in this video we'll be looking at earth leakage current how we measure it
and what we can do about it and we'll be demonstrating some of the key points using
the dcm305e earth leakage clamp meter from Megger and seeing how it can make
electrician's lives easier by spotting problems early in a non-invasive way
now we've all been there haven't we the mystery of the tripping rcd it's likely that this problem is
increasingly caused by earth leakage current so what is that well it comes in two flavors
really the one that we're most familiar with is unintentional earth leakage current you know the
type when maybe you screw back a socket front you accidentally nip the line or neutral conductor
under the fixing screw or perhaps it's pressed against a sharp point in the back of the box
and the current leaks from the line or neutral conductor to the cpc or maybe an over-enthusiastic
homeowner has pinged a nail in the wall and it's gone between the line and cpc causing current to
flow down to earth and tripping the rcd however there is also intentional earth leakage current
to consider now that may sound really weird at first because why would we want to intentionally
leak current to earth well to understand this we need to think about the types of load that
we're connecting to our electrical installations going back many years loads were fairly simple
and robust mostly loads that relied on the heating effect of electricity as in
kettles and toasters incandescent lamps things like that or perhaps they relied on the magnetic
effect as in motors pretty much the most advanced piece of technology that would have been connected
was what mynun referred to as t wireless that's a radio for the younger generation but nowadays
we're far more likely even in domestic settings to be connecting up equipment that is much more
reliant on electronics and as part of their normal natural operation these are often designed to leak
a little bit of current to earth in fact it can damage the equipment if they don't have
a good earth connection to leak current too many appliances and pieces of electrical equipment need
to protect their internal control systems from what's called electromagnetic contamination or emc
and they'll feature an electronic device called an emc filter these use what are called y capacitors
that connect from the live conductors to the protective conductors it's such a normal part
of life with things like it equipment that there's a rough rule of thumb in the industry that socket
outlets feeding it equipment are normally kept to no more than eight outlets per circuit
and it needn't be a pc or a laptop either everyday items like tvs modern washing machines dishwashers
even fridge freezers may leak a little bit of current to earth as part of their normal operation
it's not a sign of a poor quality or faulty appliance they're designed to work this way
and this is recognized by bs7671 in regulation 531.3.2 which deals with unwanted tripping
it reads residual current protective devices shall be selected and erected such as to limit the
risk of unwanted tripping the following shall be considered subdivision of circuits with individual
associated rcds rcd shall be selected and the circuit's subdivided in such a way that any earth
leakage current likely to occur during normal operation of the connected load will not cause
unwanted tripping of the device it's interesting that it uses the expression normal operation for
earth leakage current showing that this can impact on our circuits even when everything's
working healthily as it should do it goes on to state in the second part of the regulation
in order to avoid unwanted tripping by protective conductor currents and or earth leakage currents
the accumulation of such currents downstream of the rcd shall be not more than 30 percent of
the rated residual operating current now that figure of 30 is essential to making sure that
our installations comply with the regulations and when we apply it to the typical 30 milliamp rcd we
use to offer additional protection it means that the total earth leakage of circuits protected by
the rcd must be no more than nine milliamps so if you've got a split load board then the earth
leakage in all the circuits connected to each rcd must add up to no more than 9 milliamps by
sticking to this value we make sure that nuisance or unwanted tripping is minimized on a split load
dual rcd board like we've got here this can be achieved by simple measures such as keeping socket
circuits on separate rcd protection in a domestic setting with individual rcbo protection you'll
probably achieve this naturally in a commercial setting where there's a high volume of it
equipment being connected you'll need to limit the amount that can be connected to any single circuit
but let's see this in action i'm here at a typical three bed detached house it's not got very much
in the way of fancy electronic loads just the usual tvs gaming consoles a pc washing machines
dishwashers etc and we're going to have a look at just how much current it's leaking to earth
so as this incoming supply has been split to feed two consumer units i'm going to measure the total
earth leakage current within the meter cupboard here now when you first turn this clamp meter
on you may notice that it's measuring some current straight away even if you're nowhere
near any cables so if i just bring in meter cam here you can see there that we're reading about
0.023 milliamps there and that's a really important thing to remember is the fact that
we're reading on the milliamp scale so that's actually measuring about 20 millionths of an amp
