How to ground mains equipment on a +48VDC chassis?  - Page 1
Forum section: electronics / beginners

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 14, 2018, 09:34:40 am
content: HiI'm rewiring the lighting circuits of an old underground train, and was wondering how to ground the ballasts and power supplies. The whole body of the train is a positive 48VDC running of a 240V switch mode supply. Th old ballasts ran off 110VAC and where also grounded to +48VDC. The thing is if a mains live touches the chassis what happens? If part of the 48v circuit shorts then one of the fuses blow. But I have no inda what would happen if this happened with 240v. Would it blow the MCB or RCD an all is ok? If not is the a way to connect mains earth alongside the 48v? Its impossible to remove the 48v grounding as everything taps to the body work and in places we cannot reach.Thanks in advance.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 14, 2018, 12:26:09 pm
content: No question that the body of the train car should be connected to earth ground.It should make no difference what power (AC or DC, positive or negative ground) is used for the loads (lighting). If either mains power (120VAC or 240VAC) or some other power (like -48VDC) are shorted to the train car (ground) then it should blow the fuse or circuit breaker.Perhaps you are confusing yourself by thinking of the train car as being "+48VDC". If you think of the train car as ALWAYS ground/zero volts then everything else is relative to that. For example, the lighting circuit is -48V relative to ground.Are you keeping the -48V fluorescent ballasts? Or are you replacing the vintage lighting with LEDs?

author: Kjelt
author_rank: Super Contributor
timestamp: February 14, 2018, 12:42:34 pm
content: With all respect to the poster but if your company was hired to do this maintenance / replacement don't you have an expert that you can ask for advice.With high probability some official standards should be followed, this is not really a guess and trial by error situation.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 14, 2018, 12:48:34 pm
content: I have seen this question elsewhere in the past. I believe that @abdullahseba is a volunteer in a museum, not an employee of a for-profit company. But you have an excellent point. At some point a licensed electrician should review and approve of any wiring scheme. Especially if the train car will be open to the public.

author: Ian.M
author_rank: Super Contributor
timestamp: February 14, 2018, 01:37:00 pm
content: https://en.wikipedia.org/wiki/London_Underground_infrastructure#Electrificationandhttp://www.trainweb.org/tubeprune/tractioncurr.htmindicate that the wheels and chassis should be at ground potential as the running rails aren't insulated from ground. I would expect the bodywork to also be grounded, and it would be highly undesirable to have a significant positive potential on the bodywork as that would risk severe electrochemical corrosion near fastenings where a moisture film might bridge any insulating gasket that may be present.The museum should have a qualified electrician on the staff or on retainer. I would suggest getting them to check any proposed wiring diagram *before* implementation.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 14, 2018, 01:47:40 pm
content: Quote from: Richard Crowley on February 14, 2018, 12:26:09 pmThanks for the reply. The original ballasts where 110V and we are replacing them with 240v ones.So this is safe to do? Its effectively connecting the earth to the PSU's output directly. Sorry for the crappy diagram.Thats how it use to be, but without the "connect this ground" and with a 110v transformer between the mains and the ballast. Fuses and MCB's not shown.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 14, 2018, 01:50:20 pm
content: Quote from: Kjelt on February 14, 2018, 12:42:34 pmYes We are a team of volunteersWe have to get it checked and everything by a qualified electrician eventually, But we want to get as much as possible correctly.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 14, 2018, 02:00:46 pm
content: Quote from: Ian.M on February 14, 2018, 01:37:00 pmYep that is correct. But it looks like the body work is separated from the bogies/chassis. The main body is without doubt +48V (originally 50v) as equipment simply take the supply off a stud welded to the body work. The 50V and 110v originally came from a 600V generator that was in the trailer car (this been the driving unit).

