18th Edition Mythbuster

Let Cudis shine some light on the 18th Edition.
Do I need to install an 18th Edition Consumer Unit?
No. There has been no change to the design of a metal consumer unit since the 17th Edition,
Amendment 3. However the installation itself and circuit protection included must comply with
the current regulations.
Do I need to install 100A RCD’s in split load boards?
No. The use of diversity factors to determine maximum demand when designing an installation
is still an accepted practice in the 18th Edition (following the manufacturers guidelines). In
most cases a 63A or 80A RCD would be appropriate, in cases where there is a possibility of
overloading the RCD protecting the downstream circuits, then it may be more suitable to install
100A RCD’s.
(BS 7671 Chapter 314.1)
Do I need to install Type A RCCB’s as standard?
No. For general purposes, Type AC RCD’s may be used.
Certain installations require the use of Type A, Type B or Type F RCD’s to protect against DC
fault current which may become present on the installation. Such installations include Solar
PV and Car Charging Equipment. These devices can be installed at the location of the DC
installation (at the AC side) and will prevent any DC earth leakage current entering the rest of
the installation.
(BS 7671 Chapter 531.3.3)
Do I need to install a board with RCBO’s to overcome unwanted tripping of
RCD’s/RCCB’s?
No. The total earth leakage of the circuits protected by an RCD should not be more than 30%
of the rated residual operating current (eg. a 30mA RCD, regulation state no more than 9mA of
earth leakage detected on the downstream circuits).
Bearing in mind that some circuits will naturally have more protective conductor current / earth
leakage eg, circuits powering power supply units (like in PC’s) or transformers like LED Drivers.
Careful consideration of subdivision of circuits between the 2 protective RCD’s may be enough
to combat the accumulation of fault leakage.
Although installing all RCBO’s will give more flexibility in the design of installation and reduce
the consequences from tripping as only individual circuits will be affected.
(BS 7671 Chapter 531.3.2)
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