so it's commonplace now to have battery devices phones ipads tablets etc and to be charging them
up and they've incorporated now USB socket outlets within our 13 amp BS1363 socket outlet which is
fantastic idea unfortunately the plug-in section for our charging units or charging unit leads
because the charging unit is actually built into the back of the device itself is permanently fixed
into the on position so the switch itself has no correlation to the USB charging point below
meaning that it's permanently connected our problem is when we carry out our insulation
resistance test the electronic components built in here are part of the circuit
and the 500 volts that we use to test the map will damage those components so how are we going
to overcome doing our insulation resistance test or what are we going to have to do
in order to remove that part of the circuit on brand new installations or on periodic inspections
in order that we can carry out an insulation resistance test fantastic solution to our
problem that our charging units already built in problem to us as electricians is that our
insulation resistance test will pick it up and damage that part of the circuit let's go back into
the workshop and see what we can do so i'm back in my electrical workshop and we've now got our USB
socket outlet and i've connected up a cable into the back of it so we can perform the installation
resistance test to see what we're going to get with the permanently connected equipment being
the USB socket at the bottom that isn't affected by these switches these switches are only doing
the 13 amp plug tops and not the USB points so when we press for insulation resistance
they'll be permanently connected as will the electronic components in the back
manufacturers say that we cannot test that 500 volts because we will destroy them as does BS
7671 so therefore if we were to do a test to see if we could pick those up we should be doing it at
250 volts so let's see what 250 volts reading will get on insulator resistance when we're
testing between live conductors so between line and neutral therefore those electronic components
are in circuit let's see what that results going to be so we could be on a periodic
inspection or unaware of the electrical equipment connected to a new install and we may find that
we're going to do our installation resistance first of all at 250 volts to see if something
has been left in like a lamp or in this case the USB socket outlet which is permanently connected
so when we're testing between live conductors and press and hold my insulator resistance test button
we see we have a reading eventually when it settles down is less than one mega ohm
we know the smallest acceptable value of insulation resistance for the whole
installation is one mega ohm however we're testing at 250 volts we've obviously picked something
up in circuit it could be that we've left a lamp in but in this case we've got a USB socket outlet
connected across line and neutral and not a lamp therefore we must disconnect it if we were to do
an insulation resistance test of 500 volts on a periodic inspection it would be acceptable
to link live conductors together so line and neutral together and test them only to earth
because we cannot remove all of these lamps or in this case USB socket outlets with ease
on a brand new installation would be expected to do the test across all conductors
therefore we would be expected to remove the electronic components in here and carry out the
test that requires us to go into the socket outlet and disconnect either the line conductor from the
back of the socket outlet or the neutral conductor and if it's on a ring final circuit or upstream on
a radial we'd need to join the two conductors together and leave them in the back of the box
replace the front and do the insulation resistance test with the electronic components disconnected
so in other words then we wouldn't pass it through the electronic components offered by the USB
outlet and only insulation resistance test the cable in that case so we'll also see what we're
going to do when we go into the installation to have a look at what that looks like as well so a
very small radial circuit here that my students do after about 12 weeks with us at college
and somebody has inserted a USB socket outlet at this stage i wouldn't expect the learners to
have realized that that electronic equipment was in when they're doing the insulation resistance
test but it's quite a good rig to prove a point so if i was to test between line and neutral which i
will do at some point between live conductors for insulation resistance test i'm obviously going to
pick up the USB socket which is permanently connected as we said before these switches
will not turn on and off the sockets for the USB so installation resistance should be done at 250
volts where we're unsure of equipment in circuit and let's see if it picks it up again so if i test
between my neutral bar and the top of my breaker turn my instrument to 250 volts
carry out my insulation resistance test
we can see once again we have picked up the electronic components permanently connected
and our reading is less than one mega ohm what we have proved however in that circuit there
isn't a dead shore so we haven't got zero so in other words when the circuit was energized
it would obviously disconnect the overcurrent protection device but there's something in circuit
and on a periodic inspection we've got to go out and have a look and find out what we've left in
circuit it could be we've left something plugged in but in this case all the socket outlets have
been removed to the items that were plugged in but we have got a permanently connected USB
socket outlet for our initial learners coming in finding equipment that's been connected on
an initial verification of their own work so the install and they've got to test it and we're going
to have to do the installation resistance test at 500 volts similar to what we did with our
lighting circuit where we had a fluorescent lighting circuits electronic components
in we had to disconnect either the switching line out of that or the neutral out of the actual
fitting itself removing the electronic components allowing us to do the insulation resistance test
so we're going to have to take off the socket front in this case it's part of a radial circuit
there'll only be one connection in there if it was a ring final circuit there would be two
and we can either disconnect the neutral conductor and put in a connector block at the back
or disconnect the line conductor and put it in the back if it was a ring final circuit we'd have
to disconnect both neutrals or both lines and put them in a connector block replace the cover
so therefore we can do an insulation resistance test with the electronic components disconnected
so let's do that at the next stage so i've dropped my socket out that front off or maybe i would have
done it during the initial second fix but i've actually disconnected in this case the neutral
conductor wouldn't matter if i disconnected the line conductor i've inserted into a connector
block and remember if it wasn't a ring final circuit that would be two neutral connections
and i'm going to leave them in the back of the socket outlet i'm going to replace the cover
and go to the install and do the full insulation resistance as we have done previously for lighting
circuits so that would remove the electronic components which feed the USB socket outlets
this section here is now disconnected so we can pass 500 volts through the circuit
and complete a correct installation resistance test for our initial verification of our brand
new circuit so let's put the front back on carry out the insulation resistance test
with the socket outlet disconnected as such all the electronic components are disconnected
so with the insulation resistance test up to our level 2 students we're actually going to
do all three so we'll do between line and neutral between neutral and earth and line and earth in no
particular order we're going to set it now to 500 volts and we expect our insulation resistance for
a brand new circuit to be in the hundreds or thousands of mega ohms or off the scale
in other words we know the smallest acceptable insulation resistance for the whole installation
has to be greater than one mega ohm so we expect now with the electronic components disconnected
a fantastically high reading for insulation resistance so clip onto the neutral bar and
i'm going to go between live conductors first press and hold and now with the electronic
components disconnected we have a reading that's exceed the machine so greater than 999 mega ohms
moving across test between earth bar and the line connection
and once again i've got a reading greater than 999 mega ohms and finally between neutral and
protective conductor and again greater than 999 mega ohms it's important now i go back into the
installation and reconnect the socket outlet so the neutral goes back into the back of the socket
however if i forget i will pick it up on a live earth fault loop impedance test or when i'm
rechecking polarity the socket itself won't work however on a new install i'd like to think i would
remember to go back into the install reconnect the neutral before moving down the sequence of tests