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Use of Type B MRCDR with no Neutral

Lee Colin Ingram

We have Type B MRCDR's installed within our MDU/MDA assemblies. The manufacturer/model is Doepke - ELR-3BN+ residual current transformer.

They have been installed on supplies to VFD controlled equipment so we have followed manufacturers guidance.

My question is this.....The VFD equipment does not require a Neutral therefore only the 3 phases pass through the associated residual current transformer. 

We were able to test the device as both a Type AC and a Type A and the device operates, it does not however operate as type B when this is selected on the MFT.

I may be worried about nothing as they are operational.

Please share your thoughts

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  • If the Variable Frequency Drive is varying the frequency of the current will it sometimes produce Triplen Harmonics that will create high neutral currents that will not have anywhere to go?

  • in reply to Sparkingchip

    high neutral currents that will not have anywhere to go?

    Only if things are connected in star - otherwise you just end up with differing line currents but no overall imbalance (like a single phase HV transformer connected L-L under 3-phase overheads with no N).

       - Andy.

  • in reply to AJJewsbury

    I’m just trying to picture in my head what currents are following and where.

    Presumably the harmonic currents will circulate in the supply equipment possibly causing issues such as overheating?

  • in reply to Sparkingchip

    The hidden problem is that the fast switching edges (voltage & current) produce radiated and conducted emissions that affect other equipment.

    So.. The equipment hence has a load of filters (inductors and capacitors, shapes & sizes) to reduce these edges and frequencies, and most commonly that includes a link to Earth/CPC, even if no neutral is present. This provides the extra circuit loop for diverting the 'putative' neutral (aka 'unbalance') current, despite there being no physical neutral.

    It's that "most commonly that includes a link to Earth/CPC" part that catches most analyses. It's only a small proportion of the supply current but across a number of loads can be sufficient to trip RCDs or exceed allowable limits (e.g. combined 6 way extensions on the one plug)

    The choice of switching frequency in the inverters can also create that third harmonic that's nasty for neutral imbalance. Too many options!

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  • in reply to Sparkingchip

    The hidden problem is that the fast switching edges (voltage & current) produce radiated and conducted emissions that affect other equipment.

    So.. The equipment hence has a load of filters (inductors and capacitors, shapes & sizes) to reduce these edges and frequencies, and most commonly that includes a link to Earth/CPC, even if no neutral is present. This provides the extra circuit loop for diverting the 'putative' neutral (aka 'unbalance') current, despite there being no physical neutral.

    It's that "most commonly that includes a link to Earth/CPC" part that catches most analyses. It's only a small proportion of the supply current but across a number of loads can be sufficient to trip RCDs or exceed allowable limits (e.g. combined 6 way extensions on the one plug)

    The choice of switching frequency in the inverters can also create that third harmonic that's nasty for neutral imbalance. Too many options!

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