Coordination of residual current protective devices
Selectivity between RCDs 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.
Selectivity must be verified at all levels of the distribution, typically:
- At the main general distribution board
- At local general distribution boards
- At sub-distribution boards
- At socket outlets for individual appliance protection
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
Residual Current Devices are by design very sensitive to fault and shall be coordinated properly to achieve total selectivity, in addition to overcurrent protection selectivity.
Different types of RCDs are covered by different standard (IEC/EN 61009-1, IEC/EN 60947-2 Annex B or Annex M, IEC 61008). Anyway, whatever the type of RCD, the following rules apply to achieve selectivity between RCDs, as per IEC standards:
- the sensitivity of the upstream residual current device must be at least equal to three times the sensitivity of the downstream residual current device, and
- the upstream residual current device must be:
- of the selective (S) type (or setting) if the downstream residual current device is an instantaneous type,
- of the delayed (R) type (or setting) if the downstream residual current device is a selective type.
The minimum non-tripping time of the upstream device will therefore be greater than the maximum tripping time of the downstream device for all current values.
Figure F61 is an example of application of these rules to achieve RCD selectivity with 3 levels.
Some manufacturers propose RCD offers with a better earth leakage current measurement accuracy than the minimum required by standards. In that case, the sensitivity ratio to achieve selectivity between upstream and downstream RCDs may be lower than 3.
As an example, Schneider Electric offers VigiPacT and MicroLogic Vigi can be selective with ratios ranging from 1.25 to 2, depending on the other RCDs upstream/downstream. To know the exact values of applicable ratios for Schneider Electric RCD offers, refer to section "Selectivity of RCDs" in the latest "Complementary Technical Information - Selectivity, Cascading and Coordination Guide"
Example of selectivity up to 4 levels
Example of installation with 4 levels of RCD selectivity (Figure F62).
- Level 1: RCD time-delayed (setting III)
- Level 2: RCD time-delayed (setting II)
- Level 3: RCD selective or time-delayed (setting I)
- Level 4: RCD instantaneous
Note: The setting of upstream RCCB must comply with selectivity rules and take into account all the downstream earth leakage currents.
- ^ For more information on the different types and the selection of RCDs, you can also refer to the Earth Fault Protection guide