Coordination of residual current protective devices

From Electrical Installation Guide

Selectivity is achieved either by time-delay or by subdivision of circuits, which are then protected individually or by groups, or by a combination of both methods.

Such selectivity avoids the tripping of any RCD, other than that immediately upstream of a fault position.

With equipment currently available, selectivity is possible at three or four different levels of distribution:

  • At the main general distribution board
  • At local general distribution boards
  • At sub-distribution boards
  • At socket outlets for individual appliance protection

In general, at distribution boards (and sub-distribution boards, if existing) and on individual-appliance protection, devices for automatic disconnection in the event of an indirect-contact hazard are installed together with additional protection against direct-contact hazards.

The paragraphs below explain how to achieve selectivity between RCDs. But keep in mind that the sensitivity of the fault protection, if it is by RCD, must also be consistent with the maximum earthing resistance of the exposed and extraneous-conductive-parts of the equipment downstream of this RCD.

Selectivity between RCDs

The general specification for achieving total selectivity between two RCDs requires the two following conditions:

  • The ratio between the rated residual operating currents must be > 3
  • Time delaying the upstream RCD

Selectivity is achieved by exploiting the several levels of standardized sensitivity: 30 mA, 100 mA, 300 mA and 1 A and the corresponding tripping times, as shown in Figure F61.

Fig. F61 – Total selectivity at 3 levels

Selectivity at 2 levels

(see Figure F62)

Protection

  • Level 1: RCD time-delayed setting I (for industrial device) or type S (for domestic device) for fault protection
  • Level 2: RCD instantaneous, with high sensitivity on circuits supplying socket outlets or appliances at high risk for people (washing machines, etc.) See also Requirements applicable to special installations and locations.

Schneider Electric solutions

  • Level 1: Compact or Acti 9 circuit breaker with adaptable RCD module (Vigicompact NSX160), setting I or S type
  • Level 2: Circuit breaker with integrated RCD module (DPN Vigi) or adaptable RCD module (e.g. Vigi iC60) or Vigicompact NSX

Note: The setting of upstream RCCB must comply with selectivity rules and take into account all the downstream earth leakage currents.

Fig. F62 – Total selectivity at 2 levels

Selectivity at 3 or 4 levels

(see Figure F63)

Protection

  • Level 1: RCD time-delayed (setting III)
  • Level 2: RCD time-delayed (setting II)
  • Level 3: RCD time-delayed (setting I) or type S
  • Level 4: RCD instantaneous

Schneider Electric solutions

  • Level 1: Circuit breaker associated with RCD and separate toroidal transformer (Vigirex RH)
  • Level 2: Vigicompact NSX or Vigirex
  • Level 3: Vigirex, Vigicompact NSX or Vigi iC60
  • Level 4:
    • Vigicompact NSX or
    • Vigirex or
    • Acti 9 with integrated or adaptable RCD module: Vigi iC60 or DPN Vigi

Note: The setting of upstream RCCB must comply with selectivity rules and take into account all the downstream earth leakage currents

Fig. F63 – Total selectivity at 3 or 4 levels

Example of installation with selectivity at 2 or 3 levels (Figure F66)

Fig. F66 – Example of installation with selectivity at 2 or 3 levels
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