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Power Systems for Exchanges



"The purpose of the telephone power plant is to furnish energy of the required character in proper amount and available 100% of the time. The power plant is the "heart" of the system, since every line and connection will be "dead " the moment the supply of power is interrupted." From [Young].

Depending on your age, you may know that landline telephones worked (even for days) during local power failures. This has been true starting from the 1910’s. The Bell System, and others, put non-stop availability and reliability at the top of their priority list. This was strictly true for metro areas but not guaranteed for smaller exchanges (100’s of users) because of cost.


Starting in August 2022, according to FCC Order 19-72, telephone companies are not required to offer landline services. So, landline services will, over time, be completely replaced with internet/VoIP services and mobile in NA and worldwide.

‍The landline replacement services are not as durable during a power failure. On the home-side, the internet is unavailable during a power failure. Cell towers are only required to have 2-4 hours of battery backup. True, some crucial towers have backup generators, but this is not universal. Bottom line for telephone users, landline-like availability (the gold standard) is disappearing.  

How much power?

Some of the larger Bell telephone buildings contained several central offices, and in addition, administrative, engineering, and other departments. An incoming AC power service provides for such a building, of which the telephone power plant requirements may approximate 375,000 watts.  Design allows for this number to double as more exchanges are added to a building [Young].  

Next, let’s look at some Bell System power plant designs and components for large exchanges, especially in North America (NA) in the 1920-30’s. Power equipment occupies 10-15% of the total exchange floor space. In general, the principles outlined below apply outside NA too.


Rock solid availability


So, what is the strategy to guarantee 100% (really, 99.9999…) availability for landlines?  There are four pillars.

     A. Exchange systems are powered directly from batteries.

     B. Batteries are constantly charged by DC generators, turned by AC motors.

     C. If AC power fails, backup generators replace the commercial AC power.

     D. Monitoring and control panel for the power system


Fig 1 outlines these as A, B, C and D. Each area will be described in some detail below. This design is for large offices and not a small community exchange. For a small business system such as the Bell System’s 755A dial private exchange (20 lines, 4 outgoing lines), batteries are utilized but the level of availability is reduced during an extended power failure.





 Fig 1

Simplified “Metro Class” central office power plant

power room for telephone exchange overview diagram
Icon for more information

The batteries (A)


In the initial stages of telephony, both individual phones and the central office (CO) were equipped with batteries. Fig 2 [Miller] shows a basic telephone set circuit with internal batteries circa 1900.


The microphone, made of carbon granules, requires a current to function.  Speaking causes the resistance of the microphone to vary resulting in a modulation of this current. This electrical signal is sent over two wires (often called tip and ring) to the called subscriber.






Fig 2

Subscriber set with 3V battery


Fig 3 is a hand cranked magneto (to ring the called phone) wall telephone, with two batteries, manufactured in 1917. In time, it was decided that a "centralized battery" or “common battery” could supply current for every subscriber’s transmitter. This would eliminate the maintenance difficulties of batteries inside telephones.


The change to common battery came slowly and was applied initially in the large cities. In 1937 there were still about a million subscriber phones with batteries in the Bell System, especially in small towns.




Fig 3,

Early telephone with internal batteries (Wikimedia)


   For many years 24V was the standard office battery for switchboards. In 1902, after experimenting with battery supplies ranging from 24 to 110 volts, Bell engineers decided that 48V was the most practical for automatic systems. So, twenty-two battery cells (sometimes 23), in series, should be used. In 1920 it was common to use both 24V and 48V inside an exchange since switchboards and automatic coexisted. 120 years later, 48V is the universal standard in NA and most exchange locations worldwide.


According to [Shakleton], in the larger switching offices (a few exchanges sharing the same building) there may be as many as 140,000 relays which required power plants capable of handling peak loads of 4,000 amperes at 48 volts. This was in 1924.  Over time, the total amperage increased markedly when ~10 exchanges shared one large building.


To meet the amperage needs, the batteries were immense. See Fig 4, a picture from a circa 1935 central office battery room. To supply 48 volts there are 22 batteries in series. So, each cell was about 2.2 volts. Most were of the lead-acid type, but battery technology varied over the years [Martin].








Early vintage telephone with internal 3V batteries
Early vintage telephone with internal batteries

Hush, be quiet 


The same battery used to power relays and the main switches also powers the many “talking paths.” Relay drive voltage can be “dirty” but the talking battery must be very quiet and devoid of AC hum (from charging equipment) and other noises that could be heard by subscribers. The human ear is very sensitive to noise and many strategies were used to quiet the batteries.

