Machines containing
Variable Speed Drives
associated with RCCBs
DC Residual Currents:
Due to the topology of 3
phase inverters, under certain
fault conditions they will
generate smooth DC residual
currents in the AC supply.
Complying with essential installation safety
requirements!
S
mall stand-alone machines/
equipment containing
3 phase VSDs must
not be connected to
circuits protected by standard RCDs.
Connecting 3 phase inverters downstream of standard RCDs will affect the
prescribed operation of the RCD and
therefore does not meet the Electricity
at Work Regulations.
EWR 4(1) 65. The safety of a system
depends upon the proper selection
of all the electrical equipment in the
system and the proper consideration
of the inter-relationship between the
individual items of equipment.
RCD Protection with VSDs /
Electricity at Work Regulations
Where the precautions taken include
an RCCB to reduce the risk of death
or injury (Regulation 8): Three phase
VSDs must only be used with Type “B”
RCCBs. The Duty Holder (Regulation
29) makes reference to the Machine /
VSD manufactures recommendations
relating to the type and characteristics
of the RCCB required. If not clearly
stated in the operating instruction they
would need to obtain the manufactures
recommendations in writing, for safe
installation and maintenance (PUWER
Regs).
perspective cannot be ignored, as can
be seen from the following example.
Figure 2 shows the frequency range
of various leakage currents present in
a system containing a 32 amp VSD; at
50Hz the leakage current is less than
3mA , however the actual maximum
leakage current occurs at 7815 Hz and
is approaching 2000mA. RCD Types AC
and A; only detect leakage currents
at nominal supply frequency i.e.UK =
50Hz.
Understanding the risks
DC Residual Currents: Due to the
topology of 3 phase inverters, under
certain fault conditions they will
generate smooth DC residual currents
in the AC supply. DC currents flowing
through standard RCDs will result in
magnetic saturation of the AC sensing
coil. The device will not function as
intended, with regard to the safe
detection of earth leakage currents.
The Electricity at Work Regulations
makes reference to the IET Regulations
as a guidance document (unless the
site is specifically covered by another
code of practice such as Mines &
Quarries), existing Regulation 331.1
is quite clear in its requirement “An
assessment shall be made of any
characteristic of equipment likely
to have harmful effects upon other
electrical equipment”. A simple
risk assessment carried out under
Regulation 331.1 in accordance with
existing Health & Safety legislation,
on a system containing VSDs and
requiring RCCB protection, will quickly
identify that Type B RCCBs must be
used with a characteristic that is
compatible with the operational and
safety requirements of the installation,
for People and or Fire protection.
AC operational leakage currents:
VSDs and their associated EMC filters
and motor cables, generate leakage
currents at nominal supply frequency
(50Hz) and at various harmonic
frequencies. Leakage currents in
the higher frequency ranges can
be significant and from a safety
Chaz Andrews – Technical
Manager, Doep