Why do EV chargers use a type A RCD?

Short answer: An EV charging circuit needs at least a type A, 30 mA RCD; type AC is not enough. A type A RCD gives adequate protection when combined with 6 mA DC leakage detection (RDC-DD, IEC 62955) inside the charger: when smooth DC leakage reaches 6 mA the charger stops charging, so the type A RCD is never blinded. If the charger has no such detection, a type B RCD is needed. In ENGN's charger installations in Bursa, a 40 A type A RCD, a 40 A circuit breaker and a surge protection device are standard. Local rules apply.

We explain the RCD types AC, A, F and B in general in our RCD types guide. This article focuses only on the charging circuit: which RCD, why, what rating and how to check it.

What kind of leakage occurs during charging?

In AC charging (Mode 3, wallbox), the grid's alternating current goes to the car's on-board charger, which converts it to direct current for the battery. The rectifier and power electronics doing this conversion can, in an insulation fault, send leakage current to earth not only as a sine wave but also as pulsating DC and smooth DC. The charger's own electronics can do the same.

The problem: an RCD measures the difference between phase and neutral currents using a magnetic core. Smooth DC can saturate that core and blind the RCD, so it may not trip at the current it should in a real fault.

Why type AC is not enough

A type AC RCD is designed only for sinusoidal leakage and does not reliably detect the pulsating DC that can come from the charger and the car. Installation standards restrict type AC to fixed equipment whose current is known to contain no DC component. The international installation standard for EV charging points, IEC 60364-7-722, requires the RCD protecting each charging point to be at least type A. If an older board has a type AC RCD, it is not used for the charging circuit; the circuit gets its own type A (or, where needed, type B) RCD.

Why type A plus 6 mA DC detection is enough

A type A RCD detects sinusoidal and pulsating DC leakage and is designed to work correctly with a smooth DC component of up to 6 mA. The danger is smooth DC leakage above 6 mA. That is what the RDC-DD (residual direct current detecting device) defined in IEC 62955 is for: it stops charging when smooth DC leakage reaches 6 mA. The 6 mA threshold in the standard was chosen precisely so that it does not impair an upstream type A RCD.

The tasks are shared:

• RDC-DD in the charger: monitors smooth DC leakage and stops charging at 6 mA.
• Type A 30 mA RCD in the board: protects people against sinusoidal and pulsating DC leakage, and is not blinded thanks to the RDC-DD.

Most home chargers on the market today include this detection, but it is wrong to assume every charger does.

How to tell whether a charger has an RDC-DD

The charger's technical data or installation manual should mention one of these:

• RDC-DD or IEC 62955
• 6 mA DC leakage detection (DC RCM)
• A statement that a type A RCD is sufficient for installation

If you cannot find this, do not assume it; ask the manufacturer or seller for written confirmation. If the manual asks for a type B RCD, the installation follows that.

When is type B needed?

• When the charger has no 6 mA DC detection (RDC-DD).
• When the manufacturer's installation manual explicitly requires type B.
• When other equipment on the same protection can produce smooth DC leakage (three-phase inverters, solar inverters) and the design calls for it.

A type B RCD also detects smooth DC but costs considerably more than type A. With a suitable charger, type A plus RDC-DD is both safe and economical. One note: because a type B RCD may trip on smooth DC only at a value higher than 30 mA, another type A RCD upstream of it can be affected. Where the charging circuit sits in the board is therefore decided in the design.

A separate RCD and a separate circuit for each charging point

• Each charging point has its own circuit from the board; no sockets, lighting or other loads share it.
• Each charging point has its own RCD and its own circuit breaker. IEC 60364-7-722 requires separate residual current protection for each charging point.
• Connecting the charging circuit under a shared RCD that protects other house circuits is not recommended: a fault during charging would black out the whole house, and the house RCD may not be a suitable type for charging.
• Where there are several charging points in a residential site or business, each gets its own RCD; one RCD does not feed several chargers.

Portable (Mode 2) chargers have their own protection box on the cable, but the socket circuit they plug into must still be protected by its own breaker and 30 mA RCD. Ready-made socket boxes with a breaker and RCD exist for this.

Why 40 A?

An RCD's rated current must not be lower than the breaker on the same circuit; it is chosen for the current it will carry continuously. Home chargers draw at most 32 A, single-phase at 7.4 kW or three-phase at 11 and 22 kW, and they draw it for hours. A 40 A RCD and 40 A breaker leave headroom for this. Single-phase circuits use a 2-pole RCD, three-phase circuits a 4-pole one. The cable size is calculated from the breaker rating and the run length (voltage drop); it is never chosen independently of the breaker. Sensitivity is always 30 mA; 300 mA is for fire protection and does not replace personal protection on a charging circuit.

Testing and checks

1. At installation: the RCD is tested with an RCD tester by measuring trip time and trip current at rated residual current; for type A RCDs the pulsating DC test setting is also used. The results and the earth resistance are recorded.
2. In use: press the T (test) button monthly or quarterly; the RCD should trip. If it does not, stop using the charger and call a qualified electrician.
3. If the charger's display or app shows a DC leakage fault: disconnect the charger and car and try again; if it recurs, have the cable, connector and car checked.

If the RCD trips while charging

• Only with one car: the car's on-board charger or cable may have an insulation fault; it needs a service check.
• In rain or morning dampness: an outdoor junction box, cable gland or the charging connector may be taking in moisture.
• With every car: the circuit insulation, the RCD itself or the earthing is measured.
• If the charging circuit is under a shared RCD: leakage from other appliances may be adding up; the charging circuit should be separated.

An RCD is never bridged or bypassed.

ENGN installations in Bursa

In our installations each charging point has its own circuit, and a 40 A type A RCD, a 40 A breaker and surge protection are standard. During the site survey we check the charger's technical data; if it has no 6 mA DC detection, or the manufacturer requires type B, this is quoted separately. An over/under-voltage relay is optional. Our installation service is available only in Bursa, Turkey; the survey is paid and the fee is deducted from the written quote if you accept the work, and every installation is carried out with an electrical design (project).

• → EV charger installation in Bursa
• → RCD types AC, A, F and B
• → Surge protection devices for EV chargers
• → Cable size calculation
• → Single-phase vs three-phase
• → How to choose a home EV charger
• → Home EV chargers
• → Charger components