it's probably just being induced by the earth's magnetic field and it's not going to impact
on our reading at all so to measure the earth leakage you need to clamp your ammeter around the
line and the neutral of the circuit that you're measuring so here we go around the incoming tails
and what this is doing is it's effectively measuring the difference in current between
the line and neutral conductors which gives us the current that's leaking down to other places mainly
down to earth now you may be asking a couple of natural questions right away the first is why
don't we just measure around the earth conductor or the cpc of the circuit well it's because we're
concerned with how an rcd sees the circuit and the rcd only sees the difference between the line and
the neutral it doesn't monitor the earth current it's also possible that the leakage current is
passing out of the circuit via another path to earth maybe down a bonded pipe or similar so the
most accurate reading for earth leakage current is really found by the difference in current
between the line and neutral as we're doing here so again if i just bring in meter cam we can see
there that we're getting about 7.7.8 milliamps and you can tell that's not a massive difference
in this installation it's well below the value for any one of the individual rcds that we're using
for protection but what we can also do is track down and see which circuit is leaking the most
current okay so we're just going to work our way along the rcbos here measuring the leakage current
and seeing what values we get so because it's a little bit tricky and awkward to get the camera
in here we'll do these one at a time and just take some pictures using meter cam so let's get started
okay so first of all on the smokes there we've got
0.85 milliamps there 0.85 somewhere around that's fluctuating a little bit
and then on the downstairs lights we're getting about 0.9 milliamps
moving on to the first floor lighting we're getting about 0.7 there so about 0.7 milliamps
and now you can see on the immersion heater we're getting about 0.79
milliamps now that's interesting there we go so that is the ring circuit that's feeding
the rest of the house so the ring final circuit that isn't feeding the kitchen
and the garage and we're getting about 3.1 milliamps on there which is interesting
and now we're looking at the ring final circuit that feeds the kitchen
and you can see there we've got about 1.8 1.85 milliamps there
and then finally we've got the oven which is leaking about 0.2 milliamps down to earth
which is quite a good sign because the oven is actually on at the moment and that's a
fairly healthy result there for them so we can see that the most earth leakage is happening
from the ring final circuit that feeds the rest of the house that isn't the kitchen the garage
and what we can do now is establish if this leakage is happening from a specific item that
we may reasonably expect to leak some current to earth or if there's an indication of a slightly
damaged cable so what i'm going to do now is i'm going to nip around the rest of the house
and start disconnecting some of the loads that are plugged in and i'm pretty sure i know which
one is going to be the culprit for this because it just so happens that this is the circuit that has
my pc connected and also happens to be turned on at the moment so i'm going to go and switch
that off first of all and see what happens to this value of current so keep an eye on that
so there you go we can see that's dropped down quite a bit that's down by about 0.6 milliamps now
so that's by turning off and disconnecting all the stuff that's connected to the pc so that's quite
handy to see that going on so let's go and switch off some more loads and see what effect that has
so i've just been disconnected the socket that supplies all of the sort of tele italian av
equipment in the lounge there and you can see that we've lost another four milliamps so you can see
we're losing a little bit of earth leakage every time we switch off those loads that are likely
to be leaking that to earth let's see if we can find some more to turn off so there we go all the
loads are now disconnected and as you can see the earth leakage current is absolutely negligible now
meaning that there's no problem with the circuit and the loads are just leaking a natural amount
of current into the circuit with of course the biggest culprit being the pc when we disconnected
that we got that drop of six milliamps you may remember in other words we know that the circuit
is healthy now i'm sure that you don't need me to spell out for you just how useful this could be
not just in commissioning and fault finding but also in terms of taking precautionary readings for
example as part of an eicr before undertaking a consumer unit change we've all been there
haven't we we've carried out a board change and circuits that have never had rcd protection before
suddenly just won't hold when they're covered by the new protection well this device can actually
show you before you ever disconnect a cable or install an rcd what leakage current that device
will see and if it's likely to trip when installed meaning you can spot problems and work them into
your plan and your costing well ahead of time i think this is going to become an absolutely
essential bit of kit for electricians but as always we want to hear from you
do you already own an earth leakage clamp can you think of times it would have got you out of
trouble if you had one have you already used one to help you in your electrical work please leave
your thoughts and questions in the comments below and as always thank you very much for watching