IEC 61851-1 defines four charging modes for conductive EV charging, distinguished not primarily by speed but by where the control pilot (CP) function lives, what protects the circuit, and how the EVSE and vehicle communicate. Mode 2 puts the CP function inside a portable in-cable control box plugged into a general-purpose socket. Mode 3 puts it inside a dedicated, fixed AC charging station. Mode 4 hands the whole AC-to-DC conversion to an external station and shifts the vehicle-charger conversation from a simple analogue signal to a digital protocol. Knowing which mode you're standing in front of tells you which circuit, which protection device and which test instrument you actually need.
Mode 1: why it barely enters the conversation
Mode 1 is direct connection between an EV and a standard socket outlet, with no control pilot communication at all between vehicle and supply. Without a CP signal, there is no electronic negotiation of available current, no way to confirm the connection is safely made before power flows, and no dedicated means of detecting a fault condition specific to EV charging. This absence of any control and protection communication is exactly why Mode 1 is restricted or effectively unused for EV charging in most developed charging frameworks, Singapore's regulatory environment among them. It is worth naming only so the numbering makes sense; in practice, almost nothing you test in the field will be Mode 1.
Mode 2: the control pilot lives in the cable, not the wall
Mode 2 charging uses a portable EVSE, commonly called an in-cable control box or IC-CPD (In-Cable Control and Protection Device), plugged into a standard general-purpose socket outlet rather than a dedicated charging point. The defining feature is where the control pilot function physically sits: inside a control box moulded into the charging cable itself, not in a fixed wallbox or building installation. This is the "travel charger" or "granny cable" many EVs are supplied with, typically limited to a single-phase current in the region of 10 A to 13 A to stay within what an ordinary domestic socket circuit can safely and continuously supply.
What this means electrically: the socket outlet the IC-CPD plugs into was very likely designed and tested as a general-purpose socket circuit, not as a dedicated EV charging circuit. Its earth fault loop impedance, RCD protection and cable sizing were verified against ordinary intermittent domestic or light commercial loads, not against several hours of continuous near-maximum current draw. The IC-CPD's own control pilot function does provide some protection, negotiating a safe current limit and detecting basic fault conditions, but it cannot retroactively upgrade the fixed socket circuit's design margin.
What this means for testing: two separate things need checking, and they're easy to conflate. First, the general-purpose socket circuit itself needs to be verified as suitable for sustained near-maximum current draw over hours, not just for the intermittent loads it was likely designed around, which can mean re-checking loop impedance and cable thermal performance under realistic duty cycle rather than assuming a pass on a standard periodic domestic test is sufficient. Second, the IC-CPD device itself needs its control pilot behaviour verified independently, confirming it correctly negotiates state transitions and enforces its current limit, which is exactly the kind of control pilot simulation the Fluke FEV300 test adapter kit and FEV350 analyzer are built to exercise.
Mode 3: the control pilot lives in a dedicated, fixed station
Mode 3 is what most people picture when they say "EV charger": a dedicated AC EVSE, wall-mounted or pedestal-mounted, hard-wired into its own circuit, with the control pilot and proximity pilot functions built into the fixed unit rather than into the charging cable. This is the mode a Fluke FEV350 is designed around, and it is the connection type carrying single or three-phase power, commonly up to 32 A per phase, giving AC charging power up to roughly 7.4 kW single-phase or 22 kW three-phase.
What this means electrically: because Mode 3 uses a dedicated circuit rather than a shared general-purpose socket, that circuit can, and under Singapore's SS 638 and IEC/HD 60364-7-722 must, be designed specifically for the sustained current and duty cycle of EV charging from the outset. This is where the earth loop impedance and insulation resistance considerations discussed elsewhere on this site (see our guides to earth loop impedance testing at EV charging points and insulation resistance testing for EV charging circuits) apply in full: a dedicated circuit design load, a protective device selected for the actual current and disconnection time required, and RCD protection (Type A minimum, Type B where the charging system could produce smooth DC fault currents) sized for that specific installation, not inherited from an unrelated general socket circuit.
