As a quick background: older (600V) streetcar interior lights are normally in sets of five bulbs in series (58W, 120V, DC bulbs). 5 X 120V = 600V The series sets are in parallel. The number of bulbs in a car will be multiples of five. I remember once looking for a long time for the 34th and 35th bulb in a PCC car. One was in a box under the floor. Some of the bulbs could be in the roll sign, marker lights, and yes even in the fuse box. There are two downsides to this arrangement. 1. If one bulb burns out, the other four in the 5-bulb series go out. To find the burned out bulb you often have to remove and test multiple bulbs. 2. If a bulb is removed while the 600V is present the socket has 600V across it.

Now look at the diagram below for the Connecticut Co. open trolley car #838 at Seashore. Just about everything I wrote in the previous paragraph is contradicted.



Consider at the lower right side of the diagram -- the matrix. All but two of these bulbs are interior ceiling lights. They are in a 4 X 4 series/parallel matrix. Also you can always count five bulbs from +600 V to ground when you include the bulbs along the bottom of the diagram, i.e., the headlights, platform lights, and two of the end sign lights. Note the car has a Number 1 and a Number 2 end. The transfer switch at the Number 2 end is for when the car changes ends.

Now if one bulb burns out, some others burn brighter picking up part of the burned-out bulb's current. For example, if ceiling light 6R burns out its current is picked up by bulbs 5R, 4R, and 3R and they burn about 1/3 brighter, i.e., sharing 6R's current. I say "about" because the current balance in the rest of the matrix is changed slightly. Identifying that 6R needs replacing is simple. And when removed, its socket is at 120V. If 6R and 5R both burn out, 4R and 3R probably won't last long at about double the intended current.

838 has just been rewired. This circuit was retained for historical reasons though a conventional circuit, as described in the first paragraph, was an option. Oh, and it worked the first time!

I welcome your corrections and comments.

How common was this type of matrix circuit?

I would think this circuit is a winner just for the reasons you described. This circuit will not go dark in multiple places due to a single bulb failure. Of course, it is more work to wire.

p.s. is not there an error in the diagram? Should the transfer switch alternate between no. 1 and 2 street headlights? As shown, it does nothing.

Steve, note there is no dot, which means connection, where the wires cross above the #2 platform light. That fooled me for awhile.

Branford's 401 or 923 has that system. I don't think that their 1414 or 1423 have it.

Interesting circuit, indeed. I have never seen a Canadian car with anything like this for general carbody lighting although door interlock signal lights are often wired like this.

Is there a specific reason that the compressor fuse is located ahead of the compressor switch? It would seem to be safer if the fuse was after (on the load side) of the switch so that the fuse could be changed by opening the switch rather than having the fuse live at all times the pole(s) are in contact with a power source

Gord McOuat
Halton County Radial Ry.

The warning notice "Pull Pole Before Changing Fuse" appears in other places besides Philadelphia P.C.C. cars, so I think that "fuse before rotary switch" also occurs elsewhere.

LOL, if you are touching 600v circuits on a car with the pole up, you're a braver man than I, and I do overhead line work!

If there is a problem in the switch itself or the interceding wiring, the fuse would not protect it.

Not like I am going to have the opportunity to do this anytime soon, but does that mean that standard safety rules would ask that you pull the pole down before changing any fuses, switched or not?

Yes, it does. That way, the fuse can protect the switches.

If I'm working on the electrical system of a trolley car, not only do I pull the pole, I place a blue flag in the trolley catcher or retriever. If neither of those devices is present, I tie the trolley rope around the flag stick.

With PCC's, this is often because they have (or originally had) a capacitor type lightning arrestor which held a charge even after the pole was removed from the wire. Some systems had instructions to leave the body light on, then pull the pole to discharge the arrestor.

Gord

Ah. The more I learn about trolley circuits, the less I know about electricity.

If you look at the wiring diagram for the whole car, there is nothing that equates to the fusible disconnect common in industrial wiring. Pulling the pole will disconnect from the power source, of course, but only if someone does it. Otherwise it will conduct current until it burns the wire down. Since there is is no equivalent of a main breaker, each branch circuit has its own fuse upstream from any manual or automatic contacts (such as the compressor governor) so that if the contacts fail or arc to ground, they are protected.

Often there is a master fuse frequently located on the car roof between the trolley poles and the wire going into the car. It may be as simple as a short length of #6 to #12 wire inside a special fuse box. This fuse may not be shown on car wiring diagrams.

Rules of the National Board of Fire Underwriters specified that "A cut-out switch must be placed as near as practicable to the current collector so that the opening of the fuse in this cut-out will cut off all current from the car."

Cars using third rail power sometimes had a cut-out switch included in the third rail shoe mechanism.