SHOCK PROTECTION
ELECTRICAL & ELECTRONICS
DOEPKE
BS7671 18TH EDITION REVISED RCD REQUIREMENTS
If you are involved in the safety specification
and or installation of electrical equipment,
take note of changes that will come into
force in BS7671 January 2019 (18th Edition).
There are a number of recommendations
directly linked to improving the security and
safety of electrical installations protected by
RCDs.
Why is BS7671 relevant to
most industrial sites?
The Electricity at Work Regulations and the
HSE refer to BS7671 as a “Code of Practice”
for electrical installation”. Circuits may rely
on additional protection provide by RCDs,
to reduce deaths from electrocution and or
fire. Installations using Type AC RCDs may
not provide the level of protection required,
when supplying modern equipment
containing switch mode power supplies
and inverters. The 18th Edition Regs specify
the Type of RCD required, based on the
types of residual current generated by the
equipment connected to the supply see
531.3.3 and Annex A53 Fig A53. This is
a general requirement and applies to all
installations. Particular requirements see
Part 7.
531.3.3 Types of RCD
The type of RCD installed must be capable
of providing the required level of protection
based on the residual currents that may be
generated under possible fault conditions:
“Different types of RCD exist, depending
on their behaviour in the presence of
DC components and frequencies. The
appropriate RCD shall be selected from the
following”, this clause goes on to list the
various types of RCD namely A, F, B and their
suitability for different types of residual
current.
Type AC RCDs are only listed for general
purpose applications such as electric kettles,
emersion heaters, filament lamps. Many
modern appliances contain semiconductors,
and are not suitable for connection to Type
AC RCDs e.g.
The internal topology of a Single phase VFD/
VSD will determine the residual current
characteristics (Type A, F or B) and therefore
the type of RCDs that can be installed safely
upstream. 3 phase VFD/VSD must only be
connected to Type B RCDs, irrespective of
the application or location.
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PECM Issue 34
Two examples of possible fault currents in circuits containing
semiconductor devices (PECS): see BS7671 Annex A53 Fig A53.1
Circuit with
fault location
Normal load
current I L
L
~
I L
I L
50Hz
RCD
Type
Earth fault
current I F
I F
t
t
"A"
I F
N
PE
Example 1. Phase control: I F frequency is the same as the supply current I L @ 50Hz
I F1
I L
L
I L
~
M
50Hz
N
PE
t
t
I F2
=
I F1
"F, B"
t
I F2
Example 2. Inverter control: I F2 frequency is a function of the Inverter / Motor design>>50Hz
doepke cla 07/17
Annex A53.1 gives details of typical faults
currents associated with circuits containing
semiconductors and the required RCD type
(see example 1 and 2).
These recommendations along with other
changes are directly linked to reducing the
risks associated with electrical faults, namely
fire and electrocution.
For additional information
on RCDs refer to
www.doepke.co.uk/
download/Techpub-16