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Booster - Earth/Ground/Commons


This is a technical discussion of what are grounds and commons as they relate to DCC systems.

IF you looking for Best Practice information on DCC booster installation, go here: Booster Connections & Power

IF you looking for Best Practice information on Multiple booster installation, go here: Multiple Booster Wiring

In this section are:

1) BACKGROUND: WHY THIS SECTION EXIST
2) WHAT IS A COMMON?
3) WHAT IS A GROUND?
4) POPULAR COMMON AND GROUND NAMES
5) WHAT IS A BOOSTER COMMON?
6) THE GOLDEN POWER DESIGN RULE.
7) THE BAD "MULTIPLE COMMONS" PROBLEM.
8) DETAILED EXAMPLES OF BAD MULTIPLE COMMONS WHAN USING A BOOSTER COMMON AT THE SAME TIME.
9) DO and DONT'S OF BOOSTER COMMON AND GROUND WIRING.


1) BACKGROUND: WHY THIS SECTION EXIST 

Ground and commons can be a confusing topic because many people are only familiar with the concept of Earth Ground and its safety function related to AC power in their house.   However in the electronics industry, grounds and common have many more dimensions unrelated to safety while unfortunately sharing a common set of terms and names creating overlap.  Hence a confusing set of terms.

Grounds and common can only be properly understood in the context of the type of power that is being talked about. AC vs DC, High Voltage vs Low Voltage, High Current vs Low Current and Power vs Signal.

Lacking the proper context, it is perfectly understandable that many people can be unaware of the technical problems and improperly install and/or use them. 

Fortunately most DCC system are running very small or simple and you do not need to know any of this stuff.  But some people may want to know more and confirm that there is some kind of scientific reasoning behind this wiring.  Above all, do not let yourself get scared or intimidated by what I am talking about.  DCC is about having fun and not necessarily knowing all the science behind it.


2) WHAT IS A COMMON?

A common is a global electrical circuit path (wire) that is SHARED among devices.  The electrical advantage of using a common is that it can greatly reduce the amount of wiring you need to connect multiple devices together.  

A common can be what every voltage potential the design engineer wants it to be.  The term common does NOT automatically mean that has 0V or ground voltage potential.

Example: A non 0 Volt common is found in DCC decoders.  The "Blue wire common" which is shared with all the light bulbs or LEDs used to light up the locomotive.  This common's voltage is simplistically the same as the track voltage or +12V.   One could call it a "Hot common" since it is not 0V.  The return side or 0V part of the circuit is provided by the function outputs.


3) WHAT IS A GROUND?

Ground refers to the small local 0V common for a given electrical circuit.   Anything that is defined as 0V convention wise automatically gets a nickname called "ground" because of the analogy that this the electrical foundation the given circuit is built upon or stands on.  HOWEVER, just because it is called "ground" DOES NOT AUTOMATICALLY MEAN it is the same as true 0 volts which is define as "earth ground" which is what PEOPLE stand on.  The local circuit ground could have many volts on it relative to earth ground.  It is only 0V with respect to that circuit.

To be clear, Circuit ground is not the same as Earth Ground unless they are electrically tied together.  Hence the important of understanding what your talking about (context) when using the word ground.


4) POPULAR COMMON AND GROUND NAMES

Example of different types of Grounds that are not related to each other.

1) "Earth Ground"   This is the largest common in the world since its the whole planet earth!  By definition it is true 0V from a human being point of view since we walk on the earth!

2) "Common Ground".   It is a global ground that is defined to be a 0V potential or "ground" potential for all the circuits involved.  The Common Ground 0V value has NOTHING to do with 0V Earth Ground unless they are connected together.

3) "Common Rail".  This describes a rail that is electrically unbroken around the entire layout.  Typically there is a large gauge bus wire that follows the common rail where every it goes with regular track feeders running to the common rail.  With DC layouts, every DC throttle has one connection to this rail while the other side is "routed" to where it needs to go via block control switches.   Sometimes the common rail doubles as a "Common Ground" for many accessory power sources to run other devices such as switch machines.  It should be noted that Common Rail is not recommended for DCC usage.

