Which residual current device characteristic must EV charging equipment provide to detect smooth DC residual currents?

BS 7671 Section 722 requires protection against smooth DC residual current, satisfied either by a Type B RCD or by a Type A RCD combined with a 6mA residual direct current detecting device (RDC-DD to BS IEC 62955).

Detection of AC residual current only (Type AC)

Time-delayed selective operation (Type S)

Pulsating DC sensitivity only (Type A)

Detection of DC residual current (Type B or Type A with RDC-DD)

Why does BS 7671 Section 722 impose special measures where an EV charge point is supplied from a PME (TN-C-S) earthing arrangement?

On a PME supply an open-circuit PEN conductor can raise the charge point CPC and the vehicle bodywork to a dangerous potential, so Section 722 requires an additional protective measure such as a voltage-monitoring device or a TT earth electrode.

An open PEN conductor could raise exposed metalwork to a dangerous voltage

A PME supply cannot achieve a low enough Zs for a 32A charge point circuit

PME earthing prevents the use of a Type A RCD with 6mA DC detection

PME earthing forbids any earth electrode being installed at the premises

What is the maximum permitted earth fault loop impedance (Zs) for a 32A Type B overcurrent device protecting an EV charging circuit at 230V?

For a 32A Type B MCB/RCBO at 230V the maximum Zs is 1.37Ω per BS 7671 Table 41.3, which incorporates the Cmin factor of 0.95; 1.44Ω is the older pre-Cmin figure.

Free mock exam

Earthing & Protection — EV Charging Installation Practice

Free topic-focused mock exam: all 16 questions on Earthing & Protection, drawn from the wider EV Charging Installation bank. 35-minute timer, 70% to pass, worked explanation on every question. Use this when you've taken the full mock once and want to drill the topics you got wrong.

Questions: 16
Time limit: 35 min
Pass mark: 70% · 12/16
Question bank: 16

Free, no sign-up. Retake as many times as you like — the questions are reshuffled, with the answer order changed.

Sample questions

01Which residual current device characteristic must EV charging equipment provide to detect smooth DC residual currents?
- ATime-delayed selective operation (Type S)
- BPulsating DC sensitivity only (Type A)
- CDetection of AC residual current only (Type AC)
- DDetection of DC residual current (Type B or Type A with RDC-DD)
Why: BS 7671 Section 722 requires protection against smooth DC residual current, satisfied either by a Type B RCD or by a Type A RCD combined with a 6mA residual direct current detecting device (RDC-DD to BS IEC 62955).
02Why does BS 7671 Section 722 impose special measures where an EV charge point is supplied from a PME (TN-C-S) earthing arrangement?
- AA PME supply cannot achieve a low enough Zs for a 32A charge point circuit
- BPME earthing forbids any earth electrode being installed at the premises
- CPME earthing prevents the use of a Type A RCD with 6mA DC detection
- DAn open PEN conductor could raise exposed metalwork to a dangerous voltage
Why: On a PME supply an open-circuit PEN conductor can raise the charge point CPC and the vehicle bodywork to a dangerous potential, so Section 722 requires an additional protective measure such as a voltage-monitoring device or a TT earth electrode.
03What is the maximum permitted earth fault loop impedance (Zs) for a 32A Type B overcurrent device protecting an EV charging circuit at 230V?
- A0.35Ω
- B1.37Ω
- C1.44Ω
- D0.87Ω
Why: For a 32A Type B MCB/RCBO at 230V the maximum Zs is 1.37Ω per BS 7671 Table 41.3, which incorporates the Cmin factor of 0.95; 1.44Ω is the older pre-Cmin figure.

Common questions

Is the Earthing & Protection mock exam free?

Yes — 100% free, no sign-up needed. Questions are pulled at random from a substantial bank, with a worked explanation on every question after you submit.

Can I retake the mock exam?

Yes. Every attempt picks a different random selection and reshuffles the options, so each retake gives you genuinely new questions. No limit on retakes.

Does this count towards the real qualification?

No — this is independent practice material. To gain the actual qualification you need to sit the official assessment through an approved provider.