Short circuit

[lionelsoni]

John (joncoy) asked me this question:

"I am confused about a short circuit. If I put an ohm meter red end (+) on the pick up roller and the black end (-) on a wheel (train car on work bench) it pegs the meter as if I touched the leads together. I always thought that was a short in the circuit. I now (because the lights light) assume that it is the return path for the circuit. When is it a short?

"If I put the car on the track wrong and the wheel hits the outside (-) rail and the center (+) rail the transformer light blinks because it is shorted. I hope you can understand my confusion."

A short circuit is an electrical path around the intended load. Usually we assume that the resistance of the wires is zero, even though it is actually just very small compared to the normal load. But even a short circuit has some resistance. Likewise, we assume correctly that the load resistance is greater than zero. But the load resistance of one high-current circuit might be less than the wire resistance of another low-current circuit.

To measure the resistance of a circuit, in order to decide whether it has a short circuit, you need to have an idea what the normal resistance should be and to be able to measure a resistance of that value with some confidence. I just measured the resistance of a number-53 lamp with my ohmmeter. On the Rx1000 scale, it measures zero. On the Rx10 scale, it measures 15 ohms. On the Rx1 scale, it measures 40 ohms. So I cannot use the Rx1000 scale to decide whether there is a short circuit in a lighting circuit--it is too insensitive. (Could this be what you are doing?) I could use the other scales for that: The Rx1 scale would be best.

To understand why the latter two scales give different results, you need to know that incandescent lamps have a resistance that increases with voltage, in fact, approximately as the square-root. Since the Rx1 scale puts more voltage on the lamp (albeit far less than enough to light it up), it increases the resistance even as it tries to read it.

Other circuits, non-linear ones, like rectifier diodes, don't really have a resistance at all. The ohmmeter just indicates the ratio of voltage to current at the voltage and current that its own circuit happens to supply to the load. But it can still indicate a short circuit if you know what it should read when there isn't a short circuit.

You're not running with DC; so it's not really correct to designate the rails as + and -. Nevertheless, when a wheel gets between them, whether powered by AC or DC, it probably reduces the total circuit resistance to much less than an ohm. This low resistance draws a very large current from the transformer, whose circuit breaker detects the excessive current and disconnects the transformer from the load.

[Anonymous]

Bob
I'm not sure I understand your[John's] question---when you get power to and thru the workhorse motor everything is okay.
The flow of power from from the transformer thru the center rail, thru the pick-up roller, to and thru the motor and then to the common return thru the frame/axle/wheel set/outer rail and back to the transformer. That is a CIRCUIT, not a short circuit, and your megger is showing the good news that it has low or no resistance. If you bypass the motor(load) and directly connect the center (hot) and outer[common] rail via a derailled wheel set, screwdriver, watch band or whatever--that is a "short circuit".
On the other hand I may completely misunderstand what you are getting at![:)]

[lionelsoni]

John seemed to be puzzled by the fact that couldn't tell the difference with his ohmmeter between a working light circuit and a short circuit. I was trying to explain that all circuits have some resistance and that the difference between them is that the short circuit has an abnormally low resistance. In order to tell the difference, you need to have an ohmmeter that can distinguish between the two. I suspected that his ohmmeter scale was meant for measuring much higher resistance than even a normal light circuit, so that a working circuit and a short circuit looked the same on his meter.

I was incidently trying to correct his apparent confusion between AC and DC and to suggest the limitations of an ohmmeter used with other than a fixed linear resistance, except as a rough indicator of a short circuit.

Nice name!

Bob (Robert Leonard) Nelson