in this presentation we look at carrying out the perspective fault current at the origin of the
installation to come to the mains end whoever got the metering equipment and the consumer
unit and we're gonna measure the maximum amount of fault current that could flow if there was a
fault in this part of the installation earth fault loop impedances are carried out at the
furthest point of radial circuits the mid point of ring final circuits to work out if there would
be enough fault current at that position in order to operate an overcurrent protection
device but this is to see if it can handle a fault current of its highest value therefore
the prospective fault current measurement will be done within the consumer unit itself so this
test is going to prove whether our over current protection device at the origin so the incoming
cutout fuse and individual circuit breakers can carry and withstand the fault current at the
point of origin so in other words if there was a fault here in the position where had its lowest
impedance therefore the maximum amount of fault current can the overcurrent protection device
clear that fault without causing potentially the overcurrent protection device in this case to arc
across and continue to carry current or destroy the circuit breaker so again it arcs across and
continues to work so we need to see if that fault current can be cleared under fault conditions by
taking a measurement the fault current that can be withstand from the cutout fuse depends on its
type we've got a BS 1361 type two fuse in this installation however it could be and stayed on
the side here we've got a BS 88 type two fuse and the current that they can withstand and
fault conditions is different you need to look in BS 7671 for confirmation the fuse here in our
cutout can withstand fault currents up to 33 kA this one here the BS 88 can go up to thirty one
point five ka and our over current protection devices in this installation are up to 6 kA
so that's the amount of fault current they can withstand but it needs to be measured in order
to prove they can withstand that fault current at the origin itself so we're gonna carry on that
test before we start the test obviously we're going to need to isolate the consumer unit and
remove the cover isolation for the main double pole switch or linked main switch here and as
always I do it off load so turn off the individual circuit breakers for the installation the RCD's
as well and now we can turn the main switch off offload we can remove the cover and then
re-energize the circuit in order to carry out the PFC test the prospective fault current test we're
going to take several PFC readings from inside the consumer unit one for the incoming cutout
and one for the overcurrent protection devices we're gonna do both short-circuit so if we were
doing the one for the incoming fuse between the top here of the tails we knew between line and
neutral and we're going to do between line and earth for the perspective earth fault current
noting down the highest of the two readings we're going to repeat the process on top of
a circuit breaker going between line and neutral bar for short circuit and line and earth bar for
prospective earth fault current again noting the highest of the two readings remembering
we can't exceed a fault current of 33 kA 33,000 amps for the over current protection device in
this installation and these are 6 kA breakers therefore we can not exceed 6,000 amps above
the top of the circuit breaker let's make those measurements so I've removed a consumer unit cover
and I'm now ready to do my test first of all the test between line and neutral at the top of the
linked main or double pole switch for a short circuit and then between line and the earth bar
for the earth fault recording the highest of the two readings MFT needs to be set down into first
of all loop between line and neutral and as we going between line and neutral for a loop test
we need it in the red and blue sections at the top as always I'm going to probe on the least
dangerous conductor first being neutral then line off the line and off the neutral and then
repeat the process between line and the earth bar the consumer unit can now be energized and I can
probe on for the test so neutral first followed by line I'm looking at the reading at the top of
the machine being 312 amps off the line and off the neutral I now need to move my lead down one
into the green section and move it up to do a test between line and protective earth and again I'm
going on the least dangerous one first let's just change you back to 2 lead 2 lead hi so we're going
to go between line and earth bar probing on the least dangerous first in the earth bar and then
the line bar itself reading 424 amps take it off the line bar and off the earth bar so we're gonna
record the highest of those two readings which was a perspective earth fault current of 424 amps
often is higher between the line and earth because you'll have things such as the bonding conductors
to extraneous conductive parts such as water gas oil and framework of the building however you do
record the highest one whichever is in this case it was between line and the protective
conductor it also was considerably less than the maximum of 33,000 amps for the incoming fuse so
we satisfied that part of the test will now go on and do a test on the top of the circuit breaker
next I've reset my instrument back between the line and neutral and moved my probe into the
blue section at the top of the instrument turn on my RCCB and turn on the circuit breaker in
to test it wouldn't matter which of the circuit breakers we selected probe on again the least
dangerous first being neutral on to the top of the line we've got three hundred and thirty two
amps off the line and then off the neutral set it round between now line and protective
earth move across my probe but this time I'm going to need to be in the low setting because
I have got my RCCB in circuit so again earth connection first followed by the top of the line
takes a moment
got 311 so we can see this time the higher reading was between the line and neutral
and that's the one that we'll need to make a mental note of these over current protection
devices have a 6000 and breaking capacity and we know we're near even 400 amps with
our tests so obviously they're acceptable we will talk about the serviceable rating of the
devices as we move through the classroom so we've managed to carry out the prospective
fault current at the top of the switch and the switch to the earth connection top of
the breaker between neutral and earth in order to prove that both can handle the current and
the fault conditions in this case we've making sure the the incoming supply fuse can handle
anything up to 33,000 amps we were considering less and the circuit breakers themselves can
handle anything up to 6,000 amps and again we were considerably less so we've carried out the
prescriptive fault current at the origin of the installation I hope this video has been some help
you