What this means for testing: the full FEV350 sequence applies as designed, PE earth pre-test, 30 mA RCD plus 6 mA RDC-DD trip testing, nominal voltage and phase sequence, control pilot testing with waveform analysis, proximity pilot verification and error condition testing, alongside earth bond, insulation and loop/line impedance measurements via a compatible multifunction installation tester. Because the control pilot function is built into the fixed station rather than a swappable cable, a fault found here points at the charging station itself (or the dedicated circuit feeding it), not at a portable accessory that could simply be swapped out.
Mode 4: the vehicle's own onboard charger is bypassed entirely
Mode 4 hands AC-to-DC conversion to the external charging station itself, rather than to the vehicle's onboard charger, and delivers DC current directly to the battery. This is what allows Mode 4 (DC fast charging) to reach power levels an onboard vehicle charger could never handle, because the conversion electronics live in the (much larger, much more heavily cooled) station rather than inside the car. CCS2 is the connector standard Singapore's regulatory framework favours for public Mode 4 charging, and the Fluke FEV500 is built specifically to test this mode.
What this means electrically and in terms of communication: Mode 4 does not rely on a simple analogue PWM control pilot signal the way Mode 2 and Mode 3 largely do. Instead, it uses a full digital communication channel, established through SLAC (Signal Level Attenuation Characterization) power-line communication, followed by ISO 15118 or the earlier DIN SPEC 70121 protocol, to negotiate the charging session, agree on power delivery parameters, and manage the session throughout. The station, not the vehicle's onboard AC-DC converter, is doing the heavy electrical lifting, which is exactly why Mode 4 test equipment (FEV500) needs to verify power electronics, insulation resistance on both DC polarities, PE continuity at high test current, insulation monitoring device response, and digital protocol negotiation, an entirely different test surface from a Mode 3 AC station's control pilot and RCD checks.
What this means for testing: everything covered in our guides to DC insulation resistance testing and connector standards applies here specifically. A Mode 4 fault is more likely to show up as a communication negotiation failure (SLAC or ISO 15118/DIN SPEC 70121 handshake issues), an insulation or IMD response problem on the DC power path, or a load test discrepancy in actual delivered voltage, current or power, rather than the kind of control pilot waveform distortion you'd chase down on a Mode 3 AC station.
The differences, side by side
| Mode 2 | Mode 3 | Mode 4 | |
|---|---|---|---|
| Where CP function lives | In-cable control box (IC-CPD) | Fixed dedicated EVSE | Fixed DC charging station |
| Connection point | General-purpose socket outlet | Dedicated hard-wired circuit | Dedicated high-power circuit |
| Typical power | ~2.3 kW to 3 kW (single-phase, limited current) | Up to ~7.4 kW single-phase or ~22 kW three-phase | Tens to hundreds of kW, station-dependent |
| Who converts AC to DC | Vehicle's onboard charger | Vehicle's onboard charger | The external charging station |
| Communication method | Analogue PWM control pilot | Analogue PWM control pilot | Digital: SLAC + ISO 15118 / DIN SPEC 70121 |
| Typical connector | Type 1 / Type 2 (with domestic plug on supply side) | Type 1 / Type 2 | CCS2 (Combo 2) |
| Test focus | Underlying socket circuit's suitability + IC-CPD's own CP behaviour | Full FEV350 sequence: PE pre-test, RCD/RDC-DD trip, CP/PP, loop impedance | FEV500 sequence: RISO both polarities, RLO, IMD no-trip/trip, comms negotiation, load test |
| Fluke instrument | FEV300 / FEV350 (CP simulation) | FEV350 | FEV500 |
Why the mode, not just the connector, should be the first question you ask
Two Type 2 connectors, one on a Mode 2 travel cable and one on a Mode 3 wallbox, look almost identical, but they represent fundamentally different electrical realities. Testing the Mode 2 setup without recognising the underlying socket circuit was likely never designed for sustained EV charging load misses the actual risk. Testing a Mode 3 station's control pilot the same way you'd test a Mode 4 station's digital communication link would be testing the wrong layer entirely. Identifying the mode first, before reaching for an instrument, is what determines whether you're really testing a portable accessory plugged into someone's kitchen socket, a purpose-built AC charging station, or a DC power conversion plant, three genuinely different jobs wearing similar-looking connectors.