DC in AC RCDs Joe Robinson training on #e5

[Music] hello and welcome to this electrical principles training video it's my absolute privilege to be featuring on the e5 group channel in this video and I'm stood here with mr. Paul mean and the the main man who is trying to currently improve the electrical industry for the better along with E 5 so thank you so much for having me on your channel Paul is thanks for having us Joseph present a real pleasure so in this video what we're going to be doing is we're going to be considering the effect of DC currents on AC our CDs now this is something that's becoming a little bit more of a problem as time goes on why is that fun it is it is the electrical installations in modern times are engaging with more and more technology more products that are manipulating wave and current forms and the technology we have been using for the last 10 or 15 years is becoming obsolete to an extent and we have various challenges where we need to enhance our knowledge of how the new equipment that we're using you have in installations can affect the electrical safety and performance of our electrical installations very good so what we're going to see we're going to go over to some footage that we shot earlier with our learners we're gonna see what happens when DC current leaks into an AC circuit and the effect that has on an RCD and then we're going to come back here and we're going to explain with the aid of a drawing and with the aid of some software just exactly what's going on with these are CDs so cut to the footage let's give you a quick example of where DC can affect an RCD within a domestic installation everybody's worked in a domestic fuse board in some way or another you've gone in you've put a supply to mrs. Jones is shed six months later you've been called back husband's begin an electric shock and you think yourself what have I done well you know I've got it I'm gonna have to go back and check this now any electrician who works on electrical installation obviously will test the RCD because obviously is running a supply outside when he protests the RCD very simply he sets it to half times I dealt were in pushes his test button and it shouldn't trip because obviously it's not required to which is great it does that on both sides of sine wave for speed I'm not going to then does it I dealt orange so it's a 30 milli amp rated R CD and he does it again because when he's testing that's exactly what you should do and it trips out now it should trip within 300 milliseconds in accordance with product standard BS cn6 1w8 or double o 9 and he's great he's thinking you know it works it works on the ideal ring I'm then gonna do the super mega test of someone cutting straight for a cable let's go for 5 times ideal or in go for an actual trip within 40 milliseconds in accordance with the standard 25 milliseconds jobs are good and now the one recommendation I always say at the sparks before you leave a new installation do a ramp test you want to know how sensitive that RCD is because the last thing you want to do is the next day you've got a three-day job booked and you're being called back so I always recommend you do a ramp test to see how sensitive the device that the wholesalers are provided and this 125 milliamps I have occasionally found devices that have been 16 17 18 and with time and where these will also change the characteristics the device so that's fine so you've now you've now tested it you've certified it jobs are good and six months later husbands received a shock you've got to go back and you're thinking what have I done wrong first thing you do is you get your tester out and say well the RCD works not a problem at all so again you'll do your RCD test you obviously makes your powers on you go half times and it won't trip and you think yep fine there's nothing there's no problem here you'll go to one time and what you'll also do then you'll do one time and you'll make sure it trips because jobs are good and it should trip out of in 400 milliseconds so no there you go it's not tripped now what's happened there well what I've actually done is I've just injected 250 milliamps of DC current now I don't know that because I've just done my one times test so I'm thinking well it's a mystery how do i how do i solve the mystery I know what I'll do there's a problem in the RCD I'll push the trip barn that's what I'll do so I'll push the trip bar no no it's not working so okay so the trip button does not work it's saturated with DC so I've got a problem so I'll tell you what I'll do I'll do a ramp test to see if I can unseat it maybe there's something I'm not aware of I'll do a ramp test and I'll see what see what comes of the RAM test cuz around tastes a great feature at this time we know there's a problem I'm doing a ramp test Oh No and it's come back greater than thirty three milliamps so it's not tripping it's gone past the rating of the RCD I know I'll do I'll just I'll switch up the rating tell you what I'll be a bit dangerous I'll go to 300 milliamps and let's see where we go so you should trip in a minute or so oh there we go it does trip great 195 milliamps so there's enough there to kill somebody at some point so we've got it we've got a major problem so naturally what do we do we go out you're going to the van you're sitting in your scratchy region you'll go I don't say it wrong and then what you'll do is you'll look up and you'll go solar panels on the roof wow that's what that box was next to the mains there's an inverter there well let's go and have a quick look open up the box univer maybe burnt out DC terminals coming in they may be damaged with a DC enters the box because it's not been protected against mechanical damage or it may be that someone's run a bond up to the metallic framework of the PV array and there's a damaged cell which is naturally produced in DC and leaking down into the earthing system which naturally