Does anyone here bother with earth continuity round their layout, in addition to the + and - ? I ask because most of the under-board wiring on mine is done in 6242Y (of which I had several 50m coils lying around). Force of habit set me to joining all the earths and checking continuity, but when I got to rewiring the transformer with a three-pin flex I started to wonder if I am mad. Another possibility would be to use the additional core as a second signal carrier for an auxiliary DCC for signalling, but that, too, is likely an over complication.
I don't know much about the subject, but a friend who knows more than me pointed out that if I earth (ground) all my "common" wires then I run the risk of a transformer fault putting 240 volts potential on the positive rail. I certainly link all my common wiring together, specifically to avoid track voltage changes between track blocks.
On Tue, 30 Dec 2008 11:53:45 +1300, "Greg.Procter" said in :
Yes, but that's different. This is the earth connector on the twin-and-earth that runs the main bus round the layout. It's quite fat cable, since the volt drop in thin cable at 1.5A over 40ft plus can be non-trivial.
Oh, and a red-letter day: I just drove a train over the trundleduct, first time :-) Rails need cleaning and the DMU could do with a drop of oil, but Progress is Being Made.
I think that yes, you are mad. Effectively what you have is a three-core cable of which you're using two cores. I think feeding an earth or ground around on the low-voltage side of the layout, just because that's what that wire would be for if it were carrying mains, is daft.
If I were you I would leave the spare (nominally earth) core disconnected until I had a use for it. I don't think you can sensibly use it for an auxiliary DCC for signalling because that would take two more cores: in a DCC system neither rail is grounded (typically, although I suppose you could come up with a crazy booster design which grounded one of the rails, which might do well enough for signalling).
On 30 Dec 2008 10:34:56 +0000 (GMT), Ian Jackson said in :
I was thinking that one could connect both negative rails to the same wire, and then run two positives, one for signals and one for running lines. But yes, that probably would be daft even if it worked :-)
In the good old days ( last week some time I think ) Earth and 0V were the same thing. Introducing a 'switch mode' driver into the picture has removed the 0V connection, but perhaps what should be considered is the addition of a little screening around the cabling. Academic when one has a great big loop aerial attached to the end of the wires, but when other signalling cables are running around all over the place, a screen around the DCC feeds is not totally silly? That would be earth bonded.
Hanging everything off DCC feeds is not an ideal scenario in my book, so having a clean DC supply available as well makes sense. And this should be linked to a safety earth. Also the outputs of any DCC controller should have an element of safety such that a mains fault will not cause those lines to float at mains potential. So having a clean 0V reference makes perfect sense to me, with a nice quiet +24V supply in my case to take load off the DCC feeds.
I wouldn't run low noise margin high speed signalling (or hifi audio for that matter) right alongside a DCC track feed. Apart from that I think worrying about shielding the DCC feed is pointless because as you allude to the track itself is a gigantic aerial.
I did have a problem with some very sensitive analogue electronics (in my home-designed and -built booster's short circuit cutout) being spuriously triggered by noise from the track power feed but I solved it by shielding the sensitive circuit, not by attempting to reduce the surface area of the various loops around which the track current flows. The latter would have been nearly impossible in my setup and would be quite tedious in any configuration.
I doubt Guy has anything I would regard as a low noise margin high speed signal - but if he has, the best thing is just not to run it parallel right next to track or DCC feeds.
Running everything away from the control panel off DCC does make the wiring (and control) a lot simpler and I think it's quite a sensible design choice. The main downside is that _everything_ needs a DCC decoder which can get more costly.
I don't know what kind of mains transformer or power supply you are using but electrical safety regulations for mains equipment insist that either: * The mains is doubly isolated from the low voltage parts so that a single fault cannot make the low voltage side unsafe. The low voltage side is then safely floating. * The low voltage side is connected to the safety earth inside the appliance.
So unless you have extremely dodgy or years-old equipment I think this is a red herring. If you're going to get this worried about your train set why not have the same paranoia about every other bit of consumer electronics with an unearthed mains adaptor ? Do you separately ground your mobile phone when you charge it ? What about a mains-powered electric shaver ? Your DSL or cable modem ?
In Guy's case he has only one spare wire. He could use that for 0V but then he wouldn't have a spare wire for +24V. Also, what equipment
- distant from the control panel - might he have that would require an unswitched LV DC supply ?
If they're both earthed then that would short them out against each other. Depending how crazily they're constructed I suppose it might even blow them up.
I don't know what the outputs from a shop-bought DCC booster are usually labelled, but they aren't properly called `+' and `-'. Strictly speaking I don't know how a shop-bought DCC booster works but I bet it's an H-bridge just like my own:
+13v(ish) ---+------------------+ | | You must use | track | a fixed-width FET |-| FET font to see this | A |-| B | diagram. All of +----+--|-|--+-----+ the leftmost | |-| | letters of this FET |-| FET note should | | line up. | | 0V ----+------------------+
`A' and `B' are the outputs from the booster. FET here indicates a kind of transistor capable of switching the track current on or off; you can read them as if they were ordinary switches except that of course they're automatically controlled by the command station. I've left out the control wiring which turns the FETs on and off.
