TITLE OF RESEARCH

A CRITICAL STUDY OF RELIABILITY OF METAL TO CARBON RELAYS

USED IN RAILWAY SIGNALINGWITH SPECIAL REFERENCE TO HIGH

CONTACT RESISTANCE

1. PREAMBLE

Relay is an electromagnetic device, which is used to convey message electrically from one

circuit to another circuit through a set of contacts (back or front contacts) and works on the

principle of electromagnetism. The electrical symbol of a relay is as shown in figure 1.

Fig. 1 Electrical Symbol of Relay

The relays are of many types, based on their usage, construction, power source, manner of

mounting etc. A typical classification of relays can be as under:

In railway signaling, all the above variations of relays are used. For the purpose of this

study, the classification of relays as per their contact material viz. Metal-to-Metal Relays or

Metal to Carbon Relays has been considered. Although both the types of relays are

extensively used on railways, this research pertains to only Metal to Carbon Relays.

The signaling circuits are built up by using these relays. The circuits may be of a very

simple type as shown in figure 2,

Fig. 2 Simple Signal Circuit

Or they may be of a more complex variety as depicted in figure 3.

Fig. 3 Typical Signal Circuit

When the contacts of the various relays are available, as per the pre-determined sequence,

then the end relay gets picked up. The current then flows from the source to the function i.

e. to either a signal or a point or a track circuit etc. Even if one of the contacts of any relay

is not available, the path of the current shall not be fulfilled and the circuit shall not get

completed. In that case, the function viz. signal / point / track circuit shall not operate.

Thus, the reliability of relays plays a vital role in the working of the signaling functions.

1.1 METAL TO CARBON RELAYS

The Metal to Carbon relays, as shown in figure 4, used in railway signaling systemsfor

configuring various circuits of signals / points / track circuits etc.have SILVER

(METAL)and SILVER IMPREGNATED GRAPHITE (CARBON)contacts. A typical

relay has total 16 such contacts, which are used in the signaling circuits. The make and

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break sequences of these contacts simulates various selections in the signaling circuits,

consequently, governing the change in aspect of railway signals from red to yellow to

green.

Fig. 4 Metal to Carbon Relay

The expended view of the relay is as shown in figure 5.

Fig. 5 Expanded view of Metal to Carbon Relay

3

Relay Cover

Relay

Plug Board

The vital components of the metal-to-carbon relays are shown in figures 6 & 7.

Fig. 6 Components of Metal to Carbon Relay

4

SIG Contact

(Fixed)

Silver Contact(Moving)

Armature Coil &

Core Assembly

Operating Arm

Relay Base

Fig. 7 Armature & Core assembly

The Silver (Metal) and Silver Impregnated Graphite (Carbon) contacts are fixed on springs,

as shown in figure 8.

Fig. 8 Silver & SIG contacts

The salient features of these relays are as under:

(i) Common plug board for all relays.

(ii) Plug & socket kind of interconnection between plug board and relay.

(iii) Retaining clip to hold the relay firmly.

(iv) Connectors positively locked in the plug board can be withdrawn only by a special

tool.

(v) Terminating wires on the connectors is both by crimping and soldering.

(vi) Registration device with specified code combination to prevent interchanging of

relays.

(vii) Relay cover provided with gasket for moisture protection.

(viii) Maximum numbers of contact are 16 independent metal to carbon type.

(ix) Continuous current carrying capacity of contacts: 3 Amps

(x) Switching capacity: 2 Amps

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Silver Impregnated Graphite (Carbon) Contact

Silver (Metal) Contact

1.2 THE PROBLEM

‘The reliability of signaling circuits is adversely affected by High Contact Resistance

phenomenon in metal to carbon relays’.

The reliability of metal-to-carbon relays is not up to the standard, resulting in substantial

no. of signal failures, which adversely affects the train operations. As shown in figure 9, a

study of signaling equipment failures for the year 2009 – 2010 (April 2009 – March 2010)

done on Western Railway reveals that the failures of metal to carbon relays amount to

approx. 9% of the total signaling equipment failures:

AFTC8%

AAC10%

BLOCK INST.9%

CABLE10%

DAC6%

ELECT. EQPT.19%

POWER

EQPT.7%

RELAY9%

LAMPS/LED12%

MECH. EQPT.9%

PT. MACHINE1%

Fig. 9 Equipment Failures on Western Railway

This clearly indicates that the reliability of these relays is considerably low.There are

numerous reasons for the poor reliability of these relays. These are sulphation of metallic

components, shorting of relay coil, breakage of armature etc. Out of these, the most

important factor is the rise in contact resistance of silver – SIG contacts during operation.

Since the current for various selections passes through these contacts, theirconductivity is

of utmost importance. Due to high contact resistance, the current can’t pass through these

contacts. This disrupts the electrical connection between the contacts, causing the relay to

fail, eventually resulting in the failure of signaling system.It is proposed to conduct a

critical study of the failure mechanism of high contact resistance phenomenon and to

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suggest improvements in technical / quality management procedures for enhancing their

reliability.

1.3 JUSTIFICATION

Reliability of metal-to-carbon relays plays a crucial role in the overall reliability of

signaling systems. Poor performance of these relays results into increased no. of signal

failures. Since the train movement is governed through railway signals, any failure of

signals disrupts the train operation and adversely affects the punctuality of trains. This

causes passenger grievances and is a big hindrance in the customer relations of Indian

railways. This also tarnishes the railway’s image as a premier transportation and logistics

agency. Besides, since every failure is akin to accident, every signal failure is also a

potential source of disaster. During signal failure, the train movement is done in manual

mode, thus creating a possibility of human error, either through negligence or deliberation.

This may consequently lead to train accidents involving loss of human life and destruction

of property. This is totally undesirable.

In the previously referred pilot study of signaling equipment failures for the year 2009 –

2010 (April 2009 – March 2010), done on Western Railway, it is seen that the 9% failures

of metal to carbon relays resulted in detention of total 408 no. of trains with a cumulative

detention time of more than 55 hours. This is quite high and totally unacceptable.

A month wise analysis of no. of relay failures / trains detained / total detention time is

shown in figure 10.

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Fig. 10 Impact of failures of metal to carbon relays

Hence, research into enhancement in reliability of these relays shall certainly augment the

safety and punctualityof train operations, which is in the larger interest of Indian society in

general and of Indian Railways in particular, and thus, is fully justified.

1.4 BENEFITS

This research shall help in improving the reliability of metal to carbon relays by reducing

their failures, thus enhancing the performance of Railway Signaling Systems, resulting into

better punctuality of trains and additionally, improving the safety performance of railway

signaling systems. This shall reduce the public grievances and improve the customer

relations of Indian Railways. This shall also enable the Indian Railways to attain the world

standards of railway punctuality and safety.

Additionally, the relay manufacturing industry shall also benefit through this research, as

they shall be able to implement the suggested Total Quality Management measures for

enhancing their product quality resulting into greater customer satisfaction.

A summary of the likely benefits to the Society at large, Indian Railways in general and to

the relay manufacturers in particular is shown in figure 11.

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Fig. 11 Benefits

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