Is it safe to assume that if wiring passes the quarter test under DC that it will pass the quarter test under DCC. As I am wiring my layout I am checking for shorts using a buzzer and 9V battery, and I have wired about 2 1/2 out of 3 separate blocks. The power bus is less than 20 feet long and the feeders are located from 30" to 40" apart.
Yes, if you have no electrical shorts when using DC, you should have none when switching over to DCC, as long as you’re doing nothing more than hooking the DCC system up to the wiring exactly as it was connected to the DC power pack.
[banghead] The “quarter test” has NOTHING to do with eletrical shorts and has EVERYTHING to do with DCC signal strength. It is made to insure that you have enough signal so if you DO encounter a short, the base unit will shut off. If you FAIL the quarter test, you run the risk of melting an axle, or worse, a decoder if your base unit FAILs to shut off.
So the answer to bearmans comment of “Is it safe to assume that if wiring passes the quarter test under DC that it will pass the quarter test under DCC.” is a resounding NO!!! This is why DCC requires more feeders than a DC layout. Your feeders are probably too far apart and you need to consider wire gauge when wiring for DCC.
David B
I’m using 14 AWG for the bus and 20 AWG for the feeders. I admit that the 30"-40" is rather loose, nothing is farter apart than 40" and the norm is about 30". I’ve also checked the wiring with a multimeter and am not getting any kind of significant voltage drop, less than 1/2 V when I hook up an automobile light to the track and then turn on the DC current.
Actually it is a “yes”. The quarter test is designed to test the conductor resistance from the source (i.e. power pack or booster) to the location of the short (i.e. your quarter). If the resistance is too high then current will be limited (i.e. voltage / resistance) and not be high enough to trip the breaker. Now it is not a 100% guarantee because the actual voltage differential and source resistance of a DCC booster is different than DC but it is close enough.
You are assuming the trip voltage is the same bewteen the DC controller and the DCC system. In my experience, they are completely different. “Close enough” is good with nukes and horse shoes, but not with DCC systems.
David B
David is largely correct. My Digitrax manual says to lay a screwdriver blade, or a coin, across the track at various locations upon first powerup “to see if the DCC signal is available everywhere on the layout.” Also, Joe Fugate recommends separate short management using tail light bulbs, so he must understand that the short protection afforded by the base stations is inadequate to prevent burnt decoders when the current draw is sufficiently high.
Selector -I’m going to be using a circuit breaker on two blocks and a combination circuit breaker/reverser unit on the reverse loop.
Bearman, my understanding is that you should be well served. I liked the cheap and effective light bulb idea, and was convinced that it would work. So far, if not exactly elegant, it has proved its worth several times for me. A bonus is that whichever district’s light bulb lights, it makes finding the short so much quicker.
I am making no assumptions here. I understand exactly what I am saying. I am not going to start a lengthy debate here on some of the numerous erroneous information around these forums in regards to DCC wiring, electrical characteristics of wire, DCC waveforms etc… I’ve been down that road numerous times. I will simply say that there is really no such thing as “trip voltage” when thinking of power supplies and current limiting, in this realm. In fact it is “trip current”, not trip voltage. The trip current is determined by the internal characteristics of the driving source against a given load. The amount of current is limited by the maximum supply voltage divided by the resistance of the conductors to the short. This gives you the formula of I = E/R . As long as I is greater than the trip current then the supply will go into a current limiting mode. Some shut if off, some cycle to see if shorts are removed and some current limit down to the maximum output current. This is solely determined by the power source.
I absolutely agree that even if
Jeff is correct.
Don’t forget that this resistance includes the impedance of the SOURCE. DC supplies typically have higher internal impedances/resistances than DCC. One can view this as DCC systems typically have higher maximum output currents than DC. That is where the problems arise. A DC supply will usually self limit to a nondestuctive value. A five or eight amp short from a DCC supply will generate a lot of heat in parts of the circuit.
The light bulb trick is a valid one and works extremely well. I prefer it to CBs because it limits current, helps to easily pinpoint the shorted block and is a heck of a lot cheaper than a dozen breakers. [:-^]
BTW the reason that the National Electrical Code requires certain wiring sizes and good grounds is to allow overcurrent devices to trip properly thus preventing heating and fires.
Karl
The main drawbacks to the lightbulb are that it works well at lower continuous amperage (i.e. 1-2A) but at higher amperage draw you’ll exceed the current ratings of the buld and at higher amperages the internal resistance of the bulb multiplied by the amerpage will create a voltage drop that may be high enough to be noticed. On smaller layouts or in blocks where there will only be a few DCC sound locomotives, the lightbulb solution should work well. In larger blocks where there may be 5 or mor
My layout will be a lot more modest than those you describe. I’ll be using a Zephyr with the trip current set at 1.57 amps. The DC power pacs are less than 1/2 amp, I believe, but as I noted in another post, I’ll be relying on two circuit breakers (PSX1’s) and a combo circuit breaker/reverser (PSXAR) to protect the three blocks. At most, I don’t expect to run more than 2 locos and a switcher, maybe two switchers, at the same time w/o sound.
I use them and am very happy. They should serve you well.
My decision is looking better and better.
Exactly my point. The origional poster asked if this could be by-passed when moving to DCC.
True, but, one thing not mentioned in this discussion is the fact that if the short isnt tripped then you run the risk of melting axles because of the high current (or voltage…I am in no way a physics master). I have melted a few axles in my day, and have learned to make sure the signal is strong enough (hence the quarter trick) to prevent more meltages.
David b