One means used massive audio frequency “LC” filters to reduce noise. Some filters could pass 1,000 amps DC and weigh several tons. With proper filtering, one central composite battery could be used for switching and talking.


















Fig 4,

A central office battery room circa 1935


A cell may weigh about 3.5 tons (2,000 US pounds) or about 77 tons for all 22 cells. This is an incredible weight so basement rooms were needed to store the batteries. With so much current draw, the connections to the batteries were usually large copper bus bars. Large copper wires could not carry the current efficiently. Several bus bars can be seen in the upper left side of the Fig 4 picture.


Without charging, the collective 48 V battery may last three hours during heavy calling in metro offices. This falls short of 100% uptime. Next, item B in the Fig 1 diagram is considered.

A central office battery room circa 1935 telephone exchange

The chargers (B)


Almost all exchanges relied on commercial AC power (60/50 Hz sinusoidal wave) to charge the DC batteries. If 2,000 amps DC are consumed by an exchange, the chargers, on average, must supply 2,000 DC amps. AC cannot directly charge a battery; it must be converted to DC first. In the 1920’s the most practical way to convert AC to DC was to use a Motor-Generator set.


Fig 1 shows (M-G set) an AC motor turning the shaft of a DC generator. The output of the generator is 48V DC. In a large exchange there may be several M-G sets and the generator outputs are summed to supply sufficient battery charging power.


If one M-G set fails or needs maintenance, it can be taken offline knowing that the batteries can keep supplying power for many hours or even days, in part, depending on how many other M-G sets are operational. The power plant was designed to give an uninterrupted battery supply even when the usual sources of charging power have been temporarily disrupted.  


Fig 5 is a partial view of a power room supplying energy for two large panel offices in NYC in 1922. There are twelve M-G sets in this room with parts of four shown. Looking at the front left M-G, you can see the motor part and its shaft connected to the generator part.  Between the 1930s and 1940s, these machines became more compact due to design improvements.










Fig 5  [Young]

Section of a power room supplying two panel exchanges

Section of a power room supplying two panel exchanges

The backup (C)


All the essential power room machines and batteries are arranged in duplicate and will come into action automatically to ensure continuity of service in the event of loss of power or trouble with any of the equipment.”  From [Craft]


Essential to any large office were one or more backup generators. See Fig 1, C. Fig 6 is a gas powered 165 HP, 6-cylinder, motor driving a DC generator. A typical generator of this kind could supply 800 amps at 48V DC. Notice the power fail cutover switch in Fig 1. This is manually or automatically engaged to select the desired source of DC for battery charging.



Fig 6

165 HP gas engine and 48V DC generator

165 HP gas engine and 48V DC generator telephone exchange vintage

The main power board (D)



Fig 7

Central Office power board


The power board is for control and monitoring of the power systems. It includes routing switches, meters, fuses, and other equipment necessary for the operation of the plant. Fig 7 and Fig 5 show portions of large power boards. They are built from modules so each exchange can have a custom board yet built from standardized modules.




Fig 8 pictures an early board-based power station for manual switchboards [Martin]. It’s an all-in-one system with batteries, generators, routing switches, fusing and controls.  As more power was demanded by automatic exchanges, power systems evolved to meet the needs.



Fig 8

Power station circa 1890’s


The Bell System set the gold standard for telephone service reliability. Many other companies worldwide followed their lead. Using a no single point of failure (NSPOF) mentality with commercial power backup provided subscribers with unparalleled service.  




Note: Some quotes herein have been edited for length.


Craft, E. B. and others, Machine Switching Telephone System for Large Metropolitan Areas, Bell System Technical Journal, Jan 1923 page 53.


Miller, Kempster, American Telephone Practice, 1905 


Young, R, L. ,  Power Plants for Telephone Offices, Bell System Technical Journal, Oct 1927 page 702.


Shackleton, S. P. and others, Relays in the Bell System, Bell System Technical Journal, Jan 1924


Martin Jr, R. P. , Talking Battery, Bell Laboratories Record, Jan 1937

The power board is for control and monitoring of the power systems. It includes routing switches, meters, fuses, and other equipment necessary for the operation of the plant
Power station circa 1890’s
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