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
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,
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
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
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
* 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
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:
| | 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.
`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
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.
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
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
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.
On 30 Dec 2008 18:21:57 +0000 (GMT), Ian Jackson
The PSU is a transformer in a box. Not the Lenz unit, one from The
Shop On The Bridge, as two Lenz ones turned up their toes in rapid
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.
On Jan 2, 9:44 am, beamends fault will not cause
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
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.