author: richard.cs
author_rank: Super Contributor
timestamp: February 14, 2018, 02:37:34 pm
content: Quote from: abdullahseba on February 14, 2018, 02:00:46 pmSurely what this really means is that the body should be at ground potential, i.e.0 V relative to earthand the supply to the equipment is-48 V(negative!) relative to both chassis and earth. If your 48 V PSU has a floating output there is no problem here, just connect chassis to mains earth. If your 48 V PSU internally has a connection between the negative output terminal and mains earth then it is simply the wrong PSU for this application and must be replaced (because it would prevent you from connecting chassis to mains earth which will be required with mains present on the train).

author: drussell
author_rank: Super Contributor
timestamp: February 14, 2018, 02:46:13 pm
content: Yes, I believe your main problem is misunderstanding the 48 volts. The car isn't "live at +48 volts", the system is -48 volts, just like standard telecom -48 supplies. Yes, themore positiveterminal goes to the train car, but thisisground potential. The devices being powered are running on -48 volts DC.This is similar to many old automobiles where the chassis was connected to the positive side of the (usually 6 volt) battery. You still consider this to be at ground potential. Everything is running on a negative voltage from ground.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 14, 2018, 02:49:40 pm
content: Quote from: richard.cs on February 14, 2018, 02:37:34 pmNo, I know its unusual but its positive. all the supply lines are negative and its most annoying specially as they used Red wires for negative and black for positive off the emergency batteries. Its basically inverted from what you would usually expect.The PSU shows that earth is connected to the filter and it also has a parallel switch.https://docs-emea.rs-online.com/webdocs/1263/0900766b812633d4.pdf

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 14, 2018, 02:51:04 pm
content: Quote from: drussell on February 14, 2018, 02:46:13 pmWell measuring it with a meter shows + 48V with the red lead on the bodywork.

author: Ian.M
author_rank: Super Contributor
timestamp: February 14, 2018, 03:19:45 pm
content: Where's the black meter lead? If you can measure + 48V on the bodywork with the black lead on a wheel or running rail, then you are correct, but that's rather unlikely and I expect drussell's assertion that the 48V DC system is actually a -48V supply, with positive ground to be correct.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 14, 2018, 03:21:29 pm
content: Quote from: Ian.M on February 14, 2018, 03:19:45 pmBlack lead on the negative of the PSU. I'll do some more checking tomorrow.

author: Towger
author_rank: Super Contributor
timestamp: February 14, 2018, 04:01:04 pm
content: Quote from: Ian.M on February 14, 2018, 01:37:00 pmIn my experience many a 'qualified electrician' would balk at this type of stuff. Domestic single phase wiring is all they know and are interested in. They have no interesting in 'electrics' other than what is required to do basic house wiring.The conversation goes something along these lines "J*s*s, that's f*ck*ng 3 phase, I know nothing about that f*ck*ng stuff" and off they go...

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 14, 2018, 04:25:52 pm
content: Quote from: Towger on February 14, 2018, 04:01:04 pmYep, Exactly which is the trouble we are having and why we have to do it our self's. As you say a qualified electrician wouldn't know what to do with this and a company that refurbishes trains would do it so that its in running condition. Not to mention the costs.... They might not even be interested in doing just one car. All that leaves is multiple specialist company's to do the work and that would cost an arm and a leg. Thats we have opted to do it our selfs and we will probably ask the council what safety check should be carried out.Earthing is only a tiny part of this... Theres PLC's for selective lighting and door control, PA system heating Pneumatics etc.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 14, 2018, 06:35:49 pm
content: You continue to confuse yourself by calling the train car "+48V".The train car is zero volts,aka "earth" (BrEnglish) or "ground" (AmEnglish).The DC power bus is-48V relative to ground. Of course, you must use a DC power supply with "floating output" so that you can connect the positive output to the train car chassis (earth/ground). A fundamental mistake is using the red (positive" lead of your meter connected to the train car chassis. That seems to be a major source of your confusion.But the issue with the DC bus appears to be only semantics. The bigger issue is the distribution of AC mains power.Your diagram appears to show that you are distributing AC mains power (at whatever voltage 120 or 240) with only the Live/Phase wire, and expecting the return path to the train car chassis, which is connected to mains PE/ground. It is not clear what loads you have for the AC mains power? But I would urge in the strongest possible terms that you DO NOT want to do that.Since you appear to be hard-wire connecting to utility mains power, we must assume that you are working on a static display, and not on practical rolling stock. Returning the AC mains current through PE (vs. properly returning it through Neutral) will instantly trip any RCD/GFCI protection. Utility mains current should never touch the PE conductor except in a fault condition. Using the train car chassis as the return for the AC mains power is ABSOLUTELY NOT RECOMMENDED.If you can't pull in proper wiring (Hot, Neutral, PE) for AC mains power, then you should NOT be powering anything from the utility mains power.

