in this q a video we're going to answer the question what are the prescribed zones in
walls for installing cables to accessories this question relates to the video that gary and i
made on the subject of the evolve range of sockets from bg they're great looking and the fronts are
really simple to change so please go and check that video out after watching this one of course
so let's talk about the subject for this video there are two reasons we want to be concerned
about installing cables in prescribed zones one is to protect the cable from damage like having
screws or nails belted through them and the other is to protect the person doing the belting of said
screws or nails once a cable is buried in a wall behind plasterboard and surface finishes there may
not be any signs of it to the layperson or even the professional so bs7671 outlines two options
we find the first in regulation 522.6.202 where we read a cable installed in a wall or partition at a
depth of less than 50 mil from a surface of the wall or partition shell now we're going to stop
right there because that actually is our first option we can bury the cable more than 50 mil
deep in the wall and that's it no other protection is required at all however you may need to install
rcd protection for other reasons for example if it's a circuit feeding domestic lighting
circuits it's likely that at this depth even the most gung-ho of diyers won't be volleying in
two-inch screws or nails of course getting your cable buried to that depth may be a challenge
and not just for your poor apprentice's tired arms as they volley it in with a hammer and bolster
but also because it can start to affect the structural integrity of the building
for more information on that please check out the free accredited training package that we've made
on this subject on the know-how page at efix.co.uk however let's say we're installing the cable in
the real world at a normal depth it's very likely that we'll be installing at a much shallower depth
than 50 mil and so one of the options in this regulation will be required so what's the first
option the regulation continues one be installed in a zone within 150ml from the top of the wall
or partition or within 150ml of an angle formed by two adjoining walls or partitions where the cable
is connected to a point accessory or switchgear on any surface of the wall or partition the cable
may be installed in a zone either horizontally or vertically to the point accessory or switch gear
where the location of the accessory point or switch gear can be determined from the reverse
side a zone formed on one side of a wall of 100 ml thickness or less or partition of 100 ml thickness
or less extends to the reverse side or we'll look at the ore option in a moment but let's unravel
that paragraph first we can install cables buried less than 50mm deep in a zone around the top of a
room or vertically in a corner that is a hundred and fifty mil wide going off that definition i
would say where we've got coving installed the 150 mil starts from where the wall and ceiling meet
behind the coving not 150 ml down from the coving so that's pretty simple and can be super helpful
where your options are limited for instance i once had to rewire a ground floor flat with concrete
ceilings so all the cables will run around the tops of the wall in this prescribed zone
then there's the issue of getting cables down the walls to sockets and switches the regulation
allows you to run vertically or horizontally to accessories which again makes sense from an end
user point of view as hopefully they'll look at the accessory and think that the cable will
go straight out either up down left or right so can you run a cable diagonally to an accessory
well much as it makes me want to be a little bit sick in my mouth yes you can if it's buried more
than 50 ml deep in the wall or complies with the second option in this reg as an electrician of
nearly 25 years i would not be availing myself of that option it's just ingrained in me not to do it
but another question gets raised here and it's one that's been debated back and forth a fair bit on
social media in this situation let's say i've got a cooker switch and the cooker outlet plate
is below and to the left of the switch could i bury the outgoing cable in the wall and have
it change direction so it enters the outlet plate here now i really want to emphasize at this point
this isn't to be critical of anyone else's opinion and i must admit i've been backwards and forwards
over it a bit in my head but having sought advice from the eca and a member of jpel64 the answer is
yes you can if you think about it if these were two completely separate accessories then it
wouldn't be an issue to run a horizontal cable from here and a vertical one from here so it's
not a problem for the cable to change direction at this point as long as it stays in the prescribed
zone especially in this situation where the cable is connecting the two accessories together
obviously if you want to go beyond this and not change the direction of the cable in a prescribed
zone then that's also fine but to install the cable as shown here is compliant with
the regs it's also worth noting that last part where the wall is 100 mil thick or
less the zone formed on one side extends to the other face interesting that the wording there
is the zone formed indicating that the accessory defines the horizontal and vertical areas where
it's acceptable to install the cable now if you're watching this it's likely you're an
electrician and are already aware of these prescribed zones but as a chippy said to me
once on site after nailing through a cable in just such a zone where's our information
on these zones it's a good point is the wider world aware of these zones you'd hope a little
common sense would prevail but it doesn't always maybe that's why these zones are usually
referred to as prescribed zones rather than safe zones these days because they're not actually
intrinsically safe they don't offer protection to the cables from incompetent people or even to the
people themselves that's why there's another layer of protection required at the bottom of 522.6.202
where indent 1 but not indent 2 applies the cable shall be provided with additional protection
by means of an rcd having the characteristics specified in regulation 415.1.1 which basically
means a 30 milliamp device so if you're installing twin and cpc into an installation less than 50mm
deep and it's in the safe zones you have to use rcd protection for that this is in place because
in reality even getting cables into the prescribed zones doesn't mean no one will ever drill or nail
through a cable and the rcd protection ensures that if anyone does hit a cable and either create
an earth fault or get a shock to earth themselves then the rcd will disconnect and keep them safe
again it's really important to note that prescribed zones and rcd protection must be
used together to comply with the indent that we've just discussed you can't miss out rcd protection
because the cables are in the zones and you can't install cables buried just anywhere and
rely on rcd protection to comply both must be used in conjunction with each other to comply
if you choose not to use the prescribed zones or deeper than 50mm method then there is another way
we can comply with indent 2 of 522.6.202 which directs us that we should 2 comply with regulation
522.6.204 this regulation outlines a number of options indents 123 gives us the option to use
cable or cable management with earth metallic protection and closing the conductors namely swa
steel conduit and steel trunking indent four outlines a vague option for mechanical protection
to prevent the penetration of nails and screws and indent five shows us that pelve and cell
systems don't require any extra protection as they would struggle to present a dangerous voltage to
individuals coming into contact with them so in reality for a domestic setting because most
of the circuits lighting sockets showers ovens with a socket on the cooker control unit etc
will require additional protection in the form of an rcd anyway it makes sense to keep cables
in prescribed zones and protect them with an rcd rather than going to all the trouble of using swa
steel conduit or steel trunking so that's our take on prescribed zones as outlined in bs7671
if you found this helpful then check out our videos here thank you very much for watching