International Journal of Advances in Engineering Science and Technology
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41
ISSN : 2319-1120
Contact Resistance Analysis of Silver Impregnated
Graphite Contacts used in Metal to Carbon Relays
Hemant Kagra 1, Dr. Samsul Ekram
2
1
2
Director/Quality Assurance, Indian Railways, Mumbai, India
Astt. General Manager, Global Research & Development Center, CGL, Mumbai, India
1
hemantkagra@yahoo.co.in
Abstract - Metal to Carbon Relays are used in railway signaling on Indian Railways for operating various signaling
functions viz. signals / points / track circuits etc. These relays have Silver and Silver Impregnated Graphite contacts,
which make and break as per the signaling logic and operate the signaling equipment. The reliability of these relays is
very poor and its major reason is High Contact Resistance,
Resistance, due to which the relay fails. This causes disruption to rail
operations and adversely affects the train safety & punctuality.The
punctuality.The design parameters that will prohibit development of
HCR were determined through Design of Experiment.
Keywords - Metal to Carbon relays, Silver Impregnated Graphite (SIG), High contact resistance, FMEA,SEM,
FMEA
EDX
I. METAL TO CARBON RELAYS
Metal to Carbon relays are used in railway signaling systems for configuring
configuring various signaling circuits. These
relays have silver (metal) and silver impregna
impregnated graphite (carbon) contacts [1].. The make and 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
ellow to green, as per the requirement of the train operation, and as commanded by the
train controller.When the contacts of the various relays are available, as per the pre
pre-determined
determined logical sequence,
then the end relay gets picked up [2].
[2] The current then
en flows from the source to the equipment 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 [3].
The Silver (Metal) and Silver Impregnated Graphite (Carbon)
(Carb
contacts [4] are fixed on springs as shown in figure
1, while the fixing arrangement of these contacts in the metal-to-carbon relays is as shown in figure 2.
Figure 1. Silver / Silver Impregnated Graphite Contacts Figure 2. Fixing arrangement of contacts in metal to carbon on springs relays
The reliability of metal-to-carbon
carbon relays is very poor.An
poor.
analysis of signaling equipment failures during last four
years (2007 – 08 to 2010 - 11) on Indian Railways reveals that the failures of metal to carbon relays amount to
approx. 7% of the total signaling equipment failures
failures. This failure rate is quite substantial
ntial and totally
t
unacceptable.
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Contact Resistance Analysis of Silver Impregnated Graphite Contacts used in Metal to Carbon Relays
As per the Failure Modes Effect Analysis, failure modes are the errors or defects in a process, design, or item,
potential or actual [5]. A high RPN indicates that the failure shall cause maximum damage / problems to the
customer and drastically reduce the product reliability [5]. Accordingly, FMEA of metal to carbon relays was done
and the potential failure modes of these relays and their causes were examined.As per the analysis, the highest RPN
(576) was found that of High Contact Resistance. The other major causes were Sulphation of metallic parts (RPN
128) and Coil Open circuit / short circuit (RPN 120). Some failures were also due to Armature Breakage, Gaps /
Cracks in relay base etc.
Thus,it is very clear that the failures caused by High contact resistance are the most potential risk. Since the
current for various selections of signaling interlocking passes through these contacts, their conductivity 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. Thus it is imperative that development of High Contact Resistance in metal to carbon relays should be
prevented.
II. HIGH CONTACT RESISTANCE PHENOMENON
Contact Resistance is that electrical resistance, which the current has to overcome when passing through a pair of
closed contacts. This resistance is the sum of partial resistances due to physically different effects [6]. Thus,
R contact = R constriction + R layer
(1)
Constriction resistance is the increase of resistance for metallically clean contacts due to the constriction of the
electrical current when being forced through a small effective contact area. The contacts do not touch over the entire
apparent contact area but in fact, due to the roughness, touch only on a few relatively small points.Increasing contact
pressure can enlarge the effective contact area by elastic and plastic deformation of the micro contact spikes on the
surface. The larger the effective contact area and the contact pressure, the lower the constriction resistance.
Layer resistance is the contact resistance of clean surfaces due to formation of films. Even in absolutely clean
metallic surfaces, the contact surface is covered by thin filmshaving low conductivity, semiconductor properties or
even isolating characteristics. These layers of oxides, sulphides and other compounds are formed on the surface of
metals by absorption of gas molecules from the ambient atmosphere within a very short time. The thickness of the
layers and the speed of growth are dependent on the contact material, ambient atmosphere, temperature and
time.Besides, other contaminants viz. atmospheric dust or particles created by wear of the moving parts within the
relay, may be on the surface of the contacts, increasing the resistance or even preventing electrical contact. Other
contaminants can be the result of outgassing of the plastic materials within the relay or generated by catalytic effect,
through decomposition of organic vapors under the influence of arc, silicon sediments, oil or grease. The contact
resistance increases due to the film resistance.
As the Contact Resistance of relays is a function of surface physiology of SIG contacts, investigation of surface of
SIG contacts becomes highly imperative. Of particular interest is the formation of oxide / sulphide / carbide layer
formation on the contacts’ surface. Accordingly, the study of film formation phenomenon on the surface of SIG
contacts was done by Scanning Electron Microscopy (SEM) [7] and the chemical constituents of this film were
ascertained using Energy Dispersive X-ray Spectroscopy (EDX) [7].
