An AC EV charging circuit is tested for insulation resistance the same way any low-voltage installation is: isolate it, apply a DC test voltage, and confirm resistance between conductors and earth exceeds a minimum threshold. A CCS2 DC fast-charging circuit is a different problem: its power electronics typically run as an isolated (IT) system with its own continuously operating insulation monitoring device (IMD), so the meaningful test is not just what the resistance is right now, but whether the charger's own safety monitor correctly detects and responds to a developing fault while live.
Why insulation resistance matters more at an EV charging point than at a fixed panel
Insulation resistance testing catches a degrading dielectric before it becomes a live fault: cracked cable insulation, moisture ingress, contamination, or thermal ageing that reduces resistance between a live conductor and earth, eventually allowing dangerous leakage current or a short. On a fixed distribution board, exposed conductive parts a member of the public might contact are limited and well-guarded; an EV charging point changes that risk profile, since the charging cable itself is a flexible, mobile cord assembly the public physically handles, coils, drags across wet ground, and occasionally drives over, putting a compromised conductor directly into someone's hand.
AC side: standard method, EV-specific considerations
For a Mode 3 AC charging point using a Type 1 or Type 2 connector, the insulation resistance test follows the same method as any other low-voltage installation under SS 638: a 500 V DC test voltage applied between conductors and earth, minimum acceptable resistance 1 MΩ, though healthy new installations typically show hundreds of megaohms or more. The Fluke FEV350 covers the EV-specific safety functions (PE pre-test, RCD/RDC-DD trip, control pilot, proximity pilot) and, paired with a multifunction installation tester, supports insulation resistance testing on the fixed wiring side. What is specific here is less the test parameters and more the maintenance cadence: the charging cable is a wear item fixed building wiring is not, and UV exposure, repeated flexing, and daily coiling degrade insulation faster than a re-test interval set purely by general SS 638 guidance would catch.
DC side: what the FEV500 actually tests, and why the voltage and method differ
A CCS2 Fast DC charging station presents a materially different problem: voltages up to 1000 V DC carry a genuinely higher shock and arc risk than a 500 V AC test, and the DC power path itself is what needs verification. The Fluke FEV500 runs its insulation resistance test (RISO) between DC+ and PE, and separately between DC- and PE, with a selectable test voltage of 500 V or 1000 V across a measurement range of 10 kΩ to 20 MΩ, per IEC 61557-2. Testing both polarities separately matters because a fault on one rail, say moisture tracking on the DC+ side, will not necessarily show up on the other, so an asymmetric fault is an early-warning condition a combined test would miss.
IEC 62196-1 clause 21 specifies a 5 MΩ insulation resistance requirement for the DC power path measured in isolation, but the station's own IMD sits electrically in parallel with the RISO measurement, pulling the value down from what an isolated conductor alone would show, so the FEV500 sets its own threshold lower, at 500 kΩ; a reading in the hundreds of kilohms on a station with a functioning IMD is a normal result, not a marginal one, and mistaking it for a fail is a common misread. Alongside RISO, the FEV500 measures PE continuity (RLO) at up to 10 A with 0.1 mΩ resolution via the TP165X remote probe, per IEC 61557-4, confirming the earth path can safely carry fault current.
Insulation monitoring devices: testing a monitor, not just a circuit
DC fast-charging insulation testing genuinely departs from the AC side here. Many DC fast chargers operate their internal DC power conversion as an IT (isolated) system, meaning neither DC rail is intentionally referenced to earth, so a single fault to earth on one polarity does not necessarily trip a breaker immediately. The system therefore needs its own continuously operating insulation monitoring device (IMD), per IEC 61557-8, watching the live DC system's insulation resistance and raising an alarm, or initiating a safe shutdown, if resistance drops below threshold during actual operation, not just at a periodic test interval. The Fluke FEV500 verifies this monitor with a no-trip test, confirming the IMD does not nuisance-alarm under healthy conditions, and a trip test, deliberately introducing a simulated fault and confirming the IMD detects and responds within its specified parameters, since a standard RISO reading tells you insulation is healthy right now but says nothing about whether the continuous safety system would catch degradation months from now.
Worked scenario: a technician runs an FEV500 IMD trip test on a newly commissioned CCS2 fast charger and finds the IMD fails to raise an alarm within the expected window, even though RISO readings taken moments earlier were entirely healthy on both DC+ to PE and DC- to PE. This is a critical finding: the underlying insulation is fine today, but continuous protection against a future fault developing mid-session, say from water tracking into a damaged cable during a rainy charging event, is not functioning, which is why IMD verification is a mandatory Autotest step rather than optional once RISO has passed. Two further FEV500 tests round out the picture: residual voltage testing, per IEC 61851-1, confirms residual voltage on the DC pins decays to a safe level once a session ends, and error simulation exercises the charger's response to simulated fault conditions more broadly.
Test voltage and minimum value guidance, summarised
| Circuit type | Test voltage | Method | Minimum acceptable |
|---|---|---|---|
| AC fixed wiring to a Type 1/2 charging point (SS 638) | 500 V DC | Standard megohmmeter, dead test | 1 MΩ minimum (healthy new work typically far higher) |
| CCS2 DC power path, DC+ to PE (IEC 61557-2) | 500 V or 1000 V DC, selectable | FEV500 RISO test, per polarity | 10 kΩ–20 MΩ range; assess against manufacturer's acceptance criteria |
| CCS2 DC power path, DC- to PE (IEC 61557-2) | 500 V or 1000 V DC, selectable | FEV500 RISO test, per polarity | As above, tested independently from DC+ |
| Charger's own IMD (IEC 61557-8) | N/A (fault injection) | FEV500 no-trip and trip test | No alarm when healthy; correct response under simulated fault |
Common insulation faults engineers actually find at EV charging points
Moisture ingress at outdoor cable glands: a poorly rated gland or seal admits moisture over time, gradually reducing insulation resistance in a way that can pass a dry-season commissioning test and fail in the wet season. Cable jacket abrasion or crushing: cables run across ground level are vulnerable to being driven or rolled over, compromising the jacket and eventually the insulation beneath it. Connector pin contamination: dust, oil, or salt-air deposits create leakage paths without visible cable damage, a reason to clean connector interfaces as preventive maintenance. IMD desensitisation: like any monitoring device, an IMD's calibration can drift, and periodic no-trip and trip verification, not a single commissioning check, is how that drift gets caught.
For the broader connector context, see our guide to Type 1, Type 2 and CCS2 connectors compared. For general insulation resistance testing method outside the EV-specific context, see our insulation resistance testing guide.