"Grounding" Separate Power Districts

[melmerby]

Hi all

 

Following on from my question about switching track power on and off I have another query about multiple boosters.

 

Should there be any "grounding" of the boosters between different districts as when I measure the AC voltage between each of the three system's J&K outputs (From the Lv100 to the two DB4s) there is a high leakage voltage between 70 and 160v!

(This high voltage is actually floating above nominal ground.)

 

The case of the Lenz system is connected to mains earth via the ground pin on the LV100 but the two DB4s are powered by double insulated switched mode PSUs although the circuit boards do have a "ground" connexion (not connected to anything external).

 

I am concerned that a decoder might get a bit of a zapping as it crosses power districts!

 

Cheers

 

Keith

[Crosland]

Every booster needs to be connected to some form of common. Common rail is best avoided for DCC, so every booster should have a common terminal connected to the internal 0V line. I hesitate to use the terms "Earth" or "Ground".

 

See https://sites.google.com/site/markgurries/home/technical-discussions/boosters/booster-ground-commons Mark Gurries is a power supply engineer and knows what he is talking about.

 

Andrew

[melmerby]

Every booster needs to be connected to some form of common. Common rail is best avoided for DCC, so every booster should have a common terminal connected to the internal 0V line. I hesitate to use the terms "Earth" or "Ground".

 

See https://sites.google.com/site/markgurries/home/technical-discussions/boosters/booster-ground-commons Mark Gurries is a power supply engineer and knows what he is talking about.

 

Andrew

Thanks Andrew, useful reading there.

Everything is optically isolated input wise and each has a separate isolated PSU so commoning the "common" rail on the track supply looks the way to go.

 

I put "grounding" in quotes precisely because its all relative!

 

Cheers

 

Keith

 

EDIT I have got that wrong - looks like it should be the terminal marked ground on the PCBs - I'll get in touch with Peter Littfinski.

[RFS]

There is a terminal on the LZV100 called "common" - it's located on the right of the 5-pin plug that also contains UVJK terminals. If you have locos which pickup through the loco on one side, and via the tender for the other side, then these locos will stall as they cross the power district boundary.  I don't have any locos of this kind, but the Hornby lighted Pullmans use a similar method via the two bogies, each one being live only to one side. So when I installed my LV102 to provide a second power district, their lights all went out as they crossed the boundary. Connecting a wire between the common terminals on the LZV100 and LV102 resolved the issue.

 

It's documented in section 12.1 in the LZV100 manual, my copy being dated June 2008.

[Suzie]

There are several ways to do it, but it is important that you only use one method, and preferably the manufacturer's method if they provide one.

 

The most common is to connect the internal negatives of the boosters together. The Lenz 'inverted T' terminal, 0V terminals, or on some devices just connecting the metal case.

 

Another method which sometimes gets done inadvertently is to common one of the power input wires to the booster. If using transformers then you have to be sure that you get the phase correct (or common the centre tap of centre tapped transformers), and for DC supplies it is best to common the negatives. Some DC supplies use 3-pin mains connectors, and these usually have the negative connected to mains earth which gives you a common connection by default via the mains earth so make sure that the mains earth is good enough to take the full booster current - can be an issue if boosters are a long distance apart and separate earth rods are used for earthing!

 

If all else fails just common one side of the track. Just remember that the common wire needs to be able to take the full current of the booster with minimal voltage drop just the same as the track feeds.

 

Not having a common when using multiple boosters can result in problems:-

 

* Shorts between power districts can go un-noticed and present double track power to some locos (with asymmetric pickups)crossing power districts.

 

* Shorts or just high current trains crossing between power districts can be noticed when the cable feeding the track signal to one of the boosters starts to smoke (common on Roco LokMaus2/MultiMaus installations with boosters).

 

* Trains stop and lights go out when trains cross power district boundaries.

[melmerby]

Peter Littfinski (LDT) recommends using the "J" output as a common with the DB-4s.

