Auswahl von RCDs bei Vorhandensein von DC-Erdableitströmen
RCDs, die mit einem RCD vom Typ B in Reihe oder parallel geschaltet sind, können den gesamten oder einen Teil des ungefährlichen DC-Leckstroms „sehen“, der durch diesen RCD vom Typ B fließt, bevor sie auslösen. Es ist daher unbedingt zu prüfen, dass sie nicht durch diesen DC-Leckstrom geblendet werden und im Falle eines AC-Fehlers auf dem Teil des Netzes, für den sie spezifiziert wurden, immer Schutz bieten können.
Bestimmte Lasten (oder Quellen) können im Normalbetrieb möglicherweise DC-Erdschlussströme erzeugen: Typische Beispiele sind Ladestationen für Elektrofahrzeuge[1], Antriebe mit Frequenzumrichter (FU) und PV-Wechselrichter, die für den Eigenverbrauch verwendet werden.
RCDs, die diesen Lasten (Quellen) zugeordnet sind, sollten im Allgemeinen vom Typ B sein. Für EV-Ladestationen[1],ist es auch möglich, RCDs vom Typ A/F mit 6 mA RDC-DD zu verwenden. Die folgenden Absätze konzentrieren sich nur auf RCDs vom Typ B.
Der von den Lasten erzeugte DC-Leckstrom fließt ohne RCD-Auslösung durch den RCD-Typ B, wenn dieser DC-Stromwert unter 2*IΔn bleibt (maximale DC-Auslöseschwelle gemäß RCD-Produktnorm IEC 62423).
Folglich „sehen“ RCDs, die mit diesem RCD des Typs B in Reihe oder parallel geschaltet sind, den gesamten oder einen Teil dieses ungefährlichen DC-Leckstroms.Es ist daher wichtig zu überprüfen, dass diese RCDs bei vorhandenen DC-Fehlerströmen auch noch auslösen. Das potenzielle Risiko wird als "RCD-Blendung" bezeichnet: Der Gleichstrom kann die RCD-Auslösespule vormagnetisieren und sie unempfindlich gegen Wechselstromfehler in dem von ihnen geschützten Stromkreis machen, sodass sie ihre Schutzfunktion nicht gewährleisten können.
Ein einfaches Beispiel ist in Vorlage:Abb.,dargestellt , wo ein 30 mA Typ B RCD in Reihe mit einem vorgeschalteten RCD installiert ist. Der 30mA RCD Typ B lässt knapp 60mA Gleichfehlerstrom ohne Auslösung durch. Dieser Strom von 60 mA wird vom vorgeschalteten RCD gesehen. Das Problem: Andere Standard-RCDs als Typ B funktionieren bei diesem 60-mA-Strom im Allgemeinen nicht richtig.
Für dieses Szenario stehen verschiedene Lösungen zur Verfügung, die in den folgenden Abschnitten beschrieben werden:
- Select all RCDs impacted by this DC current as type B
- Schließen Sie den durch den Typ B RCD geschützten Stromkreis connect it as high as possible in the electrical architecture,an, um ihn parallel zu anderen RCDs statt in Reihe zu platzieren, um das Blendungsrisiko erheblich zu verringern.
- Check whether other types of upstream RCDs (AC, A, F) could be used instead,
- select RCDs that are designed to tolerate higher DC currents ohne an Schutzleistung zu verlieren, wie von bestimmten Herstellern vorgeschlagen
The earthing scheme of the installation can also have an impact.
The easy-to-select solution: use type B RCDs only
The simplest solution is to use only type B RCDs, as shown in Abb. F64 below, as they are designed to function correctly in the presence of DC currents.
However, using only type B RCDs across the whole installation can be costly.
And when you add additional circuits requiring type B RCDs (EV chargers, for example) to an existing installation, changing the existing upstream RCD can be difficult.
Connect the circuit that is protected by the type B RCD, as high as possible in the electrical architecture
Another possible solution, where appropriate, is to connect the circuit protected by the type B RCD, higher up in the electrical architecture, e.g. parallel with the (existing) RCDs, rather than in series (downward).
