I_Δn test therefore fails while the inverter is generating (Reg 531.3.3).

Statutory & Engineering Rationale:

Practical Rectification Procedure:

Spark LevelHarmonics & Conductor Sizing (Section 524)

Case 5: Third-Harmonic Neutral Overload on Balanced LED Lighting

Reported Symptom

A three-phase lighting board serving a retail floor smells of hot PVC after six hours of trading. Phase MCBs (100 A) are cool and have never tripped. The incoming 4-core 35 mm² SWA is hot on the neutral core only. Clamp measurements show ~100 A per phase but 180 A in the neutral. The DNO has not reported a supply fault. All three phases of LED luminaires are evenly loaded.

Plant & Installation Context

400 V TN-S commercial distribution. Sub-main: 35 mm² 4-core SWA (Cu), 100 A TP MCCB, 42 m to an 8-way TP lighting board. Load: approximately 90 kW of LED panel luminaires with cheap switch-mode drivers, balanced across L1/L2/L3 by circuit count. Neutral of the SWA is 35 mm² (same as phase). No harmonic filter, no k-rated transformer. Board installed 2016 for fluorescent; luminaires swapped to LED in 2024 without redesign.

📟 CALIBRATED MFT TEST READINGS & MEASUREMENTSGS38 PROBES ATTACHED • LIVE READINGS

U L-L V401 / 400 / 399

I L1 RMS A100

I L2 RMS A100

I L3 RMS A99

I N RMS A180

THD I PERCENT62

I H3 PERCENT OF FUNDAMENTAL60

I H3 AMPS PER PHASE60

NEUTRAL PREDICTED 3X IH3 A180

THD V PERCENT6.8

U N-E V8.4

THERMAL PHASE CORES C51

THERMAL NEUTRAL CORE C89

PHASE BALANCE PERCENT<2 % (fundamental)

PF TRUE0.71 (distortion, not displacement)

Select Your Diagnostic Hypothesis:

Unbalanced phase loading: one phase is carrying most of the lighting so the neutral is returning the out-of-balance current.Triplen (3rd-harmonic) currents from non-linear LED drivers are in phase in L1, L2 and L3 and sum in the neutral (3 × 60 A = 180 A) on an otherwise balanced 100 A load, overheating the 35 mm² neutral (BS 7671 Section 524).Neutral-to-earth fault at the lighting board is adding an earth-return current of 80 A to the true load current in the SWA neutral.The 100 A MCCB is oversizing the circuit relative to 35 mm², so the phase conductors run cool while the undersized protective device lets the neutral cook.

Statutory & Engineering Rationale:

Practical Rectification Procedure:

Diagnostic challenge complete! All 5 technical assessments attempted.

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