Engineering Guide
Built-in RCD sensor vs Type B RCD: cost, space and installation compared
A practical comparison for charger makers and installers: where the cost sits, how much space each option takes, and what you gain from fault reporting and automatic self-test.
When a charger maker decides how to handle DC fault current, the choice is not only technical. It also decides who pays, how much space is used and what the charger can tell you when something goes wrong. This guide compares the two accepted approaches from a commercial and practical point of view. If you need the technical background first, read Type A vs Type B RCD for EV chargers.
The two options in one sentence each
- Type B RCD approach. The charger has no built-in DC detection. The installer fits a Type B RCD for each charging point in the distribution board.
- Built-in sensor approach. The charger contains a Type A + DC 6mA residual current sensor (an RDC-DD to IEC 62955 for Mode 3). The installer fits a standard Type A RCD.
Both are accepted routes for dealing with DC fault current above 6 mA in IEC 61851-1. The difference is where the cost and the complexity sit.
Bill of materials for the charger maker
With the Type B approach, the charger maker saves the sensor from its own bill of materials. That can look attractive in a cost review.
With the built-in approach, the charger maker adds a compact sensor to the controller board. Depending on the model, the same part can also replace other components. For example, the 6R2Z integrates a current transformer and the 2R4Z integrates both voltage and current transformers. If your charger needs current measurement anyway, choosing an integrated sensor removes a separate transformer, its footprint and its assembly step.
The right comparison is therefore not “sensor vs nothing”, but “sensor plus any parts it replaces vs separate parts plus a more expensive installation”.
Installer and end-customer cost
This is where the built-in approach usually wins. A Type B RCD is a more complex device than a Type A RCD, and it is generally more expensive and less widely stocked by electrical wholesalers. In the Type B approach, that cost is paid at every installation.
When the charger has built-in DC 6mA detection, the installer can use a common Type A RCD. For a charger brand, that means:
- a lower total installed cost, which helps in tenders and with installers who compare complete quotations,
- fewer questions from installers who are unsure which RCD to buy, and
- less risk that the charger is installed with the wrong protection.
Panel and enclosure space
A Type B RCD is a DIN-rail device that typically occupies more module width in the distribution board than a Type A RCD. In apartment buildings, car parks and fleet depots with many charging points, panel space quickly becomes a real constraint.
A built-in sensor takes space inside the charger instead, and modern sensors are compact. The 3R14X has a 14 mm aperture and connects by harness, and the 5R15Z mounts directly on the PCB with a 15 mm aperture. For designers, the question is whether the controller board has room for the sensor, not whether the customer’s panel does.
Fault reporting and service
This difference is easy to overlook in a cost comparison, but it matters over the life of the product.
- A Type B RCD trips on its own. The charger simply loses power, and the operator sees an offline charger without knowing why.
- A built-in sensor gives a digital TRIP signal to the charger controller. The controller opens its relay, records the event and can report a “DC residual current fault” through the app or back-office system.
Clear fault information means fewer unnecessary site visits and faster diagnosis. For charge point operators, that reduces service cost, which is often more expensive over time than any component.
Self-test
A Type B RCD has a manual test button that users rarely press. EVsense sensors have a built-in self-test function triggered by the charger controller through the CLK&CHK input. The datasheets recommend running a self-test and zero calibration at power-on and again before each charging session. The charger therefore checks its DC protection automatically and can refuse to start a session if the check fails. Details are in our PCB integration guide.
Supply chain and design control
With the Type B approach, the protection device is chosen by the installer, so the charger maker has no control over its quality or availability. With the built-in approach, the charger maker selects and qualifies one sensor and buys it with the rest of the bill of materials. That gives:
- one qualified part across all markets that accept Type A + DC 6mA detection,
- predictable behaviour validated in your own test lab, and
- the option of samples, reference designs and engineering support from the sensor supplier.
Side-by-side summary
| Factor | Type B RCD in the panel | Built-in Type A + DC 6mA sensor |
|---|---|---|
| Who pays for DC protection | Installer / end customer, at every site | Charger maker, in the BOM |
| RCD the installer needs | Type B | Standard Type A |
| Panel space | Larger device per charging point | Standard Type A device |
| Space inside the charger | None | Compact sensor on or near the PCB |
| Can replace other parts | No | Yes, with integrated CT or VT/CT models |
| Fault reporting | None; charger loses power | Digital TRIP to controller; logged and reported |
| Self-test | Manual button | Automatic, controlled by the charger |
| Quality control | Chosen by installer | Chosen and qualified by charger maker |
Questions to ask in your cost review
Before deciding, put these questions to your purchasing, engineering and sales teams:
- Which parts on the current controller board could an integrated sensor replace?
- What do installers in your main markets actually pay for a Type B RCD compared with a Type A RCD, and how easy is it to buy?
- How many service visits per year are caused by “charger offline” reports with no fault code?
- Does your back office or app need to show the reason for a protection trip?
- Will one sensor cover all the markets you sell into, or will you need different hardware per region?
The answers usually make the decision clear, and they turn a component price comparison into a total-cost comparison.
Which should you choose?
For chargers built in volume, the built-in sensor approach usually gives the lower total installed cost, a smaller footprint in the customer’s panel and better service data. The Type B approach still makes sense when you install a charger that has no built-in DC detection, or when the site operator or local rules specifically ask for a Type B device.
To find the right sensor for your design, use our RCD sensor selection checklist, or contact our engineers with your charger details for a model recommendation, datasheets and a quotation.
Frequently asked questions
Is a built-in RCD sensor cheaper than a Type B RCD?
In most volume charger projects the total system cost is lower with a built-in Type A + DC 6mA sensor, because the installer can use a standard Type A RCD instead of a Type B RCD for every charging point. Exact figures depend on your market, volumes and suppliers, so compare quotations for your own project.
Does a built-in sensor replace the RCD in the distribution board?
No. The built-in sensor handles smooth DC residual current above 6 mA. The installation still needs an RCD in the supply circuit, normally a Type A device, according to local wiring rules such as the national version of IEC 60364-7-722.
Who pays for the DC protection in each approach?
With a built-in sensor, the charger manufacturer adds the sensor to the bill of materials. With a Type B RCD approach, the installer or end customer buys the Type B device for the distribution board. Many charger makers prefer to build it in, because it makes the total installed cost of their charger more competitive.
Need help choosing a sensor?
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