Tracker Posted February 1, 2013 Share Posted February 1, 2013 My electrical theory is a bit shaky so does anyone know please whether I should use an RCD when using a mains operated microwave with a suitable inverter (as per link)? Both inverter and microwave are metal bodied, albeit painted, but neither the inverter or the microwave are of course earthed? Is it sensible precaution, paranoia or pointless? http://midsummerenergy.co.uk/buy/sine_wave_inverters/Fountainhead1500WPS.html Link to comment Share on other sites More sharing options...
sshortcircuit Posted February 1, 2013 Share Posted February 1, 2013 An RCD works by detecting an in-balance in current in phase conductors to earth. If there are no earths then how would you get an in-balance? Link to comment Share on other sites More sharing options...
Tracker Posted February 1, 2013 Author Share Posted February 1, 2013 sshortcircuit - 2013-02-01 5:34 PM An RCD works by detecting an in-balance in current in phase conductors to earth. If there are no earths then how would you get an in-balance? In the event of a fault would the earth cable become live? I don't know - that's why I asked!! Link to comment Share on other sites More sharing options...
hallii Posted February 1, 2013 Share Posted February 1, 2013 I am also slightly confused by this, the inverter is not earthed but floats above ground, so any physical earth .e,g, a copper stake driven into wet soil etc, would introduce more problems than it would solve. Since you need an imbalance to earth for an RCD to operate and the inverrter has no true earth then the rcd would not work. My electrictian friend scratched his head when I asked this question and concluded that if the inverter has no earth (some do have a wing nut for an earth) then it is best left without one and so no RCD. If it has an eath then it could be used with say, an earth stake and the RCD would then work. He advised that mains and inverter should always be isolated from each other. I also believe (but stand to be corrected) that the + and - of the inverter output are at 120v each to earth, so a reduced risk of frying should a fault develop. There is a lot more to this, earth loops were mentioned, and other stuff I don't grasp. H Link to comment Share on other sites More sharing options...
Derek Uzzell Posted February 1, 2013 Share Posted February 1, 2013 Inverter earthing was discussed in these threads from other forums: https://www.motorhomefacts.com/ftopic-14307.html&postdays=0&postorder=asc&highlight=victron&start=0 http://www.ukcampsite.co.uk/chatter/display_topic_threads.asp?ForumID=8&TopicID=238391&PagePosition=1 (Seems straightforward enough to me...) Link to comment Share on other sites More sharing options...
lennyhb Posted February 1, 2013 Share Posted February 1, 2013 sshortcircuit - 2013-02-01 5:34 PM An RCD works by detecting an in-balance in current in phase conductors to earth. If there are no earths then how would you get an in-balance? You have just described how an Earth Leakage Trip works not an RCD. An RCD compares the flow and return current in the two conductors you do not need an earth for an RCD to work. How safe an inverter output is depends on the construction of the inverter. Some have have the neutral side bonded to the case which is means it's probably connected to the vehicle chassis which is not a true earth, some have the output via a centre tapped transformer. In practice it shouldn't do any harm to fit an RCD on the output if it makes you feel safer it's not going to break the bank. Link to comment Share on other sites More sharing options...
Brian Kirby Posted February 1, 2013 Share Posted February 1, 2013 I understood that the earth connection was only relevant to the earlier versions, called earth leakage circuit breakers (ELCBs). These looked for current flowing to earth, and opened the breaker if it was detected. My understanding of residual current devices (RCDs), is that they monitor current in the (+) and (-) wires and, if there is an imbalance between the two, assume an earth leak and open the breaker. I understand this is safer, as it will still give protection from shock if for any reason the earth connection is absent. Since inverters are not generally wired with an earth connection, you would have to directly bridge the (+) and the (-) wires to get a shock. Were that to happen, the current through the RCD would presumably not vary (the return still matching the flow, albeit while you light up), so it seems to me the RCD would not trip. If that is correct, I think the RCD would be redundant while the microwave is powered from the inverter, but should be retained in case you power it from the mains at some future time. Now Clive can come in and give us the correct version! :-D Sorry Lenny - was writing mine while you were writing yours! :-) Link to comment Share on other sites More sharing options...
