VSDs and RCD/ELCB

[kafelnik]

Greetings,

 

I am currently experiencing some problems regarding Earth Leakage Circuit Breakers (ELCB's) / Risidual Current Devices (RCD's) and their tripping.

 

The VSD being used has an internal EMC filter, and I have been advised that using a differing VSD with an external EMC filter may reduce the leakage current.

 

I would very much like to know what exactly causes the earth leakage, can it be quantified, and how using an external rather than internal filter may be beneficial.

 

Thanks in advance for your assitance.

Kaf

[marke]

Hello kafelnik

 

The EMC filter is made up of a combination of series inductors and shunt capacitors. The high frequency role off of the filters is affected by the values of the inductors and the capacitors as well as the filter design and physical layout.

 

Shunt capacitors will conduct A.C. Current. There will be some shunt capacitance to earth to close the EMC loop and this will result in A.C. current flowing to earth. An earth leakage protection device will interpret this as Earth Leakage Current. If the capacitance is high, the apparent earth leakage will also be high.

 

EMC filters built into a drive have limited space and so are very reliant on shunt capacitors from the DC Bus to earth and the AC input to earth. External filters will have a more comprehensive design and will be less reliant on the shunt capacitors to earth.

 

Unfortunately, I do not believe that there is any hard and fast rulle as to the best configuration, but you can look at the capacitors to earth within the drive, and get an indication of the earth current from their value.

 

External filters, often come in a standard format and a low leakage format. The low leakage format will offer an improvment in leakage current, but may not be as effective.

 

Best regards,

[Guest]

It may be beneficial to look at the type of RCD you have installed on the circuit as well.

 

Some are DC sensing, some are not. Those with DC sensing are very much more prone to nuisance tripping when controlling circuits containing electronics and inductive loads that produce dc harmonics on the Neutral or the Earth.

 

It may be that simply changing the RCD for a different one will solve your problem.