author: ScottM
author_rank: Newbie
timestamp: February 14, 2018, 07:51:34 pm
content: Richard's comments are spot on. We are talking semantics with the DC.The AC mains, are, as he said, a real issue. If you do this wrong/don't understand what is happening, people could get hurt and you could cause a fire. Or worse.You need to understand the difference between bonding and earthing. Especially in situations like this, proper bonding is essential to avoid creating points with different voltage potentials.Please use proper GFCI etc. protection.Read and understand codes for this work. Peoples lives count on this being correct.

author: IanB
author_rank: Super Contributor
timestamp: February 14, 2018, 08:36:15 pm
content: Quote from: abdullahseba on February 14, 2018, 09:34:40 amYou have many other answers in this thread, but I will add my own to reinforce what has been said.First of all, you have a misunderstanding about voltages.The body of the train must be safe to touch by people standing on the ground, both in service and in a museum. Therefore the body of the train is/should/shallbe at ground potential, so that no dangerous voltages can arise that could be a problem for any people or metal objects coming into contact. To achieve this, the train body and all other exposed metalwork should/shall be positively connected to mains earth by a solid grounding strap fastened in such a way that it cannot come loose. It is likely to fail a safety inspection if this is not done.To see why, let's first consider what happened when the train was in service. The train wheels were in contact with the running rails, which are at ground potential. The body of the car should be connected with the chassis, bogeys and other metal parts, putting the whole car at ground potential through the wheels. Power was taken up via two hot shoes, one at +420 V DC and the other at -210 V DC, giving a nominal 630 V DC supply for the traction gear. Secondly, this 630 V DC powered a motor-generator unit, the output of which provided anisolatedlow voltage for lighting and other equipment. To avoid running two wires to every lamp, one pole of this low voltage supply was connected to the rail car body for return current, just as is done in cars on the road.To emulate this arrangement in a museum setting, you need to substitute the motor-generator set with the output from a suitably isolated low voltage power supply. Note that the output of this supply must be floating and isolated from the mains, and must be safety rated for such use. You now can attach one pole of this supply to the train body, just as was done with the motor-generator. Note that the train body isstillat ground potential because it is firmly attached to mains earth with a grounding strap. There will be no problem doing this because the lower voltage supply must have an isolated output and must be specified for this kind of use.

author: Gregg
author_rank: Super Contributor
timestamp: February 14, 2018, 09:14:47 pm
content: Richard Crowley is spot on in his reply.I think there is some confusion with the drawing / schematic not clearly showing neutral connections from the mains supply to the ballasts and the 48 VDC supply. I see dots indicating connection points but no lines designating wiring to the neutral. If the neutrals are connected properly and if the neutrals are not physically connected to the chassis, ballast or 48VDC power supply enclosures it should be wired properly for the AC side. This assumes there are no faults.With so many cheap LED packages available, it may be easier to convert all of the old lighting fixtures to LED and run them on the 48VDC system and not worry about mains wiring in the rail car. You still should properly fuse the circuits and pay attention to the polarity.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 14, 2018, 10:04:19 pm
content: Quote from: Richard Crowley on February 14, 2018, 06:35:49 pmSorry about that, the Image site I used has compressed the neutral out.Quote from: Richard Crowley on February 14, 2018, 06:35:49 pmAs you say this is only semantics, but I would still like to understand it. So If I understood correctly the mistake I'm making, is measuring the voltage without taking into consideration that the body is the ground point and I should connect the black lead to the body work?I believe the PSU is a floating output if the symbol between the switching device and the rectifier is a 1:1 transformer.So what it boils down to is can I have mains earth (that comes from the utility) connected to the body work (which is effectively connecting the mains earth to the the PSU's positive output) without tripping any RCD's?I could try it out but the PSU's are quite expensive and I dont want to damage them by trying

