Using the correct Type of RCD with equipment containing VSDs
Selecting the correct RCD and sensitivity requires an understanding of characteristics of the load / machine design, installation requirements and the limits of use for the different Types of RCD.
Electrical loads are characterised by the current they draw from the supply, and the types of residual current that they produce. Linear loads produce sinusoidal residual currents. Non-linear loads produce complex leakage and residual currents with various characteristics including High frequency AC, Pulsed DC and Smooth DC components.
Selecting the correct Type of RCD for use with VSDs
The residual current under fault conditions is a function of the VSD design, EMC filters, VSD to motor cable connections, the motor and the location of the earth fault – see fig1 below. Therefore the manufacture of VSD/Machine must specify the Type of RCD to be used with their equipment. To prevent nuisance tripping the operational leakage current of the equipment should not exceed 30-40% of the RCD sensitivity. This information must be provided in the installation instructions, to enable selection and safe connection of the equipment to a supply – see Regulation 132.8: “The protective devices shall operate at values of current, voltage and time which are suitably related to the characteristics of the circuits and to the possibilities of danger. It is an offence under various UK Laws, to install equipment without the appropriate Manufacturer’s instructions. Regulation 114.1 explains the relationship between BS7671, Codes of Practice and statutory Regulations, relating to installation design / performance and equipment used in or connected to the installation – see Reg 120.3.
Types of RCD associated with VSD applications (Equipment Manufactures)
If this subject is not covered in the specific standard for the equipment, guidance is given in BSEN 62477-1 4.4.8
Single Phase VSD
Some single phase inverters can be used with Type A or Type F RCDs, see general limits below.
RCD Type A*: Smooth DC component < 6mA / Pulsed A.C. at 50Hz / No mixed frequency or transient components.
RCD Type F*: Smooth DC component < 10mA / Pulsed A.C. at 50Hz / Mixed frequency and transients <3kA.
*Note: Type A & F RCDs do not detect DC fault currents / Use Type B if there is a risk that these values will be exceeded.
Three Phase VSD
All 3 phase VSDs produce smooth dc (> 10mA) and variable frequency leakage currents under certain fault conditions and therefore must only be used with Type B RCDs.
For further information on this subject Doepke have an updated 60 page guide on the application of AC-DC sensitive RCDs, which can be downloaded from; http://www.doepke.co.uk/download/Techpub-17.
Chaz Andrews – Technical Manager, Doepke UK Ltd