To explore the possibility of presence of film on SIG contacts, total 2 samples from two different metal to carbon
relays, which were declared failed in the field due to High Contact Resistance, were analyzed by Scanning Electron
Microscope [Model: JEOL JSM 6380 LA]. For temporal diversity, samples were selected from relays manufactured
in 1998& 2006 and to maintain geographical variedness, the relays were picked from various locations on Indian
Railways viz. Chennai and Allahabad. The electrical parameters of both the relays were measured and found similar
and within the parametric limits as per the specification. Their contact resistance was measured and was found to be
higher than the specified value. Thus, it was confirmed that the failure of relays was due to High Contact resistance
and not due to any other cause.
ISSN: 2319-1120 /V2N1: 41-46 © IJAEST
IJAEST, Volume 2, Number 1
Hemant Kagra and Dr. Samsul Ekram
For comparison purpose, SEM
EM & EDX of fresh unused SIG samples were also done. The results of SEM &EDX
analysis [8] were as shown in figure 3, 4 and 5.
Fig. 3Fresh (Unused) SIG Contact
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Contact Resistance Analysis of Silver Impregnated Graphite Contacts used in Metal to Carbon Relays
Fig. 4 Failed SIG Contact from Relay No. 1998
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IJAEST, Volume 2, Number 1
Hemant Kagra and Dr. Samsul Ekram
Fig. 5 Failed SIG Contact from Relay No. 2006
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Contact Resistance Analysis of Silver Impregnated Graphite Contacts used in Metal to Carbon Relays
From the above micrographs, it is seen that the Carbon content has increased from 1.12% to 3.23% (by mass).
This clearly indicates the formation of carbide layer on the SIG contact surface. Besides, other chemical impurities
viz. silicon and Potassium have also increased.
Hence, through the metallurgical analysis of silver and SIG contacts, it is concluded that the development of High
Contact Resistance is due to the formation of film on the SIG contact surface. This film is most probably made up of
carbon and is formed through a complex process. The continuous opening / closing of contacts during the course of
operation of relay causes arcing between the silver and SIG contacts. This increases the temperature of contacts’
surfaces. Besides, the SIG contact is softer in comparison to the silver contact, and the continuous hammering action
of the hard silver contact on the softer SIG contact dislodges some graphite material from the surface of SIG contact.
As has been seen from EDX, some chemical impurities are also present on the SIG contact surface. This dust /
debris / impurities, under high temperature and arcing, fuses together and forms a complex carbon film on the SIG
contact surface. The thickness of the film and the speed of growth are dependent on the graphite material properties,
ambient atmosphere, temperature and time.
III. CONCLUSION
The development of surface film is an intrinsic property of the SIG contact functioning. Due to high temperature
and arcing, the film shall always be formed. In hot and humid areas, the process shall be faster and consequently,
recurrently generate High Contact Resistance. The resistance of film depends upon the depth of the layer, effective
contact area and the specific resistance of the graphite material. In absence of film, the contact resistance remained
within reasonable working limits. Hence, in order to prevent development of High Contact Resistance, the surface
film should either be prevented from being formed or should be removed / broken every time it forms.
Preventing the formation of surface film is an unmanageable task, for as long as the dust / debris is present on the
surface, it shall always fuse under high temperature and arcing to form the film. An easier task is to remove / break
the film every time it is formed. This shall provide reliable electrical contact and thus, help in maintaining the
contact resistance within reasonable limits, consequently preventing failure of metal to carbon relays.
Implementation of modifications in Silver / SIG contact design parameters shall certainly prohibit the
development of High Contact Resistance, consequently enhancing the reliability of metal to carbon relays.
Improvement in reliability of metal to carbon relays by reducing / eliminating their failure modes shall enhance the
performance of Railway Signaling Systems, resulting into better punctuality of trains and additionally, improving
the safety performance of railway signaling systems and reducing public grievances.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Metal-to-carbon Relay Specifications:
(i)
British Rail Specification 930 & 931 A
(ii)
British Rail Specification 932A, 933A, 934A, 935A, 937A, 938A
(iii)
Indian Rail Specification RDSO/SPN/84/88
(iv)
Indian Rail Specification STS/Relay/AC Lit LED Signal/99/2002
Australian Rail Track Corporation Ltd., “Relay Design Standard”, Australia,.
Indian Railways, “Handbook on Signaling Circuits”,India.
M/s Westinghouse, “Relay reference Book’,U. K.
D. H. Stamatis, “FMEA – From Theory to Execution”, 2nd ed. ASQ Quality Press, Wisconsin, USA, 2003
R.Holm, “Electric Contacts,” Springer – Verlag, 1967.
B. Viswanathan, S. Kannan and R. C. Deka, “Surface characterization techniques”, Narosa Publishing house, India,2010.
Joseph Goldstein, Dale E. Newbury et al.,“Scanning Electron Microscopy and X-ray Microanalysis”, Springer Publications, USA, 2003.
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