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 14, 2018, 10:15:20 pm
content: Quote from: IanB on February 14, 2018, 08:36:15 pmThanks for that, so looking at the diagram of the PSU in the previous post would you say its suitable? Heres the datasheethttps://docs-emea.rs-online.com/webdocs/1263/0900766b812633d4.pdf. It looks Isolated to me but not too sure if thats supposed to be a 1:1 transformer as it doesn't show the coils on either side of the core. Looking inside there is a large PCB mount transformer so I assume it is so.Heres a basics diagram of the power of the train circuits.It actually had a few more odd voltages like 62V off the gen but thankfully we dont need those.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 14, 2018, 10:27:52 pm
content: Quote from: abdullahseba on February 14, 2018, 10:04:19 pmGood. That looks proper.QuoteThe body of the train car is "Circuit Ground"/Zero BY DEFINITION for measurement purposes.And the body of the train car is Earth/Ground/Common because it MUST be connected ("bonded") to PE. Perhaps using a ground rod driven in to the crust of the planet between the ties.Someone on this forum has a signature line that says something like "pick a point and call it ground".Electrical (and electronic) convention is that SOME point in a circuit (or a system) is "ground".And also by convention, all voltage measurements are made with reference to ground (except in some special cases which should be explicitly identified).When you are using a DMM, the black/"negative" lead should be connected to circuit ground/common.Unless there is some special case where you are measuring a "differential" voltage between two non-grounded points.So in your case, the DC power rail should measure NEGATIVE 48V relative to circuit ground (which is the train car chassis).Back in the days of mechanical meters, if you wanted to measure a negative voltage, you would connect the red lead to ground as you have been doing.Because old mechanical meters could not handle positive/negative. But we assume you are using a modern DMM.QuoteYes it looks like the output is completely isolated from the input. With that transformer, and also with those optical isolators.QuoteYes. It appears that there is no path for any fault current, so the RCD should be happy.QuoteYou could simply measure between the output terminals of the PSU and input PE to see if there is any resistance.It should be very high (100s of kilohms to megohms).But the PSU diagram shows pretty clearly that the output is isolated. So I would have no doubts that it was safe.

author: IanB
author_rank: Super Contributor
timestamp: February 14, 2018, 11:40:17 pm
content: Quote from: abdullahseba on February 14, 2018, 10:15:20 pmThanks for that picture. So we are preserving the history of the Victoria line?Note that the 115 V AC output from the motor-alternator set is just as isolated as the 50 V DC. The way you would emulate the 115 V AC in a museum setting would be by the use of a suitable 240 V to 115 V isolation transformer. Happily, such transformers should be relatively easy to procure. With such an isolation transformer you could also connect one pole to the car body without any worries of short circuits or trips.From an authenticity perspective it would be nice to keep the original fluorescent lighting intact and not replace it with LEDs as suggested by some.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 14, 2018, 11:50:54 pm
content: Quote from: IanB on February 14, 2018, 11:40:17 pmYes, 240/120V mains transformers should be readily available in a variety of different forms And then you wouldn't need to change out the ballasts.However, we don't know what was the frequency? Was it 50Hz? (probably not 60Hz like here in the Colonies)400Hz was (is?) popular in avionics because it greatly reduces the size/weight of the iron core.If the ballasts were designed for 400Hz (or something significantly above 50 or 60Hz, they might not like mains frequency.QuoteIn which case, better stock up on replacement tubes (and ballasts) because they will go out of production soon (if not already).