Tracker Posted February 1, 2013 Author Share Posted February 1, 2013 lennyhb - 2013-02-01 6:44 PM In practice it shouldn't do any harm to fit an RCD on the output if it makes you feel safer it's not going to break the bank. Thanks Lenny - the cost is even less as I already have three RCDs!! The point is - is there any point - and if there isn't what would happen in the event of the microwave becoming live!! I want to cook my lunch not my lovely darling wife - on the other hand..........................................? Link to comment Share on other sites More sharing options...
T8LEY Posted February 1, 2013 Share Posted February 1, 2013 If I understand correctly what you said in your OP the microwave oven does not have an earth connection, which suggests it's double insulated and therefore doesn't need earthing. If your power source (inverter) is not earthed an RCD won't help you. An RCD needs somewhere for a % of the current to go instead of back along the neutral to the source. Without an earthed supply there is no route for the current to return to the origin; think of an electric shaver socket with it's unearthed secondary winding. Link to comment Share on other sites More sharing options...
Tracker Posted February 1, 2013 Author Share Posted February 1, 2013 T8LEY - 2013-02-01 10:12 PM If I understand correctly what you said in your OP the microwave oven does not have an earth connection, which suggests it's double insulated and therefore doesn't need earthing. If your power source (inverter) is not earthed an RCD won't help you. An RCD needs somewhere for a % of the current to go instead of back along the neutral to the source. Without an earthed supply there is no route for the current to return to the origin; think of an electric shaver socket with it's unearthed secondary winding. The microwave has a three pin plug and the cable is heavy enough to be 3 core and feels like heavy duty 3 core but as the plug is one of those one piece sealed on things short of cutting it off I have no way of knowing whether it is or isn't? Assuming the microwave is not double insulated I wonder what would happen if there were a fault, unlikely as it is maybe, and the casing became live, so seeing as I already have several RCD devices doing nothing would it be worth using one - or pointless? Call me old fashioned but I have a sort of mistrust of things I don't understand - especially things that can kill me if they go wrong!! I know I am protected at home by the circuit breakers in the main fuse box and I know they work because they pop off from time to time when there is a fault and I know they work on site on an EHU - I just want to have the same confidence in the van on an inverter? Link to comment Share on other sites More sharing options...
lennyhb Posted February 1, 2013 Share Posted February 1, 2013 Rich I think you have had too much wine tonight even you know how to check if the microwave has an earth. get your multimeter out set it to ohm and check between the earth pin on the plug and the chassis if you get a reading its earthed if it's open circuit you can assume it's double insulated. If it don't make sense it's 'cause I just got back from the pub. (lol) Link to comment Share on other sites More sharing options...
T8LEY Posted February 1, 2013 Share Posted February 1, 2013 Tracker - 2013-02-01 11:29 PM T8LEY - 2013-02-01 10:12 PM If I understand correctly what you said in your OP the microwave oven does not have an earth connection, which suggests it's double insulated and therefore doesn't need earthing. If your power source (inverter) is not earthed an RCD won't help you. An RCD needs somewhere for a % of the current to go instead of back along the neutral to the source. Without an earthed supply there is no route for the current to return to the origin; think of an electric shaver socket with it's unearthed secondary winding. The microwave has a three pin plug and the cable is heavy enough to be 3 core and feels like heavy duty 3 core but as the plug is one of those one piece sealed on things short of cutting it off I have no way of knowing whether it is or isn't? Assuming the microwave is not double insulated I wonder what would happen if there were a fault, unlikely as it is maybe, and the casing became live, so seeing as I already have several RCD devices doing nothing would it be worth using one - or pointless? Call me old fashioned but I have a sort of mistrust of things I don't understand - especially things that can kill me if they go wrong!! I know I am protected at home by the circuit breakers in the main fuse box and I know they work because they pop off from time to time when there is a fault and I know they work on site on an EHU - I just want to have the same confidence in the van on an inverter? If you're going to go down the earth everything route, you need to know if your inverter is earthed also. Do as Lenny says and test the circuit, but this time check if the inverter output socket is earthed. If it is I think an RCD would be useful. Don't know about any other protection on your inverter, for the output, but an RCD will only give you earth fault protection; they don't do short circuits. For that you need an mcb. Alternatively, you could do both jobs with an RCBO. Link to comment Share on other sites More sharing options...
sshortcircuit Posted February 1, 2013 Share Posted February 1, 2013 lennyhb - 2013-02-01 6:44 PM sshortcircuit - 2013-02-01 5:34 PM An RCD works by detecting an in-balance in current in phase conductors to earth. If there are no earths then how would you get an in-balance? You have just described how an Earth Leakage Trip works not an RCD. An RCD compares the flow and return current in the two conductors you do not need an earth for an RCD to work. And how on earth would you get an imbalance if an earth path is not there? Link to comment Share on other sites More sharing options...
