so i'm going to pose a question to Rob does resistance decrease proportional to the amount of
torque you add to a screw and to do that rob you look like you've brought in the world's fanciest
MFT tester and one i've never seen before just explain to me what we've got in front
of me before we start doing this test right this is a four terminal tester it's a micro
ohmmeter okay so it just records resistance it's nothing more than that but it's more accurate than
a digital multimeter it passes current through the two outer terminals the blue ones here yep
and it measures the voltage using the small inner terminals there so we can measure the voltage
across a small part or part of the circuit and it automatically negates the resistance of the leads
so by doing it this way we get a very accurate measurement of the resistance at a point in the
circuit and not the whole circuit all right and the points we're going to do is from the front of
the screw on the neutral to the conductor behind it when we add the other lead in making it the
fourth lead we'll measure the resistive path here and i've currently set it to one newton
meters of torque so we'll measure its resistance and then we'll start increasing it and see what
happens to that resistance now we would expect it wouldn't we as we get tighter to probably be a
better electrical connection and therefore a lower resistance but let's see if we can prove that and
see what happens when we over torque the terminal as well so the first one is set to one newton
meter torque am i am i allowed to use your shield okay so i found a good use for the Wiha torque
it looks like a badge yes it's a torque bar it works well so we're going to probe onto that
and then we wait a second or two and we get a reading of so that's a
reading there just tell me that reading Rob 138 micro ohms okay so that's the resistance between
the conductor and the front of the terminal at one newton meters of torque that's right yeah okay
is there a way of working out maybe what what that would dissipate in power wise is there a
way of coming up with the calculations how much energy that would dissipate if the terminal was
at one newton meters of torque yeah well now we've got the resistance reading mathematically
it's I squared R yes so if we're passing 100 amps that squared that that's 10 000
multiplied by 138 micro ohms gives you about 1.3 watts okay so that that would be if i left it in
that situation with 100 amps running through the circuit that's it would that cause any
issues over a longer period of time that's it would cause moderate heating of the terminal
probably and i'm not sure but 1.3 watts could probably be dissipated to the environment without
significant temperature rise maybe one or two degrees okay so it depends on number of
factors okay that's all right so so because of the way the enclosures et cetera it will
give it a chance to dissipate that heat okay we're gonna now set it to two so i'm gonna bring it up
and we're not gonna quite be there yet so i've gone from one newton meter to two next
so i change my bar around so i can get all the way to two so we're not the two five required so there
we go so we've set that one now with our Wiha torque screwdriver to two new meters of torque
and i'm just going to bring that terminal back up to the required torque setting of two
one click no more than one needed and you're going to allow me to do it again
i'm just going to bring in bring in the bar and we would expect this to be lower
wouldn't we i'd expect this to be a smaller number yep just not unlike so so we had 138
you wanted to prod me on the side there and say stand still yeah okay so so it has come down to
that makes logical sense doesn't it we've got a tighter terminal and we've come down
so that would mean now under the 100 amp rule you've just gone yeah it'd be 0.93 watts so so the
problem we might have had or might not have had is now obviously less of a problem it wouldn't get
quite so hot in service okay so now we're going to set it to 2.5 which is the required torque setting
and just before anyone asks when we measure the uh the voltage here using this
clever probe it doesn't matter whether this lead is at that end or that end
because we're actually measuring the vault drop between those two points okay
okay so there we go the one click it's slightly tighter now and that's at 2.5 newton meters of
torque as required this is what the manufacturer under their instructions specified to be
oh wow that's a big change 54.6 that's almost yeah let's use easy maths together it's almost
half isn't it yeah it is yeah so we're now the power being dissipated now by that terminal
would be almost negligible would you agree yeah yeah half a what or less okay but there's
no movement and we've said in other videos that we've created and with created with you
because your background is being and sort of that forensic investigator when everything's gone wrong
obviously with a little bit more movement maybe an under tight terminal we could see that resistance
greatly increasing generating more effectively heat and causing a problem isn't it yeah yeah
now we're going to take it beyond it i'm going to i don't know whether i can get up to five
so i'm going to go for four i'm gonna go for four newton meters and let's see what happens
to that reading of 54.6 so go back in with my bar and again logic suggests that reading will drop
so we're nearly there just whip it around
and then we are at four okay considering more than a manufacturer has set it and
let's see what happens to that reading if i can get it to a required torque setting
i'm just going to check that i know we shouldn't do two clicks just to check yeah i'm happy with
that one so there we go so that's a four we bring it back in let's mind it out
we probe on let's see how much the resistance has changed now okay near enough so 58
so it hasn't made any difference so so that's that's what's got so tight is better the more
power i put in it the better the electrical connection have we proved that it's kind of what
we said earlier you're flattening the conductors you're getting a bigger contact area up to a point
and once you've achieved that there's very little gain in increasing that contact pressure but what
i have done is i've put the screw head under more stress than maybe the manufacturer wants you to
because i've said it at 2.5 news i don't know what's happened to those threads i've obviously
rearranged the conductor pattern in there haven't i haven't improved it by any stretch have i no
you may even have taken out some of the springiness of that copper that maintains
the pressure and you gave a really good analogy before you talked about a jelly baby one of my
favorite sweets and you suggested when we got it to a certain uh springiness that copper believe
it or not it springs about and again we've mentioned this in a previous video but once
you squash it too hard the jelly baby being the copper conductors yeah what's really happening
then well effectively it's it's starting to yield the copper moves and work hardens yeah you may get
microscopic fractures in that copper right you may for smaller conductors you may squeeze the copper
out from the contact area yeah so there's less material carrying the current and it can actually
make things worse okay and you talked about maybe going like a cherry red colour a hundred
amps over a long period of time and other factors coming into play and that's where we start having
the really big issues aren't we yeah i mean you can find the resistance with some connectors and
and this is an example it might increase over time and if it does increase over time it can get hot
and the heat can help degrade the connection even more right so it becomes cherry red it can oxidize
it can increase the resistance further and then you get this runaway situation
just give us a brief term on this runaway just for the people just that runaway situation what's
happening then that run away well we said here for example a loose connection might give you one watt
it does one or two what's dissipated if that generates heat over time that resistance might
increase so you might find after a year or two service you might get two or three watts
dissipated because the increase in the resistance yeah and so as a consequence it's then running
hotter and it may then start to deteriorate even quicker that may then develop to five or ten watts
and it will escalate it will go up as a slope versus time and when it gets to a certain stage
that heat can't be dissipated or it leads to a large temperature rise cherry red oxide colour
is something that forensic investigators might look for after an event and that's a clue that the
conductor's been running hot for a period of time okay so when we say forensic investigator
that's at the point that the installation has failed obviously there's been an electrical fire
hopefully no one's perished in the disaster but we have talked off-camera about people that
have not survived these events so we can see there how important it is from all of the stuff that
we've looked at that we use the correct kit so in this case the Wiha torque screwdriver we set it to
the manufacturer's torque setting because we trust our manufacturer so they've done all their tests
all their product tests on it and have said if we go for 2.5 newton meters of torque that connection
it links into everything that you do when it isn't that case they've obviously got that data as well
in order that we can be reliable with our electrical connections and i think that fits in
nicely with the stuff that we've done on torque and again if torque is the first time you've
actually come across it we've done a CPD on it i'll leave a link in the description for that as
well Rob thank you ever so much for all the videos you've done with us the last couple of days it's
been brilliant and again any comments uh anything just leave them below and i'll try and get back to
you as many as i can the really difficult ones rob i might have to ring you up it's a pleasure