Devices and RCD typesType AC, A, F or B. Which RCD?

Chapter 53: devices for protection

Chapter 53 covers devices for protection, isolation, switching, control and monitoring. Know which job each device does — the exam tests exactly that distinction.

And a related one: the protective conductor current shall not contribute to the residual current measurement. An RCD compares line and neutral only.

Chapter 53 · protection, isolation, switching, control, monitoring 531 · devices for fault protection by ADS

RCDs: types, the 30 % rule and Table 41.5

Types of RCD

RCD types are marked on the device as pictograms, not letters — a plain sine wave for type AC, a sine wave plus a pulsating DC symbol for type A, and so on. Guidance Note 3 shows them in Figure 2.32 (p.97); BS 7671 (531.3.3, pp.165–166) describes Types AC, A, F and B in words only: a Type A still works with smooth DC up to 6 mA, a Type F up to 10 mA, and a Type B up to 0.4 × I_Δn or 10 mA, whichever is higher. Type AC is effectively obsolete; type A is the normal choice now.

The 30 % earth leakage rule

Keep the normal standing leakage on one RCD below 30 % of its rated residual current, or you get nuisance tripping. An RCD may trip at any value above 50 % of its rating (531.3.2(c), p.165).

30 % of 30 mA = 9 mA

That is 9 mA across every circuit on that RCD,
not 9 mA per circuit.

For scale: a single computer can leak about 2 mA.
Four computers (8 mA) and you're nearly there;
five (10 mA) and you're over.

Table 41.5 and the 50 V touch voltage

On a TT system the RCD is doing fault protection, because the earth electrode gives a much higher resistance than a supplier's earth. Table 41.5 (p.77, under 411.5.4 in Chapter 41) gives the maximum earth fault loop impedance Z_s for each RCD rating.

Table 41.5 · p.77 · maximum Z_s per RCD rating 531.3.2(c) · p.165 · standing leakage ≤ 30 % of I_Δn · may trip above 50 % 531.3.3 · pp.165–166 · RCD types AC, A, F and B in words Guidance Note 3 · Figure 2.32 · p.97 · RCD type pictograms