Brian Kirby Posted February 2, 2013 Share Posted February 2, 2013 sshortcircuit - 2013-02-01 11:19 PM lennyhb - 2013-02-01 6:44 PM sshortcircuit - 2013-02-01 5:34 PM An RCD works by detecting an in-balance in current in phase conductors to earth. If there are no earths then how would you get an in-balance? You have just described how an Earth Leakage Trip works not an RCD. An RCD compares the flow and return current in the two conductors you do not need an earth for an RCD to work. And how on earth would you get an imbalance if an earth path is not there? In a normal, mains fed, installation the earth and the neutral are one and the same, because they are connected at the transformer. All current travels back to the power station from there via (literally) the earth, there is no neutral wire. Therefore, if an earth wire had become disconnected and you touched a live, the current would flow through you to earth. An RCD would then see more current flowing on the live than was flowing on the neutral (because of what was passing direct to earth via you). That would give the imbalance, and that imbalance would trip out the RCD. So, on a mains installation, providing there is an RCD, it is pretty much fail safe, and an earth pretty much belt and braces. But note, only where there is a RCD, in all other cases the earth is vital. However, from an inverter, there is a fully wired path from the battery to the inverter, then from the inverter to the appliance, from the appliance back to the inverter, and from the inverter back to the battery, and there is no "earth" wiring. Therefore, there is no path through you back to the inverter, or the battery, should you touch a live wire. So, even with Rich's live casing, you would get no shock. However, if you were to simultaneously touch a live and a neutral, the current would pass through you, so you would get a shock, but then the RCD would see no imbalance, because nothing had "leaked", so would not trip. The best advice would be never to touch anything fed via an inverter while the inverter is connected to the battery. Then, you can't bridge live to neutral, so can't get a shock. Link to comment Share on other sites More sharing options...
Brambles Posted February 2, 2013 Share Posted February 2, 2013 If the invertor had one of its outputs tied back to the negative supply, or as is common had filter capcitors on both the outputs tied to its frame which is subsequently connected to the chassis or negative feed then a fault in the appliance making the case become 'live' could indeed give you a shock, the path being through you and possiby because you are also holding onto some other item which is connected to the chassis. Hence the case of the microwave should be connected to the chassis via its earth lead in the appliances mains supply cable. Then the RCB would serve a purpose. In a microwave oven there is a large diode connected between the case and the capacitor. This diode connection also connects to the high voltage input of the magnatron which has its other connection to the case. There is about 2000 volts negative generated across the magnatron. Under certain fault conditions which whilst are unlikley could result in the case charging up to 2000 volts in respect to its mains supply input. Therfore the a microwave oven case MUST be earthed to the world around you which in this case is the van chassis. This is achieved very easily by connecting the earth lead in the supply cable to the chassis which of course the battrey negative is also in turn connected to the chassis. Most invertors will have the earth pin on the output socket connected to its frame which in turn is connected to either the negative supply or chassis. Now you have the earths sorted out, which you must do, the RCB will have a purpose and will function should a fault occur. Apologies for any errors and typos...early hours of the morning and time I retired to the cottony cotton. You must have an 'earth' or should it maybe now be called a ground, or even chassis, connection to the case of the microwave. Link to comment Share on other sites More sharing options...
