so i've come to a property today with um the new lab for work experience Eric
say hi Eric he's behind the camera now um and basically we've come to an installation to do
some testing um it's an empty property so i thought it was a good idea to maybe get some
video footage on earth fault loop impedance testing and maybe some reasons why we have
different settings i use a Megger tester so it'll be three lead low etcetera two lead high and we're
just going to go over the difference between the two and why we actually use them settings
so as you can see i've got my tester set to measure my earth fault loop impedance
and as you can see it comes standard as three lead low okay we can also change that to two lead high
and we can change that to two lead low as well now this has become ever more important since the
introduction of things like RCDs and RCBOs um and how we have to test them in any issues we may come
across if we're trying to test on the wrong set so what we're going to do is we're just going to test
on the different settings okay and then we will look at what happens when we now have our RCBO's
in the circuit so i'm going to start off by using the two lead high test okay now we have our RCBO's
in this installation so what we're going to do is we're going to run this test and see what happens
as you can see the voltage has appeared the test will work back
and straight away we have an error and now we have no voltage and you may may not have heard it the
RCBO did trip downstairs so now we've had the problem of the RCBO tripping what we're going to
do is we're actually going to change the setting on our tester from two lead high to three lead low
okay now the reason we are undertaking it as a three lead low now is because the test is
actually going to use another conductor okay so we're going to introduce the neutral conductor
so what the test is going to do is it is going to balance the load current between the line
and neutral to protective earth therefore the RCD should not see an unbalancing line neutral
and should not trip and allow us to carry out this test and as you can see we have a loop
value of 0.89 ohms so now we've talked about two lead high and what that does to an RCBO three
lead low etcetera we're just going to switch this now to two lead low now the two lead low
isn't necessarily something you would do on a socket circuit however it does have its uses if
you're doing loop testing and let's say you have an RCBO on the circuit or an RCD
and you take off let's say a switch cover to test behind that switch cover
sometimes if if depending on how it's wired it may not have a neutral there present so what you can
do is you can use the two lead low in the things like light switches where you don't have a neutral
and as you can see it won't cause the RCBO to trip it's not necessarily using a neutral conductor now
to balance in line and neutral however it will be applying a smaller load so the RCD um and
this doesn't work with every RCD sometimes it will still trip it depending on how sensitive they are
it'll apply a small load um where it's negligible to the RCD where it doesn't see it as that
imbalance so again just a quick recap okay now you see Eric's in shot so Eric's doing work experience
with me um there's a cool story behind that which we will do a video of a later date um but just to
recap on this really so if we're thinking about a fault loop impedance test and the settings we
need to be on we know now that a two lead high okay or a high current test will actually trip or
disconnect an RCD or RCBO so we generally only use that when we're doing a Ze okay when we're doing
our external earth fault loop impedance because we generally don't have in a domestic dwelling
an RCD down the circuit if we're doing socket outlets generally we have it on a three lead
low okay that will balance the line neutral and stop the RCD from tripping and again for things
like light switches where there's not a neutral present we can use that handy two lead low okay
so Eric you're a found found watcher of GSH Electrical right now so you know what happens
at the end of every video yeah hope this video has been some help i know that's fine that's fine