earth, with frequency components
between 100Hz <1kHz plus leakage
currents in the resonant frequency
range of the filter.
If the switching
frequency of the
inverter is in the
resonant frequency
range of the external
EMC filter (poor
design specification),
extremely high
leakage currents can
flow.
Variable Leakage Currents: The
length of cable between the inverter
and motor has a significant effect on
the operational leakage current, and
should be kept as short as possibletake note of the VSD manufactures
recommendations. If the switching
frequency of the inverter is in the
resonant frequency range of the
external EMC filter (poor design
specification), extremely high leakage
currents can flow. Changing the
switching frequency of the inverter
may not elevate the problem as it will
still be a multiple of the filter resonant
frequency. From this it can be seen
that the resultant leakage current
seen by the RCD will be complex;
vectorial sum of the capacitive,
inductive and resistive
elements of the leakage
currents plus the various
frequency components.
Transient Leakage
Currents: These occur
during switch-on / switchoff and as result can
cause the RCD to trip if
the transients exceed
the defined tripping
characteristics of the
selected RCD. Type B RCCBs
have a higher resistance to
inrush residual currants with
a maximum duration of 10ms
<3KA.
RCD protection to meet existing
Regulations
When there is a requirement to include
RCD protection to meet existing
regulations, inverter manufactures
recommend the use of Type B RCDs
for 3 phase VSDs. Doepke offer a free
64 page Technical Application Guide
covering the selection of RCDs for
use with 3 phase VSD installations
and measures for reducing associated
Remote pumping stations/
distributed process
installations subjected to
wide temperature changes
create conditions for
condensation on de-energised
electrical equipment. A
build-up of dust on insulated
surfaces subjected to moist
damp conditions, can create an
additional fire hazard.
leakage
currents.
http://www.doepke.co.uk/rcd/
rcdB.html
Chaz Andrews – Technical Manager
Doepke UK Ltd.
Email: [email protected]
Web: www.doepke.co.uk
Issue 16 PECM
65