Type A RCDs cannot detect smooth DC fault currents from three phase EV chargers and solar PV systems. BS 7671 Regulation 411.3.3 and Table 7.3(ii) require Type B RCDs where DC fault currents may occur. This guide explains the difference, when Type B is required, and how to earth DC circuits correctly.
Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians
Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
1,000+
UK electricians
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer — DP Electrical
The rapid growth of solar PV, battery storage, and electric vehicle charging has introduced DC circuits into domestic and commercial electrical installations that were previously purely AC. DC circuits require different earthing, bonding, and protective device considerations to AC circuits — and the wrong protective device selection can leave a DC fault completely undetected.
The critical distinction is the type of residual current that a fault can produce. Standard Type AC and Type A RCDs are designed for AC and pulsating DC fault currents. They cannot detect smooth DC fault currents — which can occur in three phase EV chargers, solar PV inverters, and DC-coupled battery storage systems. BS 7671:2018+A4:2026 Regulation 411.3.3 and Table 7.3(ii) require Type B RCDs where smooth DC fault currents may occur.
Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.
AC earthing for standard electrical installations follows the requirements set out in BS 7671 Chapters 41 and 54. The earthing system (TN-S, TN-C-S, or TT) determines the available earth fault loop impedance and the type of protective device required. For AC circuits, the RCD types in common use are:
DC circuits from solar PV and battery storage are typically ungrounded (IT earthing system) on the DC side. This means the DC positive and negative conductors are not connected to earth. This is different from AC earthing, where the neutral is earthed at the transformer.
The metallic components of the DC installation (panel mounting structure, inverter enclosure, DC cable trunking) must be bonded to the installation protective earth via the PE conductor of the AC supply cable and additional bonding conductors where required. This bonding does not ground the DC conductors — it ensures that metal parts are at earth potential to prevent shock hazard from accidental contact.
BS 7671 Regulation 411.3.3 states that where a circuit includes equipment that can produce DC residual currents that might impair the operation of a Type AC or Type A RCD, a Type B RCD must be used, or a Type A RCD with additional DC fault current detection provided by the equipment itself.
Why Type A RCDs Fail on DC Faults
A smooth DC fault current (from three phase rectification) passes through the toroidal core of a Type A RCD without causing any magnetic imbalance — the DC field is constant, not alternating, so the RCD core does not detect it. The RCD will not trip. A person in contact with the fault current path on the DC side will receive an electric shock that the RCD cannot interrupt.
Type B RCDs use additional sensing windings and a DC-sensitive measuring circuit to detect both AC and smooth DC residual currents, triggering the relay within the specified 300ms (or 40ms for 30mA instantaneous trip).
BS 7671 Table 7.3(ii) (in the EV charging section, Appendix 7) identifies where Type B RCDs are required for EV charging applications. The same principle applies to other DC-generating equipment — always check Table 7.3(ii) and the equipment manufacturer's installation manual for the specific RCD type requirement.
EV charging equipment (EVSE — Electric Vehicle Supply Equipment) has specific earthing and RCD requirements under BS 7671 Section 722. The requirements depend on the charger type and power level:
PME earthing considerations for EV chargers are addressed in BS 7671 Regulation 722.411.4.1 — EV charging equipment connected to a PME supply must have additional protective measures to guard against the PEN conductor break risk. Many charger manufacturers include a PEN conductor monitoring device in their products for this purpose.
Elec-Mate's EIC app includes fields for EV charging circuit details, RCD type, earthing system, and DNO notification.
Try it free for 7 days
16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.
Start free trialSolar PV systems require careful attention to both the DC side (panels and inverter DC input) and the AC side (inverter output to consumer unit). Key requirements:
The protective earth (PE) conductor must be continuous throughout both the AC and DC sides of a solar PV or battery storage installation. For DC circuits:
DC earthing and RCD selection for solar PV and EV chargers are among the most complex areas of BS 7671 for domestic electricians. The consequences of incorrect RCD selection — particularly using a Type A RCD where a Type B is required — can leave a DC fault completely undetected and uninterrupted. Always check the specific equipment installation manual for the required RCD type and earthing configuration before commencing the installation.
Elec-Mate's Solar PV and EIC certificate apps capture all required details: RCD type, earthing system, insulation resistance test results…
Try it free for 7 daysVerified reviews from the UK App Store.
Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.
I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.
I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.
Real feedback from real sparks
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer
Sole Trader · DP Electrical
“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”
Nathan Perry
Electrician · NP Electrical Services
“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”
Jake Pizey
3rd Year Apprentice · Apprentice
Elec-Mate's Solar PV and EIC apps capture all earthing details, RCD specifications, and test results for compliant certification. 7-day free trial, cancel anytime.
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer, DP Electrical
From £6.99/mo after trial — less than a coffee a week
We use cookies to improve the app and measure what works. Cookie Policy