The DC leakage current let through by the type B RCD in normal operation remains the same (up to 2xIΔn), but only a fraction of this current will be “seen” and could impact the RCDs when they are installed in parallel with this type B RCD.
In the TN earthing system, the RCDs in parallel do not see the leakage current (see below), so any type of RCD is suitable.
In the TT earthing system, the risk of blinding is significantly reduced compared to a scenario where RCDs are installed in series. It is still essential to verify that the RCDs in parallel are not blinded by this potential DC current.
Check whether other types of RCDs than B type can accept DC leakage current without being blinded
The maximum DC current value acceptable by RCDs (other than type B) without any blinding effect is defined by the IEC standards for RCD products. This max DC current value depends on the type of RCD, as shown in Abb. F66.
| Type of RCD | Related standard | Max. DC current |
|---|---|---|
| Type AC | IEC 61008 / 61009 | 0 |
| Type A | IEC 61008 / 61009 | 6mA |
| Type F | IEC 62423 | 10mA |
Specifically, as an example, it is only possible to install a type B RCD upstream of another 30mA type B RCD: the current that may be let through by the downstream type B is 60mA, which is higher than the max 10mA acceptable for a non-type B RCD.
However, certain manufacturers such as Schneider Electric offer type A and type F RCDs that can tolerate a higher level of DC residual current, thus eliminating the risk of blinding, as detailed below.
Select non-type B RCDs with better “non-blinding” performance, from manufacturers like Schneider Electric
Certain manufacturers propose non-type B RCDs that tolerate a higher level of DC residual current without being blinded. This makes it possible to use non-type B RCDs and thus to optimize the installation
To give a specific example, Abb. F67 illustrates the “coordination table” for Schneider Electric RCDs that are connected in series with a Schneider Electric type B RCD.
With a Schneider Electric 30 mA type B RCD positioned upstream, based on this table, it is possible to use the following non-type B Schneider Electric RCDs:
- 300mA RCD: any type (AC, A, A-SI, F) with the exception of one product[2]
- 100mA RCD: types A, A-SI, F (AC = not possible)
These are more cost-effective solutions compared to using only type B RCDs, and have no impact on the performance of the RCD protection.
For complete and up-to-date RCD coordination tables in the presence of type B RCDs, please refer to the Earth Fault Protection guide. This guide also embeds digital selectors to verify coordination, even for more complex scenarios (several type B RCDs in series and/or in parallel to other RCDs, specific case of EV charging …).
Impact of the earthing system in terms of RCD blinding risk
Coordination between type B RCDs and other RCDs is easier to guarantee in the TN earthing system:
- Generally, the protection of people is achieved by a circuit breaker. The circuit breaker is not disturbed by these DC currents
- No blinding of RCDs that are installed in parallel: the residual DC current going through type B RCDs flows back through the PE conductor and is not seen by these RCDs in parallel
- RCDs installed in series are impacted. Refer to previous paragraphs for recommended ways of ensuring protection coordination. Note that the impact is the same as with the TT earthing system.
Conversely, the TT earthing system is less favorable, because:
- there are RCDs at every level of the electrical installation
- RCDs installed in series are impacted, exactly as with the TN earthing system
- RCDs installed in parallel may also be blinded (though less so than if they were in series), which also requires correct protection coordination
Synthesis
In short, if you have the choice:
- opt for the TN system
- connect the circuit protected by type B RCDs as high as possible in the electrical architecture (e.g. in parallel with other RCDs within the installation)
And finally, make use of enhanced non-type B RCDs that can operate correctly even in the presence of higher DC leakage currents (compared to the values demanded by IEC standards), as offered by manufacturers like Schneider Electric, for an optimized solution.
Anmerkung
- ^ 1 2 see dedicated chapter for EV charging application
- ^ 1 2 the figures and examples are provided for illustration purposes. Always refer to the latest version of the Earth Fault Protection guide for valid and up-to-date coordination tables