Brian Kirby Posted February 2, 2013 Share Posted February 2, 2013 Thank you Jon. I sort of understand that. However, could you please answer a few more questions, for my edification? Is it always the case that the mains 230V circuits in a van are earthed to the chassis? I seem to have heard for and against doing this. Is it normal good practise to ensure the inverter case is grounded, and that the 230V circuit "earth" is connected to that ground? If 230V AC is generated from a 12V DC battery via an inverter, what happens if the 230V current is shorted back to the battery? Does the current flow into the battery at 230V, or would the fact that point of origin is the battery negate the effect of the inverter, reducing its voltage back to 12V, or do both circuits just "die"? Hope this is clear! :-) What I don't understand is that the 230V power is generated via a transformer, which is a separate, insulated, circuit to the 12V circuit. Accepting that there must first be a fault, how does power on that broken 230V circuit actually flow back onto the 12V circuit? Can't get my head around that at all! All I can visualise is an open 230V circuit, that must somehow be completed back to the transformer for current to flow. Blind spot, I'm afraid. Link to comment Share on other sites More sharing options...
Brambles Posted February 2, 2013 Share Posted February 2, 2013 My reply made sense to me last night but today I realise it is very confusing and I harrdly know what it means myself. . There is a proviso in my reply -- If one of the outputs is connected back to the negative of the battery or to ground. This is actually unlikely as most inverters ( got the spelling correct now) as you say are indeed via a transformer and are indeed floating in respect to the supply from the battery. A fault in an appliance to its case just connects one of the inverter outputs to that case and as the output floats there is no path back to the battery so it is safe to touch. However this fault could go undetected for years until you decide to use mains hook up and then hopefully the fault would be detected as a short to the mains earth. But back to microwaves. The problem with mcrowaves is they are generating high voltage inside and via the use of a diode. The problem with this is you now have 2000 Volts kicking around and potentially under fault conditions you could charge the case up to 2000 volts compared to its surroundings or the supply. Inverters will have insulation to protect against 230 vac but not 2000 volts dc so there is the risk of break down of insulation and the case of the microwave charges of up to 2000 volts in respect to the battery volts...under fault conditions. As battery -ve is connected to the chassis we now have a situation where if we touch the case we will discharge the voltage through us to the chassis. By connecting the case of the microwave to the chassis we have made it safe under fault conditions. The problem is the high voltages generated by the microwave. I am going to stop here. I have a real fundamental problem reading the font MMM now use and have for some time now for the mesages. I need to change the font I display which so far I have been unsuccessful in doing or try some other browser which will allow me to change it. I do not know what it is about this font but I just cannot read it easily at all and spend all my thoughts in desciphering text rather than what I am trying to actually say.. the frustration of it is why rarely post nowadays as I just get so confused and often have to delete paragraphs I have typed as just find impossible to correct. Gripe and moan over and am sure i must just be weird. Link to comment Share on other sites More sharing options...
Tracker Posted February 2, 2013 Author Share Posted February 2, 2013 Whilst I appreciate the advice from all, I am really struggling to get my head around all this and I suspect that I am not alone! Sorry, but I don't follow how earthing the microwave to the van chassis will help because the chassis is isolated from the ground by it's rubber tyres which means that the earth has nowhere to go other than stay in the van? If the casing became live with 2000 vac how can it run through me unless I am touching something metal at the time (not likely in a mainly wooden van with carpet on the floor?) or unless I get hold of both poles of the supply? Forgive me if I have got it all wrong. Agreed Jon - the font is small and unclear. I too would prefer a clearer font like maybe 'arial' but I too don't know how to enlarge or change it? Link to comment Share on other sites More sharing options...
Brian Kirby Posted February 2, 2013 Share Posted February 2, 2013 Thank you for that Jon. If we can get the font right for you, I'd still be interested in your thoughts on the remainder of my questions. Font. Have a look at your forum control panel. About half way down, you should see a yes/no option for "Use rich edit box when composing messages?." I suspect that is presently set to "No". Your messages are therefore set in ordinary text, and you can't change the font. If you set that box to "Yes", you will be able to change the fonts, (type, bold, italic, etc), their colour, and their size. However, for some reason the black has always tended to come out as a palish grey on-screen, which some find difficult to read. Mine is set to "No" for that reason. You may need to fiddle with it for a while to get something you can work with. However, please do, as I can't afford to lose you! :-D :-D Link to comment Share on other sites More sharing options...
sshortcircuit Posted February 2, 2013 Share Posted February 2, 2013 Returning to my original posting "An RCD works by detecting an in-balance in current in phase conductors to earth. If there are no earths then how would you get an in-balance?" The OP indicated there were no earth connections either at the appliance (micro-wave, although interesting, will not get side tracked about voltages within) and the source (inverter) which would result in the RCD having no effect. In normal domestic situations an RCD would operate at 30mA. As far as "Earth Leakage Trip" is concerned my understanding is that this was an early, now obsolete, protection device which measured the current in the main earthing conductor and therefor did not work as I described. Fitting an RCD for the situation as described in the OP would have no benefits and where to fit it would be another problem as the consumer unit in a MH normally has an RCD already fitted. Addition of an extra RCD is just providing another piece of equipment to go faulty Link to comment Share on other sites More sharing options...
