Coordination of residual current protective devices

Discriminative-tripping coordination 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 discrimination avoids the tripping of any RCD, other than that immediately upstream of a fault position:

• With equipment currently available, discrimination 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.

Discrimination between RCDs

The general specification for achieving total discrimination between two RCDs is as follow:

• The ratio between the rated residual operating currents must be > 2
• Time delaying the upstream RCD

Discrimination 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 F34.

Fig. F34 – Total discrimination at 2 levels

Discrimination at 2 levels

(see Fig. F35)

Protection

• Level A: RCD time-delayed setting I (for industrial device) or type S (for domestic device) for protection against indirect contacts
• Level B: RCD instantaneous, with high sensitivity on circuits supplying socket-outlets or appliances at high risk (washing machines, etc.)

Schneider Electric solutions

• Level A: Compact or Acti 9 circuit-breaker with adaptable RCD module (Vigicompact NSX160), setting I or S type
• Level B: 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. F35 – Total discrimination at 2 levels

Discrimination at 3 or 4 levels

(see Fig. F36)

Protection

• Level A: RCD time-delayed (setting III)
• Level B: RCD time-delayed (setting II)
• Level C: RCD time-delayed (setting I) or type S
• Level D: RCD instantaneous

Schneider Electric solutions

• Level A: Circuit-breaker associated with RCD and separate toroidal transformer (Vigirex RH)
• Level B: Vigicompact NSX or Vigirex
• Level C: Vigirex, Vigicompact NSX or Vigi iC60
• Level D:
  • 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. F36 – Total discrimination at 3 or 4 levels

Discriminative protection at three levels

(see Fig. F37)

Fig. F37 – Typical 3-level installation, showing the protection of distribution circuits in a TT-earthed system. One motor is provided with specific protection