4) "DC Ground" Typically another name for Common Ground.  However the DC ground does not carry any AC current.

5) "RF Ground"  A local ground used in RF (Radio Frequency) circuits.  DC and radio frequency AC currents are involved.

6) "ESD Ground"  An highly resistive ground circuit with sole purpose it to absorb the ESD shock current and return it to Earth Ground.  Typical resistance to Earth Ground is 100K or 1Meg Ohms.  ESD straps on human are part of the ESD grounding system.   The high resistance prevents any hazardous high current flow permitting working on live power equipment.

Without the extra word to help define the specific type of common ground, you really do not know what it means.  Even then when working on a large scale wiring system, there can be more than one type of ground that you do not want to mix up unless you fully understand the consequence both good and bad.

Why is this important to know?  Because it goes directly to the electrical problems one can create in wiring a layout including its AC power.


5) WHAT IS A BOOSTER COMMON?

A Booster Common is a single wire connection between boosters that allows them to see each other from an electrical voltage and power point of view.  The common makes sure that all the boosters voltages are referenced to each other.  Think "Level Playing Field".  This prevents the problem of voltage doubling which can destroy decoders under the right circumstances.


6) THE GOLDEN POWER DESIGN RULE.

The goal in any power distribution system is to ONLY HAVE ONE COMMON in the entire electrical system.   
A single common means you have 100% control of where the current flows.

Why?  Read the next section.


7) THE BAD "MULTIPLE COMMONS" PROBLEM.

 When you have multiple commons shared among multiple circuits, it means:

1) there are multiple paths that current can take.

2) you are not in control of which path the current may take.

3) you have the potential for current to flow in circuits or paths that it was NEVER intended to flow.  Can you smell Smoke?

In power, multiple commons can be a serious problem if things go WRONG in the power system.  The key word is wrong.  In other words, it will work fine and may do so for a long time.   There in lyes the issue, people assume everything is OK simply because it works and "I have no problems".   That not engineering, thats just luck.   It is the potential for big problems when things go bad.  Multiple commons can quickly take things from bad to worse!

A typical multiple common in the Audio Industry is called a "Ground Loop" where the AC ground and the Audio ground form two paths between to audio devices.  What you hear is the AC current flowing in the audio return in the form of HUM on your speakers! Just like a Ground Loop is bad in audio, things really start to come "alive" when power is involved.

Do not assume that because a multiple common appears to be working it must be OK.  See Anecdotal Engineering


8) DETAILED EXAMPLES OF BAD MULTIPLE COMMONS WHAN USING A BOOSTER COMMON AT THE SAME TIME.

BOOSTER COMMON IS INSTALLED.  Completing this connection per the DCC manufactures recommendation, you then decide, already have or accidentally do one of the following too:

1) COMMON POWER SUPPLY:  To save money, one decides to use one big power supply and share its output between multiple boosters.   This creates an "Transformer Common"  or a "Power Supply Common" between all the boosters.   Two commons have now been created.   

What can go wrong?  Go here:  System One Disaster Story

Solution: There are two:

a) Only use one dedicated power supply per booster as recommended by the manufacture.

b) Install current protection for each individual booster on both wires feeding to each booster.  An example of such a protective cable is the one that
Digitrax sells with its 20 Amp DC power supply "PS2012" intended to power multiple boosters.  People sometime site this as an example of "proof" that it is ok to use a common power supply for multiple boosters.  WHAT PEOPLE MISS ABOUT THIS POWER SUPPLY SETUP is that Digitrax includes a very special (YC52) "Y" cable with this power supply.  The YC52 is NOT JUST A SIMPLE WIRE "Y" CABLE.  It contains inline thermal based PTC fuses in every power lead and satisfies the electrical protection requirement.