Brian Kirby Posted February 2, 2013 Share Posted February 2, 2013 Tracker - 2013-02-02 6:15 PM Whilst I appreciate the advice from all, I am really struggling to get my head around all this and I suspect that I am not alone! Sorry, but I don't follow how earthing the microwave to the van chassis will help because the chassis is isolated from the ground by it's rubber tyres which means that the earth has nowhere to go other than stay in the van?.................... I can help with this bit, Rich. As your current is being generated by an inverter, and not coming from mains, the term earth is misleading. If you were connected to mains, there would be (should be! :-)) an earth path provided by the earth wire in the EHU. In your van, with an inverter running the 230V, there is no connection to earth as such, so the term is misleading. All that can be done is to give a safety "earth" path back to the inverter by connecting the "earth" wire back to either the chassis - in which case the inverter neutral must also be somewhere also connected to the chassis, or to the inverter casing - in which case the neutral must be somewhere connected to the inverter casing. However, in either case I don't think an RCD would trip unless it is wired specifically into the inverter circuit so that the live and neutral pass through it, but the "earth" by-passes it. To trip, the RCD must see in imbalance between the currents flowing on the live and the neutral. I'm now really guessing, but assuming a fault in the microwave (about which I know nothing) and 2,000V available, I suspect at that voltage many substances that are normally insulants would become conductors, and a shock might be possible. However, I still don't understand where those 2,000 Volts would go to as their "earth". Very interesting question, incidentally. I'm learning lots. I just hope teacher comes back with all the answers! :-D Link to comment Share on other sites More sharing options...
Brambles Posted February 2, 2013 Share Posted February 2, 2013 ... Link to comment Share on other sites More sharing options...
Tracker Posted February 2, 2013 Author Share Posted February 2, 2013 Brian Kirby - 2013-02-02 6:22 PM About half way down, you should see a yes/no option for "Use rich edit box when composing messages?." I suspect that is presently set to "No". Your messages are therefore set in ordinary text, and you can't change the font. If you set that box to "Yes", you will be able to change the fonts, (type, bold, italic, etc), their colour, and their size. However, for some reason the black has always tended to come out as a palish grey on-screen, which some find difficult to read. Mine is set to "No" for that reason. You may need to fiddle with it for a while to get something you can work with. However, please do, as I can't afford to lose you! :-D :-D Yes indeed, thanks Brian, that does indeed make things changeable although I have to get the font to change and stay changed.However I can click on the 'customise and control' button on the top right hand corner of Chrome and use the 'zoom'feature to enlarge the print as much as needed - I find 110% works fine - but we all have different eyes - some of us even have two different eyes themselves!! Link to comment Share on other sites More sharing options...
Tracker Posted February 2, 2013 Author Share Posted February 2, 2013 sshortcircuit - 2013-02-02 6:23 PM Fitting an RCD for the situation as described in the OP would have no benefits and where to fit it would be another problem as the consumer unit in a MH normally has an RCD already fitted. Addition of an extra RCD is just providing another piece of equipment to go faulty Fitting it is easy enough as it is a plug in type, and if it goes faulty removal is easy enough too! The inverter is wired directly to a new socket dedicated for the use of the microwave only as, rightly or wrongly, I didn't much feel like running the whole van electrics and the Sargent control unit and panel from the inverter which it was not designed to do? I could easily change it if there was ever the need to run the heating or fridge from the batteries in the event of a gas failure? Link to comment Share on other sites More sharing options...
sshortcircuit Posted February 2, 2013 Share Posted February 2, 2013 If you are happy with it Rich fine, but it ain't go to do anything. I have a similar set up as yourself with an easily accessible socket which by unpluging the microwave allows connection of the hair dryer :-D Link to comment Share on other sites More sharing options...
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