CPL signals and how they operate

NW Mailing List nw-mailing-list at nwhs.org
Thu Jan 9 15:05:34 EST 2020

Abe, imbiber of turnip nectar,

Thanks for the attachments and particularly PRR S-855.  I would like to go
through my interpretation of the schematic shown in Figure 2 on page two
and solicit your corrections and elaborations.

In attempting to decipher the nomenclature, it appears that the signal
designation is 2A and that R is used as a suffix for Relay and E for

So, the “H” relay (2A_H_R) routes positive battery voltage (E_B_X [don’t
know what the X stands for}) through the de-energized contact to wire
2A_R_1_E.  This wire is connected to a potentiometer and after the pot, the
1 is dropped from the name.  2A_R_E is then connected to lamps 0 + 1.  Is
“R” always used to for the voltage designation to these lamps?

If the “H” relay is energized, battery voltage is routed to the “D” relay
through wire 2A_H_2_E.  If the “D” relay is de-energized, battery voltage
is routed via wire 2A_H_1_E through a pot and then wire 2A_H_E to lamps 45
+ 46.  If the “D” relay is energized, battery voltage is routed via wire
2A_D_1_E through a pot and then wire 2A_D_E to lamps 90 + 91.

As you pointed out, EBX is constantly supplied to the neutral lamp through
wire 2A_N_E (it seems likely the “N” is for neutral), and the “low” sides
of all the lamps are tied together through E_N_X to the negative battery

I would be very interested in any diagrams/information on how the track
relay, track relay repeater and the H and D relays were interconnected.

Also, still looking for how block occupancy information is passed between
blocks.  Was is just a presence or absence of voltage on a line pole wire
or was it encoded?

On Wed, Jan 8, 2020 at 12:41 PM NW Mailing List <nw-mailing-list at nwhs.org>

> Commodore Cochran of the Roanoke River Coast Artillery, and Associates :
> You have everything right in your understanding so far !
> Off the contacts of the track relay was wired another relay called the
> Track Relay Repeater, which controlled signal aspect lighting.
> I am attaching two documents which may help as you work through the
> lighting of PL Signals.
> The first attachment is a JPG called PL Signal Lamp Numbers.  It will be
> helpful if you realize that the lamps on a PL signal have formal names, and
> using those names avoids having to jump through convoluted verbal
> constructions like **Top Arm, Lower Left Lamp.**  Just say  **the 45
> lamp.**  I apologize for the less than elegant graphics.  Decades ago I had
> the graphics gal in our company publications department made up the graphic
> for a Time Table Special Instruction.  She had probably never seen a signal
> in her life and was, no doubt, misled by my sloppy pencil sketch, so it is
> what it is.
> The second attachment is the goodie.  It shows the relay controls and the
> wiring of aspect lighting for PL signals:  single-arm, two-arm, old style
> PL dwarf, new style PL dwarf, and pedestal signals, in every possible
> combination of top arm and bottom arm aspects.
> Notice that every relay in the scheme of signal control has a name, and
> every wire has a name, too.  These are the **nomenclatures** which a big
> railroad simply MUST have when dealing with complex and repetitive
> situations, to avoid confusion.  Every relay and wire in a railroad signal
> case is labeled in accordance with that railroad's standard nomenclatures.
> Two things to call your attention to, as helps.
> (1)  On high signals (i.e. signal arms having a **Neutral** lamp in the
> middle,) the outer lamp pairs (e.g. the 0 + 1, the 45 + 46, the 90 + 91,
> and the 450 + 460) are each wired with common a feed and with a common
> ground, and all the grounds are connected together for a single-wire return
> to battery.  The N- or Neutral lamp is fed separately.  Thus, with a
> 6-conductor cable you can light 9 lamps on a signal arm (one conductor for
> the ground; one conductor to light the Neutral light; and one conductor for
> the lighting of each of the four possible **row** combinations, 0 + 1, 45 +
> 46, 90 + 91, and 450 + 460.)
> (2)  On the dwarf signals, energy for each single lamp is switched over a
> discrete contact in a relay, and therefore three relays are required to
> light four aspects on a dwarf signal.   This seems a little complex, but in
> the days before diodes this is the way they had to do things.
> Railroads draw their relay circuits differently from the electrical
> industry, but the drawings are very unambiguous when you study them.
> Hope this helps. Let me know if you need more,,, there are about 600 more
> pages in the Standard Plans !
> I will also attach a photo of the style relays which generally served as
> track relays on the N&W, the Union Switch & Signal DN-11 relay.  In the
> photo, overlook the big Ziegler relay with a white porcelain top.  The
> DN-11s are are on either side of the Big Zig.  The DN-11 is a neutral relay
> with two front and two back contacts.  These particular DN-11s are used on
> my approach bell circuit.  Track relays are normally wound to 4 Ohms
> resistance (2 Ohms each coil,) but for this application I used 500 Ohm
> relays because I do not like low resistance, high current relays... too
> many high current relays in a circuit blow the cartridge fuses protecting
> my power supplies.
> -- abram burnett, CEO
> Deplorable Turnips, LLC
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