Coordination of residual current protective devices: Difference between revisions

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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.


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


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.
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


Such selectivity avoids the tripping of any RCD, other than that immediately upstream of a fault position:
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.
* 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 ==
== Selectivity between RCDs ==
The general specification for achieving total discrimination between two RCDs  requires the two following conditions:
* The ratio between the rated residual operating currents must be > 3
* 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 {{FigureRef|F62}}.
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.


{{FigImage|DB422244_EN|svg|F62|Total discrimination at 3 levels}}
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.


== Selectivity at 2 levels ==
{{FigureRef|F61}} is an example of application of these rules to achieve RCD selectivity with 3 levels.
(see {{FigureRef|F63}})


=== Protection ===
{{FigImage|DB422244_EN|svg|F61|Total selectivity at 3 levels}}
* 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 [[Residential and other special locations|Recommendations applicable to special installations and locations ]].


=== Schneider Electric solutions ===
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.
* 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.
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 '''[https://www.se.com/ww/en/download/document/LVPED318033EN/ "Complementary Technical Information - Selectivity, Cascading and Coordination Guide"]'''{{fn|1}}


{{FigImage|DB422245_EN|svg|F63|Total discrimination at 2 levels}}
== Example of selectivity up to 4 levels ==


== Discrimination at 3 or 4 levels ==
Example of installation with 4 levels of RCD selectivity ({{FigureRef|F62}}).
(see {{FigureRef|F64}})


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


=== Schneider Electric solutions ===
'''Note:''' The setting of upstream RCCB must comply with selectivity rules and take into account all the downstream earth leakage currents.
* 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
 
{{FigImage|DB422246_EN|svg|F64|Total discrimination at 3 or 4 levels}}
 
=== Specific case for the coordination with RCD 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 {{FigureRef|F65}}, 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 {{FigureRef|F65}}.
 
{{FigImage|DB431019_EN|svg|F65|Coordination between RCD 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 {{FigureRef|F66}}, 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.
{{FigImage|DB422247_EN|svg|F62|Example of installation with RCDs selectivity at 4 levels}}


These high security RCD type A include:
{{footnotes}}
* Acti 9 RCD iID and Vigi 300 mA, 500 mA
<references>
* Vigi NG 125 300 mA, 500 mA, 1 A
{{fn-detail|1|For more information on the different types and the selection of RCDs, you can also refer to the [https://www.se.com/ww/en/download/document/CA908066E/ Earth Fault Protection guide] }}
* Vigi C120 300 mA, 500 mA, 1 A
</references>
* Compact NSXm, NSX 300 mA, and up


{{FigImage|DB431020_EN|svg|F66|Coordination between RCD type A and RCD type B }}
{{Related-guides-intro}}
{{RelatedGuide
|image=Hp-highlight-selectivity-guide.png
|title=Selectivity, Cascading and Coordination Guide
|text=Get all required information to verify your electrical distribution design's robustness, considering overloads and short circuits.


Example of installation with discriminative protection at 2 or 3 levels ({{FigureRef|F67}})
Combine the benefits of selectivity and cascading to maximize power availability of your LV design at optimized cost.


{{FigImage|DB422247_EN|svg|F67|Example of installation with discriminative protection at 2 or 3 levels}}
Find Schneider Electric's coordination data for ACBs, MCCBs, MCBs, switches, busbar trunking (busways), motor starters and more.
|btn-text=Download the guide (.pdf)
|link=https://www.se.com/ww/en/download/document/LVPED318033EN/
}}


[[fr:Protection contre les chocs et incendies électriques]]
[[fr:Protection contre les chocs et incendies électriques]]
[[de:Schutz gegen elektrischen Schlag]]
[[de:Schutz gegen elektrischen Schlag]]
[[ru:Защита от поражения электрическим током]]
[[zh:电击防护]]

Latest revision as of 08:36, 27 April 2022

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.

Fig. F61 – Total selectivity at 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"[1]

Example of selectivity up to 4 levels

Example of installation with 4 levels of RCD selectivity (Figure F62).

Protection

  • 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.

Fig. F62 – Example of installation with RCDs selectivity at 4 levels

Notes

  1. ^ For more information on the different types and the selection of RCDs, you can also refer to the Earth Fault Protection guide

Related technical guides

Download the guide (.pdf)
Selectivity, Cascading and Coordination Guide
Get all required information to verify your electrical distribution design's robustness, considering overloads and short circuits.

Combine the benefits of selectivity and cascading to maximize power availability of your LV design at optimized cost.

Find Schneider Electric's coordination data for ACBs, MCCBs, MCBs, switches, busbar trunking (busways), motor starters and more.
Download the guide (.pdf)
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