obviously earth and neutral are combined in some form as it leaves the installation however DC does what it wants and goes where it wants it doesn't perform or behave the same way AC does so we need to be aware of that and this is why you guys are being taught about the de hazards of AC type our CDs when they came out they were fine they were fit for purpose we had resistive loads everywhere in our houses now we have low-energy we have electronic loads so those protection characteristics can cause effectively unseen hazards that we need to be now considering now if you walk into a house and it's a crossroad from a railway station and it's a third rail railway then and there's no PV then there's probably something being picked up onto the supply cable from the railway but more unlikely it's probably going to be an Eevee or a PV or even some Inca silly as a USB socket outlet where you've fitted it onto a 25-mile back box it's got hot the rectifier is cheap quality it's got hot or melted but it's sufficient just for DC to leak out the back of the rectifier because these things don't switch off I found that once so henceforth my first video wasn't going to be about us bees but it ended up being about us bees and sockets so yeah so there you go so this is this is a real serious problem it does not trip it's saturated that magnetic core it interferes with that sine wave so the easiest way I've ever explained it and I actually did this in my charter ship interview was I said to the champion of you and me I said I've done a lot of research on how DC effects are CD's I mean or how does that work to explain how an RCD works so I said to him and I'll give you the the detail version and the simple version and he said okay I'll have a simple version so I said look I'm an RCD and I'm looking for a fault so Gary you're a fault when you call my name then I'll react as an RCD word okay so I'm looking for a fault Gary call my name Paul yes straight away I've reacted and that device would then react in trip and cut the supply I am now an RCD that's saturate with DC please call my name pull pull pull sorry did you call me no eventually they can trip if you do it five times ideal or in it's very hit-or-miss as to whether they actually trip depending on the magnitude of the fault cart but it's a real eye-opener so if you do have that this is why I suggest you go through that sequence of tests check the button a perfect example of this was I worked on Docklands Light Railway third railway traction system during the day I could walk up to an RCD I can push the button and it did nothing but when we did the electrical inspection and testing when would you do that on a railway when it's closed and there's no traction power so when it's closed and there's no traction power guess what pass is the functional test great operational interruptions can give false readings and this is the problem I've got certificates handed to me where they've said it's great I've gone out there push the button and gone no it's not it's not my contractor's fault it's the operational restrictions of the EIC are but I'm doing but it's well worth noting I always say to guys if there's two regulations you learn 130 to 160 operations additions bonding parity for safety but also appendix five external influences I work on a railway that goes parallel with a seat so if you come onto my railway and you select an erect some conjuring trunking it's either going to be GRP or stainless because when you look into the detail of external influences it will actually describe salt miss testing and pollution categories which as sparks we don't get involved with but when you start looking at working from the regulations you'll find your knowledge will immediately start opening up and what you'll find is you'll actually provide a far better quality product for your clients so it's just something to be aware of so we just saw some really really worrying effects of what happens when you apply DC current into an AC RCD pool and we discussed how potentially that could be happening inside typical domestic commercial industrial installations I'd say that if you weren't worried by what you just saw in that piece of video you didn't really understand the implications of what we were looking at there some pretty serious effects there poor indeed and for the video you've just watched as well for speed we didn't test on both sides of the sine wave because on both sides of a sine wave when we tested it it comes out with the same result so it was just for speed that we just measured it on one side of the sine wave just for speed and gravity of the guys watching but the effects are still the same on both sides of the sine wave very important point now so what we're going to do now is we're just going to try and explain some of the science behind what's happening inside that RCD to understand this fully we need to understand how a healthy RCD operates so here we've got a very simplified drawing of the internal workings of an AC type LCD and what we've got is inside the RCD we've got this iron core which is easily magnetized which is really important it we've got the line conductor passing through and that's wrapped around the iron core sometimes it goes straight through the middle that's what the principles exactly the same yeah and then the return path of the neutral is behaving in the same way and then wrapped around this side of the iron core we've got a small loop here again inside an actual RTD this might be looped around many times yep we've just simplified it here and that's what communicates with the little tripping device inside the RC bi or the RC D which is going to disconnect the circuit under fault missions we Hadid so when it's healthy one circuits happy current flows down the line through that coil and then back into the circuit goes into the circuit to the load and then the current flows back down the neutral again through