DC leakage is a fault current with a DC component, flowing when charging equipment or the car’s electronics develop an insulation fault. It matters because a smooth DC fault current can blind an ordinary AC-type residual current device (RCD), leaving it unable to trip on a later, dangerous fault. EV charging protection is therefore built to detect DC residual current too — typically at 6 mA of smooth DC.
Key Takeaways
- DC leakage is a fault current with a smooth DC component, from an insulation fault in charging electronics.
- Its real hazard is blinding an ordinary AC-type RCD so it cannot trip on a later fault.
- Protection lives in one of two places: a Type B RCD upstream, or 6 mA DC detection built into the charger.
- It is not insulation monitoring — that guards the DC circuit of a fast charger, a different job.
Why a Charger Can Produce a DC Fault at All
A home charger feeds AC to the car; the car converts it to DC for the battery via power electronics. If insulation fails on the wrong side of that conversion, the fault current leaking back toward the supply does not alternate cleanly like the mains — it can carry a steady, smooth DC component. Nothing has to be visibly broken; deteriorating insulation is enough. The leak may be small — the problem is what it does to the protection upstream.
The Blinding Problem
An ordinary RCD watches for an imbalance between current going out and coming back, and trips when the difference suggests current escaping. Its sensing core is designed for alternating fault currents. A smooth DC component can magnetically saturate that core, so the device stops responding properly to the AC imbalance it exists to catch. The RCD has not tripped and shows no obvious failure; it has been blinded. That is the hazard DC leakage protection exists to prevent — separate from the ordinary ground-fault question covered in our article on GFCI protection.
| Fault type | What detects it |
|---|---|
| AC residual current (ordinary ground fault) | A standard RCD / GFCI device |
| Smooth DC residual current from charging electronics | A Type B RCD, or DC detection (6 mA) built into the charging equipment |
| Overcurrent and short circuit | The breaker or fuse — a different job entirely |
The Two Places the Protection Lives
Installations solve the problem in one of two ways:
- A Type B RCD upstream. Built to sense residual currents including smooth DC, it covers the circuit whatever charger hangs off it.
- DC detection inside the charger. Many modern wallboxes include a residual direct-current detecting device that reacts to smooth DC leakage at the 6 mA level and shuts charging down. With that function certified inside the equipment, the upstream device can be a simpler type — which arrangement your installation needs is your electrician’s call under local rules.
Which device belongs in the consumer unit is a separate question; this article’s subject is the fault itself. The owner-level check is simpler: the charger’s documentation should state its DC protection plainly, alongside its rating label and markings.
Not the Same as Insulation Monitoring
DC fast chargers use a related but different function: insulation monitoring watches the DC circuit’s insulation resistance while a session runs. AC-side leakage protection watches residual current at the supply. Both exist because DC behaves differently from AC in a fault — but they guard different circuits in different equipment. Neither replaces the installation’s earthing and bonding, verified by your electrician at install time — a sequence our guide to what a standard EV charger installation includes sets out.
Equipment With the Protection Built In
EVDANCE builds its wallboxes for this protection model: the EV-ACT3204PE 22 kW wallbox is a three-phase Type 2 unit with RFID and app control, made for IEC markets where DC detection is written into installation rules. For shared sites, the EV-AC03201T dual-connector AC station delivers up to 43 kW across two Type 2 connectors. With either unit, installation belongs to a licensed electrician, who confirms the upstream protection matches the equipment.
Frequently Asked Questions
Is DC leakage dangerous on its own?
Usually small in itself. The danger is indirect: it can blind an ordinary RCD, so a second, larger fault may not trip the protection. The protection is designed against that combination.
My charger mentions “6 mA DC protection”. What does that mean?
It means the equipment watches for smooth DC residual current and reacts at the 6 mA level, the threshold the standards set for this function. It is the in-charger alternative to a Type B RCD upstream; your electrician confirms which arrangement your installation uses.
Can I test the DC protection myself?
No. A device’s test button checks its basic trip function, not the charger’s DC detection. Verifying them belongs to the installer, with proper test equipment, at installation and later inspections.
Related Reading
- Does an EV Charger Need GFCI Protection?
- Rating Labels and Required Markings on EV Chargers
- EV Charger Certifications Explained: CE, UKCA, UL, ETL, CB and What Buyers Should Ask For
- What Is a Standard EV Charger Installation?