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 15, 2018, 12:00:04 am
content: Quote from: IanB on February 14, 2018, 11:40:17 pmYeah we are keeping the fluorescent tubes. just replacing the original ballast with 240v ones. We use to run them off a 110V transformer but they are 20 years old now and a lot of them are dying. 110V ballast are hard to get hold of so. We hire it to a lot of filming companies too who have requested selective control repeatedly so we are adding that but its all behind the seens stuff that doesn't affect the look and feel of the car.Heres an old pic of the full windings:They use a lot of odd voltages probably to account for voltage drops.I can take a hi res pic of the full thing if you like

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 15, 2018, 12:08:11 am
content: Quote from: Richard Crowley on February 14, 2018, 11:50:54 pmAlmost half the ballasts have died after 20 years of use so replacing them with 240 is a better option. Most of the train circuits where actually 850Hz including the ballasts which run fine off 50Hz until now. Quite possible that 50Hz has shorted the lifespan of them though.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 15, 2018, 12:27:18 am
content: Quote from: abdullahseba on February 15, 2018, 12:08:11 amHave you analyzed the failure mode? I would expect that 50Hz would significantly overheat a transformer designed for 850Hz.

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2018, 12:31:07 am
content: Quote from: Richard Crowley on February 14, 2018, 11:50:54 pmGood point. However I have an idea that the on-train electrics in the London Underground has traditionally been 115 V 60 Hz due to the American heritage of the system. A higher frequency would not have been a benefit in previous decades since the supply from the motor-alternator went directly to the lighting and no transformers were involved (except for the magnetic ballasts in fluorescent lamps). Also the practical frequency would depend on the speed and number of poles in the motor-alternator set.The earlier trains like the 1938 stock used incandescent lamps (much more inviting than fluoros).

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 15, 2018, 12:32:56 am
content: Quote from: Richard Crowley on February 15, 2018, 12:27:18 amTransformer we used was a 50Hz one. 50Hz in and out.https://docs-emea.rs-online.com/webdocs/1531/0900766b815319c1.pdfIts the ballast that was rated for 850Hz

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 15, 2018, 12:50:41 am
content: Quote from: IanB on February 15, 2018, 12:31:07 amI believe the 1957 tube stock was the first to use 850Hz systems not sure if older ones where 60Hz. Even the 240v fan supply generated by 50VDC inverters are 850Hz for some reason.

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2018, 12:56:42 am
content: Quote from: abdullahseba on February 15, 2018, 12:50:41 amIf it's truly 850 Hz AC then any magnetic equipment like ballasts or transformers will absolutely not be compatible with 50 Hz electricity. It would be so incompatible that instant failure would be predicted if attempting to connect 850 Hz equipment to a 50 Hz supply. So while you clearly have access to better information than I do, I still wonder how you managed to get things to work.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 15, 2018, 01:02:48 am
content: Quote from: IanB on February 15, 2018, 12:56:42 amThe ballast are electronic and we ran them off 50Hz for about 7 years. We are not using any 850Hz transformers... the one that came off the alternator is in the second trailer car which we dont have.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 15, 2018, 01:07:25 am
content: Quote from: IanB on February 15, 2018, 12:56:42 amHeres an image of one:The original original ones as in (pre refurbishment) where magnetic. Keeping in mind that these where refurbished in 1995.

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2018, 01:12:49 am
content: Quote from: abdullahseba on February 15, 2018, 01:07:25 amI see. Cool. Yes, those are probably not very sensitive to supply frequency.

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2018, 01:19:54 am
content: Quote from: abdullahseba on February 15, 2018, 01:07:25 amIf we are talking about 1967 stock, it interesting to recall that these were brand new shiny trains when I first rode them in the early 1970's. At the time their speed and acceleration was impressive.(And what happened to Thorn?)

author: abdullahseba
author_rank: Frequent Contributor
timestamp: February 15, 2018, 01:48:02 am
content: Yep these are 19Quote from: IanB on February 15, 2018, 01:19:54 amYep this is the 67 tube stock. We actually have half of one of the pre refurb ones that had an accident.Thorn is still goinghttp://www.thornlighting.co.uk/