 

Keith

[Phil S]

'Generic Readers' [attracted by the Thread Title] should be aware that the guidance for earthing, or not, or how, can be dependant on the differing systems that people may be using - these were specific answers to specific equipment. Practices might also vary between countries.  What follows is not IEE regulations, just some observations of related thought...

 

Some similarity may be observed to interconnecting equipment to TVs - which used to have a live mains chassis  (less so now as many might have a separate low voltage power supply)

The advice in the manuals was always to connect with the equipment turned off   .... what was not mentioned was the '120V shock', and spark, that might have been 'experienced' when connecting switched-on items together ... and then the 'problems' introduced with 'home computers' of a mixture of 2-wire and 3-wired mains devices... perhaps the worst case scenario ??

 

2-wire. LN, devices, with no earth wire but with exposed output connections (such as the dc output from a switched mode power supply) 'could' be totally 'floating'... having been isolated from the high voltage input side through either a traditional transformer or a miniature hf transformer and the SMPS process.....  but are more likely to be 'clamped'/held to a mid-rail potential through a VERY high resistance ... so that only a safe current could flow through YOU when you touch a connector .... but in the 'absence of current flow', the voltage felt would be 120V ( and better than what might have been induced if totally isolated - although a spark may still be seen).;   Once the connection has been made, the units are linked, and only a low current ( <1mA) hopefully flows between them, if any.   RCCB devices protect (typically at 30mA) by measuring the DIFFERENCE in current between the Live and Neutral wires, and do not require an earth wire  ... hence SMPS's with large input capacitors, and poor surge suppression, can cause them to trip when power is applied 8-(

 

3-wire LNE devices would usually reference the output to the local Earth, by means of the 3rd wire.  Usually to the -ve rail of the output voltage (unless it is described as having a negative voltage).   Of course, you may have a 'poor earth' - especially if relying on it going via plastic water pipes!!!

 

When interconnecting 3-wire devices, one can therefore probably assume that the output -ve sides can be linked together, and the positive voltage would then be referenced to the same 0v.

So with 3-wire power supplies, and  boosters - the only 'problem' is matching the phase so that sections may be crossed with no difference.

Using identical 2-wire devices ( I use 4 identical SMPS units for my 4 power districts) should produce a similar result, but at the 'mid-voltage' (120V) via the high impedance - considered safe.

The potential problem can come when a mixture of power supply types is used ... mixing 3-wire (E='0') and  2-wire (mid, via high) can cause 'static damage' type failures if connected when live, or have higher residual currents ....and this was the worst case scenario wth home computers (3-wire) connencting to 2-wire monitor/tvs etc.

 

'Balanced' or 'Single-Ended'?: This sometimes comes up in the discussions about layout wiring: As far as RFradiation is concerned, balanced should cancel out ... but a dc offset of a few volts won't make much difference ....   dc supplies are often run through aerial cables eg to pre-amplifiers or LNBs at satellite dishes.   Again, if connecting back at the Central unit / Booster, it is best to keep to one make or follow the advice of the manufacturers as to whether to connect, and how.

 

In the US, the 2-wire plug is now polarised in many instances.  Continental plugs, even when placed into '13A adapters' are random in phase.    Our Campervan Diesel/Electric heater control circuitry is sensitive to incoming polarity to 'detect' the external mains .... and some camping electrical equipment includes warning neons about 'phase reversal'. (German Sites with 6A limits often use 2-pin sockets)

 

Provided I have matched the phase of the DCC outputs from each booster to that of the master, I have no problems transferring trains across section breaks: both rails cut in each case..

The 2 sections become linked as soon as 1 rail is bridged by a wheel, or more continuously, by 2 wheels on the same rail 8-)

If in doubt - you could always opt for a single TRACK power district, perhaps from a high-power booster, with plenty of PSX protected sub-districts (note the PSX only breaks 1 rail)