in this electricians Q&A video made in association with IDACs solutions we'll be answering
the question do i need to fuse down a buzzbar connection if i'm connecting to a smaller cable
this video relates to the fascinating content produced by gordon and gary on the Woertz flat cable
system that can simplify installation work for example where you've got maybe several ev charge
points installed around a car park the idea is that you have a flat cable mounted on the wall
and then you can tap off it using specific connectors that screw
through the outer sheath and the insulation so it's very much like a buzz bar arrangement
now one question that was brought up was that the conductors connected from the tap off point down
to the charge point had a smaller cross-sectional area than the conductors in the flat cable
and there's no fuse in the connector to protect that smaller conductor so have gary and gordon
messed up or is there another explanation well let's think about it logically what kind of
protection do we need to offer to this conductor once it leaves the flat cable fundamentally we
need to provide overload and fault protection so we need to stop the load from drawing more current
than the conductor can handle and to disconnect if a short circuit or an earth fault occurs
now let's think about overload first normally we like to see an overload device at the point where
the conductor size changes the classic example being a consumer unit the thick buzz bar goes into
the bottom of the mcb and the spindly delicate lighting conductor comes out of the other side
protected at say six amps or perhaps we may think of a plug top where we go from the 2.5 millimeter
squared behind the socket to the flex that comes out of the plug protected by maybe a 13 amp five
amp or three amp fuse and it's fair to say that's good practice and it means that we can avoid
some challenges but it's not the only way we can provide overload protection to a conductor chapter
43 of bs7671 covers protection against over current section 433 deals with overload current
and 433.2 deals with the position of devices for protection against overload 443.2.1 states except
where regulation 433.2.2 or 433.3 applies a device for protection against overload shall be installed
at the point where a reduction occurs in the value of the current carrying capacity of the conductors
of the installation seems simple enough put the fuse or mcb where the cable gets smaller right
so how come we didn't need to do that with the tap off point on the verts cable system
well that regulation mentions exceptions to this rule one of which is in 443.2.2 which reads the
device protecting a conductor against overload may be installed along the run of that conductor
if the part of the run between the point where a change occurs in our case in cross-sectional
area and the position of the protective device has neither branch circuits nor outlets for connection
of current using equipment and fulfills at least one of the following conditions indent one it is
protected against fault current in accordance with the requirement stated in section 434
indent two its length does not exceed three meters it is installed in such a manner as to reduce the
risk of fault to a minimum and it is installed in such a manner as to reduce to a minimum the risk
of fire or danger to persons so this regulation is telling us that the overload protection device
doesn't have to go at the point where the change in cable size occurs it can go somewhere along the
length of the smaller cable now if you think about that from an overload point of view this makes
perfect sense because the section of cable before the protective device can never have more current
flowing through it due to being overloaded as the protective device will prevent that from happening
now with the charging point that was used in the video that gordon and gary made
there is overload protection provided in the ev charger and so we can apply this regulation
however we do still need to meet one of the two indents and these are really addressing the issue
of fault current as in what's going to protect the cable if two of the conductors touch each other
or live touches earth well we can apply indent two in this case because the length of the
drop to the charging point is less than three meters long but what about the rest of that reg
if you can install the cable so the risk of a fault fire or danger to a person is reduced to
a minimum then you can still have the protective device along the smaller conductor instead of at
the beginning it points us to regulation 434.2.1 where we find a little bit of advice in this note
which tells us that we can reduce the risk of fault to a minimum by for example reinforcing
the protection of the wiring against external influences so by providing extra protection to
the conductors between the tap off point and the charge point to prevent faults occurring
we can use the provision in regulation 433.2.2 to place the protective device in the charging unit
now another thing just to bear in mind if this is perhaps still troubling you a little bit
just think about this you may well be able to meet both indents one and two of regulation
433.2.2 as the device protecting your main distribution cable may well be sufficient to
provide fault protection for your tap off cable as well careful planning and calculation at the
design stage may well mean that the tap off cable will have sufficient fault protection
so it may seem a little unusual to be placing protective devices in positions that aren't at the
start of a circuit but there is an allowance to do it and still keep your circuits compliant and
safe of course as always we'd like to hear your thoughts on this topic do you feel comfortable
using tap off points with overload protection at the load end or do you have further questions
about this type of application if so then please leave your questions and comments in the relevant
section below all that remains in this video is to say thank you very much for watching