From: WiringForDCC@... [mailto:WiringForDCC@...] On Behalf Of JohnOn Jan 1, 2014, at 9:55 PM, John wrote:
I was in the NCE group and realized it probably wasn't the right place to be asking this question. There they had recommended using a detector like NCE's BD20 on the main bus,and stated the following advantages:You have not clearly stated what your desired intention is for the use of occupancy detection. To address the question about detectors, one must take a step back and ask why you need any detector at all. Normally people add detection to the layout because they want to add signaling to the layout or some subset of that such as setting up some working crossing gate animation. To be clear, there is NO DCC requirement that says you must install signals or animation. These are all options you can add if you desire.
If YES you need a track occupancy detectors and the track elecrtrically broken down in to signal blocks. One detector per signal block and each signal block should be longer than the train. Even if you are not ready right now to install the signal system, planning the layout wiring to support it is a good thing to do BEFORE you build your layout. Otherwise there will be a big rewiring later. Beware the cost of installing a signaling system can cost as much as the what you spent on the layout before you added signals. Cost is the biggest reason why it is not done most of the time. Of the types of occupancy detectors the monitor track current, there are two types. Diode based and transformer/coil based. The NCE occupancy detector mentioned is transformer/coil based. Compared to the diode base detector, the list provided is correct in terms of the advantage over the diode type.
If NO, there is no need for any occupancy detectors at all and the question about detectors can be dropped.
How would a circuit breaker work in DCC? I'm very familiar with house wiring since my dad was an electrician.DCC circuit breaker work just like they name but have some additional features. At a minimum, they detect current draw above a set level and trip as in enter the off state. To understand the need for them is to first understand the problem they fix.
Like a DC PowerPack/Throttle, DCC booster shuts down when there is a short on the track. This is because all DC Powerpack and DCC boosters all have a circuit breaker function built in and it self resets itself when the short is removed. The current trip point is the same the DC throttle or booster current rating. With DC, this was not a big problem because there was only one DC Powerpack per train. Only one train was effected. However, with DCC the single booster is running multiple trains and when the booster trips due to a short, all power is lost and ALL of the trains stop running. If there is only one operator, it not so bad. But if you have lots of operators, the other that did not cause the short will not be happy for they must wait on the operator with the shorted train to fix the problem. With a global short, the biggest advantage of running multiple trains with DCC becomes it biggest detractor when you have more than one layout operator and running multiple trains.
The 2nd issue that pops up with multiple train running is that during the fixing of the short, the short will come and go and OTHER running train will start and stop in a jerky fashion potentially creating more derailments. This generates even more displeasure.
Like a house with its multiple circuit breaker protecting certain equipment or areas of the house, a short at a given outlet will not cause the entire house to lose power because the main breaker tripped. Likewise if you device the layout into what is called in the DCC word "Power Districts" created by DCC circuit breakers, you can control how much of the layout is effected by a given short.
The DCC circuit breaker worked because it is designed to shutdown BEFORE the booster does. Most DCC circuit breakers have a Auto-Reset feature turned on by default. In this mode, the DCC circuit breaker return power to the track once the short is removed. The key point is that if you choose you power districts carefully, everyone else can still be running trains while you fixing your own short. Everyone is happy other than guy fixing the short.....which most likely he cause by himself running against a turnout....a self inflicted wound.
So the need for DCC circuit breaker depends if you have
1) more than one person running you layout
2) the layout designed to allow multiple trains to run safely unattended.
If the answer is yes to any of the two question, then you can benefit from the use of DCC circuit breakers.
Keep in mind that most DCC circuit breakers are designed to be used with 4 to 5Amp boosters which are normally found on large layouts.
You should also again plan you layout wiring to support Power districts which by the way are not necessarily the same as signal blocks but have almost the same impact on the wiring design.
DCC Website & NMRA DCC Clinics: www.markgurries.com