Using the correct Type of RCD
with equipment containing VSDs
Selecting the correct RCD and Linear loads produce sinusoidal residual
sensitivity requires an understanding currents.
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.
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.
8 FDPP - www.fdpp.co.uk