RCD selectivity — Type S and cascading residual current devices

The 531-rcd guide and the distribution-board-selectivity guide (§536) already briefly mention selectivity between residual current devices. This article goes deeper into the two specific conditions that IEC 60364-5-53 sets for selectivity between two RCDs in series — a topic separate from the general circuit-breaker/fuse selectivity covered in §536.

Two conditions, both required

Selectivity between an upstream and a downstream RCD is only guaranteed if both conditions are met simultaneously:

1. Time discrimination: the non-actuating time/current characteristic of the upstream RCD must lie above the total operating time/current characteristic of the downstream RCD — the upstream RCD must therefore have a built-in, standardized time delay.
2. Current discrimination: the ratio between the rated residual operating current (I∆n) of the upstream and the downstream RCD must be at least 3:1 (for example a 100 mA RCD upstream of a 30 mA RCD, since 100/30 ≈ 3.33 ≥ 3).

Both conditions are needed: a 3:1 current ratio without a time delay guarantees nothing, and a time-delayed RCD with too small a current ratio (for example 30 mA vs. 30 mA) does not either.

Type S: the upstream RCD must be designed for it

An ordinary (instantaneous) RCD trips as fast as possible once the operating current is exceeded — suitable as the final (downstream) protection, but unsuitable as an upstream selective RCD, since it could then trip simultaneously with or even before the downstream RCD.

A type S (selective, time-delayed) RCD is specifically designed and tested (IEC 61008-1/61009-1) to not trip for a standardized minimum period even once the operating current has already been exceeded. At 1×I∆n this minimum non-actuating time is typically at least 130 ms — ample time for a downstream instantaneous RCD to trip first. As the fault current increases (for example at 5×I∆n) both the minimum non-actuating time and the maximum operating time shorten, but the upstream type S RCD remains designed to always let the downstream RCD go first.

Note: it is not enough to place "an RCD with a higher I∆n" upstream. Without the type S classification (or a product explicitly certified as time-delayed/selective) there is no guaranteed time discrimination, regardless of the current ratio.

Practical relevance

In a home or commercial building with a main RCD above several circuit-group RCDs (for example to protect the main connection against an accumulation of leakage currents, see also the 30% cumulative-leakage rule in the 531 guide), that main RCD must have both a sufficiently high I∆n (≥ 3× the downstream circuit-group RCDs) and be of type S — otherwise a leakage current in one circuit group may trip the entire installation instead of just that one group.

Common mistakes

1. Placing an instantaneous RCD with a higher I∆n upstream and assuming the current ratio alone guarantees selectivity — without a time delay (type S) that is not the case.
2. Not maintaining a 3:1 ratio — for example 30 mA upstream of 30 mA, or even 100 mA upstream of 100 mA: insufficient margin for guaranteed selectivity, even with a type S RCD.
3. Confusing RCD type discrimination (AC/A/F/B, see the RCD-types guide) with time/current discrimination — these are two independent requirements: a correctly selected type S main RCD does not solve a problem if the downstream RCDs fail to detect the actual fault-current waveform.

Related

• §531 — Residual current devices (RCD)
• §536 — Distribution boards & selectivity between protective devices
• RCD types AC/A/F/B — classification and selection