RCDs selection in presence of DC earth leakage currents

From Electrical Installation Guide

RCDs that are connected in series or in parallel to a type B RCD are able to “see” all or part of the non-dangerous DC leakage current passing through this type B RCD before tripping. It is therefore essential to check that they are not blinded by this DC leakage current and can always provide protection in the event of an AC fault on the part of the network for which they were specified.

Certain loads (or sources) can potentially generate DC earth leakage current in normal operation: typical examples are EV charging stations[1], Variable Speed Drives (VSD), and PV inverters used for self-consumption.

RCDs associated with these loads (sources) should generally be type B. For EV charging stations[1], it is also possible to use RCDs type A/F with 6mA RDC-DD. The following paragraphs focus on type B RCDs only.

The DC leakage current generated by the loads will pass through the RCD type B, without RCD tripping, if this DC current value remains below 2*IΔn (maximum DC tripping threshold according to RCD product standard IEC 62423).

Consequently, RCDs connected in series or in parallel with this type B RCD will “see” all or part of this non-dangerous DC leakage current. It is therefore essential to verify that these RCDs do not lose any performance in the presence of this DC residual current. The potential risk is called "RCD blinding": the DC current may pre-magnetize the RCD tripping coil and make it insensitive to AC fault on the circuit they protect, therefore they will not be able to ensure their protection function.

A simple example is shown in Fig. F67, where a 30mA type B RCD is installed in series with an upstream RCD. The 30mA RCD type B lets almost 60mA DC residual current pass through without tripping. This 60mA current is seen by the upstream RCD. The problem: standard RCDs other than type B will generally not function correctly in the presence of this 60mA current.

Fig. F67 – Example of architecture where DC earth leakage current may blind non-type B RCD(s)

Different solutions are available to deal with this scenario, which are detailed in the following paragraphs:

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 Fig. F68 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.

Fig. F68 – The simplest solution = choose a type B RCD upstream of another type B RCD, to simplify selection and prevent the risk of blinding

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.

Fig. F69 – Connect the circuit that is protected by the type B RCDs, as high as possible in the electrical architecture. The RCD in parallel will only see a fraction (<< IDC) of the leakage current let through by the type B RCD (IDC), compared to an RCD in series (100% of IDC)

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 Fig. F70.

Fig. F70 – Maximum DC current acceptable by RCDs according to IEC standards
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, Fig. F71 illustrates the “coordination table” for Schneider Electric RCDs that are connected in series with a Schneider Electric type B RCD.

Fig. F71 – Coordination table. Schneider Electric RCDs that can be connected in series with a type B RCD, with no blinding effect.[2]

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.

Notes

  1. ^ 1 2 see dedicated chapter for EV charging application
  2. ^ 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
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