TT system - Coordination of residual current protective devices
From Electrical Installation Guide
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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 Multi 9 circuit-breaker with adaptable RCD module (Vigi NSX160 or Vigi NC100), setting I or S type
- Level B: Circuit-breaker with integrated RCD module (DPN Vigi) or adaptable RCD module (e.g. Vigi C60 or Vigi NC100) or Vigicompact
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 RH328AP)
- Level B: Vigicompact or Vigirex
- Level C: Vigirex, Vigicompact or Vigi NC100 or Vigi C60
- Level D:
- Vigicompact or
- Vigirex or
- Multi 9 with integrated or adaptable RCD module: Vigi C60 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
