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 occurring are installed together with additional protection against direct-contact hazards.

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 (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

Specific case for the coordination with RCDs type B

When there is a possible DC earth-leakage fault current, a RCD type B need to be used for protection against electric shock. In this case, the upstream RCD should not be blinded by the possible DC residual current and should provide its normal protection when any residual fault current happens in any other part of circuits.

For example, in the circuit of Figure F64, the 30mA RCD type B at level 2 could have a maximum DC tripping threshold of 2*IΔn, according to RCD product standard IEC 62423. That means this 30mA RCD type B could let pass through almost 60mA DC residual current without tripping and the upstream RCD should not lose any of its performance with the presence of this high level of DC residual current. That’s why it’s often proposed to use a RCD type B at level 1 to avoid any blinding effect by DC current, as shown in Figure F64.

Fig. F64 – Coordination between RCDs type B

However, Schneider Electric has proposed another possibility

Some RCD type A in Schneider Electric are qualified to be not sensitive to DC residual current until 60mA. As shown in Figure F65, they can be used upstream a 30 mA RCD type B without any blinding risk. Its type AC and type A protection behaviors are guaranteed even in the presence of a 60 mA smooth DC residual current.

These high security RCD type A include:

  • Acti 9 RCD iID and Vigi 300 mA, 500 mA
  • Vigi NG 125 300 mA, 500 mA, 1 A
  • Vigi C120 300 mA, 500 mA, 1 A
  • Compact NSXm, NSX 300 mA, and up
Fig. F65 – Coordination between RCD type A and RCD type B

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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