2) COMMON RAIL: To leverage the layouts existing "common rail" wiring, each booster is treated like a DC power pack and one terminal of the track output is wired to the common rail bus.   Two commons have now been created.   If you do not know what common rail wiring is, chances are you do not have it.  If you wish to know what common rail wiring is, go here: Boosters & Common Rail Wiring

What can go wrong?  
If the booster output that connects to the common rail is broken or become highly resistive connection, then the common rail connection will allow current from this booster to flow through another booster that should not see this current.   To understand this, we need to understand the current path details:  
a) Current from booster #1 flows out to the "Hot Rail"  (not the common rail)
b) "Hot Rail" current flows through a engine or a short circuit directly to the common rail.
c) Since the path from Booster #1 to the common rail is broken, the current finds it way to nearest 2nd booster (#2) via the common rail all the booster share.
d) Booster #1 current now enters Booster #2 and finds a return bath to the booster's #2 internal power supply ground.  
e) Booster #2 internal power supply ground is externally tied to the "Booster Common" or Ground and booster #1 current flows out to it.
f) Using the "Booster Common", the current gets back Booster #1 and find it way back to #1's internal power supply. 
This complete the circuit starting from booster #1 through booster #2 and back to booster #1.

Here is the problem: IF the 2nd booster has to handle it own current PLUS the current of #1 booster, it can potentially see current flowing well above it current rating.   When you exceed the booster's current rating, you can overload booster.  One may say: "No that is not possible because the other booster has its own current limiting" with the implied assumption that the booster will see the other booster overloading current and shutdown.   That is a bad assumption.  I know this is not true with NCE booster and many other booster depending one WHERE they sense the current inside the booster.   If the booster monitors the current in the power supply ground path, then yes it will see the other boosters current.   But many booster only monitor the powers supply's positive rail such as NCE.   No current sensing is done on the negative power supply rail.   This will permit current excessive current to flow WITHOUT the 2nd booster even knowing its flowing!!  The second booster can become damaged.

Solution:  Do not use a Booster Common with Common Rail. 

3) EARTH GROUND COMMON:  The Boosters have a terminal called "ground".   One mistakenly thinks they must tie this ground terminal of each booster to the house ground wire other wise known as Earth Ground.  One goes off to buy a power supply for each booster that also provides a wire to make a easy Earth Ground connection.  The booster are all distributed around the room and plugged into different AC outlets.  However the house's AC Earth Ground (Green Wire) wiring ties all these booster together back at the House AC fuse or circuit breaker panel.   Simply put a Earth Ground Common has been created.  Two commons have now been created.  

What can go wrong?  In your house the neutral and earth ground wires are connected together inside you AC power panel of your house.  The purpose of the earth ground wire is to back up the neutral wire in the event there is a failure or weak connection of the neutral wire.  If there is such a break in the neutral in some given appliance that current will flow in the Earth Ground wire back to the AC power panel keeping the appliance safe in term of electrocuting you.  

At face value, having a Booster Common and each booster having its own Earth Ground connection to it own AC outlet would seem to be a good idea.  Booster #1 is Earth grounded to to Outlet #1 and Booster #2 is Earth Grounded to Outlet #2.  You got everything earth grounded so you will not get a shock....However electrically what you have done is place the Booster Common in parallel with the House's Earth Ground wiring.  

Here is the problem: Given Outlet #1 is electrically far away from the AC power panel than Outlet #2, then the resistance in the earth ground of outlet #1 to the AC power Panel will be higher due to the longer wiring than the resistance of the Earth Ground wiring in outlet 2 to the same AC power panel.  If an appliance plugged into Outlet #1 loses it Neutral wire connection due to some internal wiring failure unknown to you, current then start to flow into outlet #1 earth ground.   With outlet #2 closer to the house AC panel, the current will see TWO paths back to the AC panel.   

a) The long earth ground wiring path of outlet #1 back to the AC panel as intended by the safety codes of house wiring
b) A shorter lower resistance earth ground wiring path via the "Booster Common" to outlet #2 back to the AC panel.

What does this mean?  AC appliance current will now flow through you booster common.  Your booster common wiring is now potentially hazardous.  The EXACT OPPOSITE of what you thought you would be getting if you earth grounded everything!!!