the coil in the RCD and back out again now we know Paul when current flows through a coil what is generated around the coil electromagnetic field an electromagnetic field now in this case because the electromagnetic field in the line and the neutral are going to be acting in opposition to each other because the currents traveling in opposite directions whichever part of the cycle or on the magnetic fields are kind of balanced if you like they have no effect they appear within the iron core but they do not create any kind of current inside this little coil here so under healthy conditions there's no current being generated inside this little coil and therefore the tripping unit is quite happy would you agree with that yeah yeah I agree well that's good it's nice and I'm explaining it right so far huh yeah yeah just confirming ah yeah yeah and if you're wondering because you probably will be wondering if I push the circuit for tripping all that does is that pushes out to a position where it induces enough a current onto that coil where it does cause the device to disconnect really good point okay so that's great now when a healthy AC circuit develops an earth fault some of the current that's flowing down the line disappears off down a different path down the earth path either through a person literal earth or down a CPC and out to the earth that way yeah now when that happens generally speaking we will have more current flowing in the line conductor than we will have flowing in the neutral because some of this current flowing back is going somewhere else therefore the strength of the magnetic field generated around this coil is weaker than the magnetic field generated around this coil and because this one's lot slightly stronger Paul what effect does that have on this little coil here it effectively used me it effectively causes an imbalance on that coil yeah absolutely because we've got a stronger magnetic field coming from this coil it creates a little bit of current inside that coil which then triggers the tripping circuit if it's of sufficient deliverance so if more than 30 milliamps is disappearing down the CPC from the line or down through a person to earth then that will generate sufficient current inside the coil to trip this out absolutely excels is the trick fantastic yeah now the problem occurs when we start to have DC current being injected back into this AC circuit by some kind of load that we've got connected up to that and what would be typical loads that would generate that DC flowing back into the ocean with in a domestic installation a faulty e V charger inverter a direct PV leak of DC it could be a variable speed drive on a domestic washing machine that's faulty it could be yeah EVP V or inverter driven devices probably more than likely and you may get a faulty phone charger which could accidentally earth out and cause some leakage in USB sockets potentially so again you work primarily in the sort of industrial industry on the right although heavy rail head and rail I've read line and we see that problem happening there which is obviously a concern yeah but what's even more of a concern is that we're seeing this happening in domestic properties places where people who are not skilled who don't have training probably don't test their RCD as often as they should are very much susceptible to this kind of thing happening this is not something that's happening remotely it's something that's happening in people's homes yes so it is a real concern now to understand how this DC electricity is affecting our AC type RCD we need to understand a little bit about what happens when we put DC current into an AC supply so what we're going to do is we're going to break we're going to snip over to our graphing software and we're going to see what happens when we put DC into an AC circuit so what actually happens when DC current starts to get added into an AC circuit well we can illustrate it on the board here what we've got here and don't worry too much about the numbers over on the left these are just kind of random numbers that were chosen to illustrate a point here you can see the red waveform is a healthy AC waveform now the important point is that on the positive cycle it's peaking at 10 and on the negative cycle it's peaking the same value now a smooth DC waveform is generated by some of the things that Paul mentioned earlier look something like this so there's our DC waveform so that looks quite comfy that sits quite happily there it's just in theory a straight line okay now obviously we do get other types of DC we do indeed so that straight line could be directly from a leaking battery a UPS a PV system that's generating a smooth DC output stuff like TVs washing machines they use switch mode power supplies rectifiers which chop the DC but for this example it's just constant smooth DC current now what I'm going to do here is I'm very simply going to look at what happens if we bring that DC waveform back down to zero so we'll get that down to zero there and then what I'm going to do is I'm actually going to tell the software just to add those two waveforms together so what we're doing here now is every time I increase the size of the DC waveform we're going to see the effect of what happens when we add that into the sine wave and essentially when we inject DC into an AC system this is what happens to the supply so if I start now adding DC current into this supply watch what happens to the AC waveform can you see that it starts to rise up and up and up now that's an interesting effect there because what we're actually seeing what we're actually seeing here is the fact that we still got an AC waveform but can you see how on the positive part of the cycle the current goes to a much higher value and then on the negative part of the cycle it's not quite as strong as it was now don't get this mixed up by thinking this represents what's flowing down the