For one half of each DCC squarewave, the top-left and bottom-right FETs are turned on and the other two turned off, so A is connected to
+13V and B to 0V. On the other half, the bottom-left and top-right FETs are turned on, connecting A to 0V and B to +13V.
On 30 Dec 2008 14:27:13 +0000 (GMT), Ian Jackson said in :
Aye, a dig around the circuit diagram persuaded me that this was not a good idea. Of course, I asked the wrong question: more on my mind was, what use can I find for this copper that runs all round the room? I may have an answer: I think I can use it to construct some kind of emergency stop circuit.
Arh yes, cut the copper wire at any point and the signals (read as system) will be returned to danger and an alarm will sound - just like the Seven Tunnel...
Come on - none of those have yards of LIVE metalwork exposed. Double insulation rules relate to not being able to touch live metal. I HAVE had problems with the occasional mains pack that I use and supply to others NOT providing the necessary protection and giving mains belts. So it is something as a responsible supplier I do take care to check, and flag to my suppliers when a safety problem arises.
On 30 Dec 2008 14:10:13 +0000 (GMT), Ian Jackson said in :
describes the styles of extra-low voltage circuits. I believe that railways are usually considered SELV, but I can't really remember and I'd probably have to find my copy of the wiring regs to find out.
I'm not sure what you mean by `LIVE'. If you mean `at hazardous mains voltage' then neither does a model railway. If you mean `electrically connected to the low voltage circuitry' then what about all those shiny aluminium cases and exposed metal parts in consumer electronics ?
Good grief! Have you talked to the HSE ?! I'm surprised you're still dealing with those manufacturers! Supplying a `power pack' which `gives belts' on the low voltage terminals is criminal (quite literally) on the part of your suppliers.
I appreciate that you say you `do take care to check' but even so I'm astonished to find you admitting continued dealings with suppliers of dangerously shoddy merchandise! (What do your checks consist of?)
Have you ever had a `belt' from a mobile phone charger or an electric shaver or a desktop computer or an almuminium-clad lapop or the aerial of a mains-powered radio or indeed any piece of consumer electronics not part of your stock-in-trade ? I've had an electric shock from an appliance exactly once, and that was one whose mains lead had (unbeknown to me) been extended by an idiot twisting wire ends together and wrapping the joint in electrician's tape.
Perhaps I'm just applying normal (ie not-model-railway-land) rules to this situation. My layout is powered from an 80W 18V general purpose switched mode power supply brick supplied by Farnell and I have complete confidence in its safety, and in particular in its safeguards for properly isolating the low voltage side from the mains.
But maybe other people's railways are powered by crazy cobbled together `power packs' with ad-hoc arrangements of the internal circuitry and inadequate insulation.
Well, surely whether it's SELV or PELV depends entirely on the mains power supply unit. I wouldn't be surprised to find both types.
If it's an earthed PSU it should have an earthed mains lead. If it's a double-insulated PSU it may well not. I don't even know what kind mine is without looking it up although it does have a three-pin mains lead so I think it's probably earthed.
0V is a tested connection to physical earth, it has a specific meaning when wiring panels, transformers etc. I think confusion arises because, for say out trains, all we are interested in is whether there is a 12v difference in potential between the rails. In theory this could be 100v and 112v (not necessarily a good idea though). In the home most mains electrics potential between live and neutral is actually the voltage between two of the three phases, meaning under some (unlikely) fault conditions neutral could actually be at approx 230V (415v x cos(Fi) or something)- hence the necessity of turning off the mains completely before playing with wiring (a tiny risk, but worth bearing in mind) and having a proper earth at the property.
UK domestic mains are connected between *one* of the three phases and neutral. The neutral is Earthed at the substation and the situation you describe cannot occur except under a gross fault condition.
Maybe that's what 0V means in the US when talking about mains wiring. But in this (UK) group and in the context of model trains, it doesn't mean anything of the kind.
0V in this context just means the nominal baseline against which other voltages (in the low voltage system) are quoted. It may or may not be earthed inside the mains power supply, but there is no need to earth it separately.
Indeed in some systems with multiple power supply rails, a rail other than the nominal 0V may be earthed inside the PSU although I would expect most model railway systems to avoid that - since in such a setup a naive user who earths the 0V output PSU terminal effectively shorts out the PSU.
(Assuming of course you're using reputable equipment!)
I think you missed out the words *Normally in the*, whilst it's uncommon to have 3ph in a residential property it's not unheard of, nor is it unusual for neighbouring properties to be on different phases - meaning that, for example, an un-isolated shared TV aerial / sat dish or extension cable run between properties could result in a ~450 volt single phase shock hazard should a fault occur.
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