Solution:  There are two.

a) Do not use a Booster Common with Earth Ground wiring. Get ride of the earth ground connections going to the booster.  The only reason one may need to connect the booster common to earth ground is ESD and as such an ESD ground cable should be used.  True ESD cables have a very high but known resistance between it two connection that it will allow ESD to find it way back to earth but not allow any dangerous current to flow.  The alternative is to connect Earth ground to the booster common with a 1 megaohm resistor to the booster ground.

b) Pick one AC outlet and power everything that is connected to the layout, booster, computers, power supplies, from that one AC outlet.


9) DO and DONT'S OF BOOSTER COMMON/GROUND WIRING.

DO USE A "BOOSTER COMMON" IF YOUR:

1) Layout does NOT have common rail wiring.

2) The Booster you have is NOT not optically isolated on the control input.
 
DON'T USE A BOOSTER COMMON IF YOUR:

1) Layout is using common rail wiring.  Further more, you must buy boosters that have an "Optically Isolated" control input.  Digitrax, for example, only sells a optically isolated version of their booster on special order.

2) You have a common Power Supply for all the boosters.  Use protection on each booster power input.

AVOID AT ALL COST:

1) Connecting the booster common to house AC/Earth ground.   If you live in a high static electricity (ESD) environment, you can Earth Ground the booster common but do so with one wire connecting to just one earth ground point though a 1 megohm resistor. (Same procedure used by ESD straps in the electronics industry)

2) A common power supply for all the boosters.  
The only time there can be an exception to this is if you have a power supply SPECIFICALLY designed for this.  The electrical requirement is that some form of current overload protection (fuse or PTC device) is provided for on BOTH inputs terminals of each booster being fed by the common power supply.


A current loop is defined as there is MORE THAN ONE electrical path for current to flow.  AKA more that one wire.  The 1st intended path and the 2nd or more unfortunate UN-intended path(s).   

It means one does not have control over WHERE the current flows and as a result the current finds paths in layout wiring that was NEVER intended to carry that current.  (Cab bus, Loconet, Earth Ground, Computer connection)   This leads to ongoing unexplained mystery problems including booster failures if extreme enough.  This is also know as a "Sneak Path" since you are not aware of its existence.   Current ALWAYS finds a way to flow in any wire it can.  The only variable is how much goes down each given wire path.  This unintended current flow can corrupt signals and the information they represent or worse go through a part of device that was never designed to handle that level of current resulting in its destruction.  Only by carefully maintaining a clear SINGLE PATH for track current flow for a given booster will you have control of the current flow. 

Controlling The Current Path = Electrical Operational Stability & Reliability.



EXAMPLES OF BAD CURRENT LOOPS.

1) "EARTH GROUND" CURRENT LOOP

The following assumes at least a Command Station with a single booster (Inside) is Earth Grounded.  It can also apply to multiple boosters setups with them all being Earth Grounded too.

A computer or device that is Earth Ground is connected.

The Loop:

Command Station Ground to Earth Ground
Earth Ground to Computer Ground
Computer Ground to Serial/USB Ground.
Serial/USB ground to Throttle Bus Ground (Loconet, Cab Bus, ExpresNet..ect)
Throttle Bus Ground to Command Station Ground.  (Includes Booster Common)

If the computer introduces any high frequency electrical noise into Earth Ground, and they do generate a lot, it will find its way running down the Throttle Bus ground potentially corrupting the communication on the Throttle Bus.  Data is lost.  Commands are lost.

Solution 1:  Break the command station's Earth Ground connection.   This eliminates the current loop path preventing any computer noise from going down the throttle bus.
Solution 2:  Insert a Serial or USB isolation device.  Expensive and specialized device compared to removing the Earth Ground.


2) "COMMON RAIL" CURRENT LOOP.

The following assumes there is at least two boosters.


To use common rail with DCC correctly, it requires:

1) Each booster must have "optically isolated" inputs to prevent ground loops where track current finds paths in the wiring that the current was never intended to flow.  This feature is 
2) No common power supply.

2/16/14