line and what's flowing down the neutral we're simply changing the direction of the current flow and that is key to understanding what's happening inside our RC D so you can see there very clearly the more current that we add into our AC waveform the more and more we end up with a higher value on the positive part of the cycle and when the current changes direction in the negative part of the cycle the current actually lower so what we'll do now is we'll knit back over to the board and we'll have a little bit of a look at why that then affects the RCD so Paul we've just seen on the computer screen the effect there of what happens when we add DC into an AC current we end up with the positive part of the cycle having a much higher current than the negative part of the cycle now the reason this becomes a problem is it it has an effect on the iron core this is actually the bit that kind of causes is the problem because the iron core can become shocked stunned or saturated yeah by that higher current in the positive direction now the reason that happens is that inside this iron core that little molecules of iron are all shaped like little tiny bar magnets so they all sort of look a bit like this now when current is applied in a positive part of the cycle the molecules all flip to follow the path of that magnetism that is applied to it and then when the current changes direction they flip back the other way again there's this kind of constantly changing their polarity in line with the changing of polarity of a current flow now the problem arises because we've injected DC back into the circuit the positive part of the cycle becomes a higher current and the effect of that is that all of the molecules inside this core become lined up with each other in one direction but then when the current changes direction into that smaller negative part of the cycle there is not enough magnetism generated to flip those back round the other way completely and because of that the iron core remains slightly magnetized does it's that expression again being saturated which is when a magnetic core become all the molecules and lined up in one way okay the technical term for what's happening here is remanence so if I use the word remnants I just mean leftover magnetism in the core so then what we're looking at is as the current is flowing we end up with this iron core remaining slightly magnetized and then if a fault occurs because this iron core is still slightly magnet there is not enough magnetism generated by the imbalance in the two coils to fight against that remaining magnetic field and generate enough current inside this coil to trip the RCD and that is the big problem the RCD has to generate a much higher imbalance between the line in the neutral has to be a bigger flow of current to earth to generate the tiny amount of current that is required to trip the RCD would you agree with that absolutely so the devices themselves if you look at the IDL or n of a 30 million par CD if this device was the basic device for a 30 million I don't write the number of windings would be proportionate to the current that was needed to trip it if you are saturating that device you're effectively stunning or delaying the performance of the balance of the electromagnetic field henceforth it will trip but you need a huge current as we saw in the video we did a ramp test on a 30 milli amp RC D and it took a hundred and ninety-five milliamps to take the device out so what that means is if you have devices which are saturated by DC not only will the test button not see the the shunt coil but it will also delay and hinder the performance of the device to the British Standard it's made to which is a problem it happens a lot on industrial environments but more domestic now because of the new technology we're bringing in hence beer seven six seven one introduced Taipei's type F type B's we now have to consider electro magnetics as an external influence in in our farm or sterna light now absolutely so hopefully we can see from the information that we've watched in this video that if you have the types of load which are becoming increasingly common common you have that feed DC back into an AC circuit either under their normal operation or even worsened when they've gotten faulty and then a typical domestic fault occurs where line and neutral or neutral ncpc come into contact with each other or worse still somebody gets a shock then the RCD is not going to try when just 30 milliamps flows through a person now if we get more than 30 milliamps flowing through a person that is a serious problem as it goes above the level where someone can die and this is fault conditions guys now a lot of you will be watching thinking but I have a C type our CDs are they all dangerous no not necessarily however what you need to be mindful of is the connected loads of in the installation because they're micro electronics and powerful electronic they can saturate they can leak they can perform in a way which we're not used to hence they can perform in a way which can affect connected components of the electrical installation now as this knowledge grows their knowledge of how we select an erect and the wine regulations changes to inform us accordingly hence this video today absolutely so I think Paul we've covered that material really nicely both in practical presentations and also with the science behind what's actually happening inside the RCD so I think you probably don't know how in my videos because not many people get to the end of my videos so I normally say thank you very much for watching so well let's do it to you oh thank you so thank you very much for watching [Music] you [Music]

Channel: Team Electrical, e5 Group

Views: 12,725

Rating: 4.9669423 out of 5

Keywords: #e5, Joe Robinson, RCD, DC, Electricians, joerobinson, Joe Robinson training

Id: 9qX89cPGgp4

Length: 23min 38sec (1418 seconds)

Published: Mon Apr 08 2019