Friday, July 10, 2009

Third rail changeover switch

The 1005 is being restored to its 1934 configuration and appearance. In 1934, the Sacramento Northern operated on both overhead wire and third rail.

Electrically, it would be simplest to wire the trolley poles/pantographs to the third rail shoes at all times. However that is illegal and unwise. There must be a changeover switch that connects car power to the active one while isolating the other.

On the 1005, this is fully automatic. This is done with a pair of relays in the baggage compartment of the car. One relay picks up if power is present on the trolley wire. The other picks up if power is present on the third rail. If "trolley" is on and "third rail" is off, it energizes a wire which runs down to the changeover switch and says "switch to trolley". Or vice versa.

Alongside this relay box are four resistors. Two are dropping resistors for the two relays present in the box. The other pair are dropping resistors for the two wires going down to the changeover switch. One pair of resistors was fine (it was made of cartridge resistor elements which removed like a fuse.) The other pair was totally defective and new resistors had to be made. The order came in; it was wrong and had to be remade. They were further modified to replicate the original mounting style. This work was completed last week and the relay box and resistors are remounted in the baggage compartment.

The changeover switch is underneath the car. It processes each signal further. For instance if the "switch to trolley" signal is energized, it will switch to trolley, but only if it's not already in trolley, and only if the car is coasting (not motoring). If all conditions are met, the signal reaches a magnet valve, which applies air to muscle over a big rotary switch similar to a reverser. There is yet another interlock, which assures the switch will only operate when the car is configured for 600 volts DC (not 1500 volts DC).

As to the changeover switch itself, the restoration had a challenge.

Normally on a third rail car, all third rail shoes on the car are connected together electrically. Third rail shoes on one side are touching the third rail; on the other side they stick out, energized and dangerous. Crews in third rail territory are trained on this, and civilians are kept away. That is how SN 1005 was configured in 1934. The Key System bought the car in 1942. The Key used third rail to come over the Bay Bridge and into the San Francisco terminal, but they had low level platform loading. Passengers could easily touch a third rail shoe. So, Key required a more sophisticated changeover switch which isolated third rail shoes on each side of the car. They removed and scrapped the original changeover switch!

The correct changeover switch would have to be remade from scratch.

Changeover switches are very similar to reversers. The air motor, magnet valves and some other parts were scrounged from an old reverser. Outside of that, the changeover switch was built from scratch over the last two years, using the reverser on another car as a model. This was a considerable undertaking. It took about two years, and is complete. It has been installed and the heavy power connections have been made.

What remains is the small signal connections. At this writing, wiring has been pulled through conduit, between the changeover relays, the changeover switch, and the 600/1500 interlock.

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