major contributors of relay failure are interlock burnt /pitted and operating coil open circuited .

These failures warrant  review of the current maintenance practices and plan for additional value input beyond existing maintenance paradigm. 

Quality of workmanship and checking must be improved along with proper record keeping of testing and checking dovetailing  Predictive maintenance and trend monitoring relay parameters.

The action plan is given below-

1)  Relay interlock burning and contact surface pitting-

  reasons of contact burning or contact surface damage are –

A)     Timing of drop out and pick up –

When the contacts are closing, the metal surfaces will collide and hit against each other several times (bouncing), causing elastic deformation of the contact area and mechanical destruction of the thin layers. 

Again successful "breaking" of a DC load requires that the relay contacts move to open with a reasonably high speed. A typical relay will have an accelerating motion of its armature toward the un energized rest position during drop-out. The velocity of the armature at the instant of contact opening will play a significant role in the relay's ability to avoid "tack welding" by providing adequate force to break any light welds made during the "make" of a high current resistive load (or one with a high in-rush current).The more drop out time means more heat generation resulting extensive damage to contact surface and failure.

The pickup and drop out time may vary with the following factors-

a)    Defective armature spring (EMD part no 8373528) due to loss of property ,corrosion etc .

b)    Restricted movement of armature or contact carrier assly

c)    Loss of property of core material causes delay in drop out time.

d)    Increase or decrease of resistance value of operating coil.

 Suggestion-

·        Armature spring tension should be checked with digital gravimeter during overhauling . Spring length (Free length 1.421” and stretch length under 1-1/2 Lbs is 1.646”)should be measured.

·        Armature spring should be checked for any deformity and crack with illuminated magnifying glass.

·        Armature or contact carrier assly should be checked for wear and restricted movement. Lubrication should be done if needed,

·        Sometimes core material loses its property and start to behave like permanent magnet. It should be checked.

·        The coil resistance value should be remained as per MI of manufacturers and temperature correction chart should be used. If the value achieved is beyond the limit as per MI it should be discarded.

·        Measurement of contact temperature rise during drop out of the relay should be done .

B)     Contact failure due to high contact resistance –

The contacts are the most important element of relay construction. Contact performance is influenced by contact material,  voltage  and  current  values  applied  to  the  contacts,  the  type  of  load,  frequency  of  switching,  ambient  atmosphere, form of contact, contact switching speed and of bounce. Contact resistance is one of major factor for failure of relays.

     Contact resistance mainly depends upon-

·                  Contact pressure.

·                  Cross sectional area of Contact surface ( Contact matching)

·                 Formation of non conductive film of oxides, sulphides and other    compounds due to flash over at the time of contact breaking.

Checking and adjusting relay contact pressure-

    Check the pressure required to open all normally closed contacts with a    

    digital gram gauge (5 to 150 gm range).

v  Connect the normally closed contacts in series with a simple low voltage (6 volt) lamp circuit.  The reading should be taken at the position the lamp is de-energized.

v  Place the probe of the gauge up close to the movable contact. A minimum reading of 40 grams is acceptable before contact opening .With a DC voltage of approximately 2 to 3 volts above the rated pickup voltage, energize the relay coil.

v  Check the pressure required to open all contacts which close when coil is energized. A minimum reading of 40 grams should be obtained on this test.

Ø   If the minimum reading of 40 grams is not obtained, the contact brush assembly will   have to be adjusted. Using a bending tool make gradual adjustments along the length of the contact brush assembly.

Note- Do not make any sharp creases or bends in the assembly. When making a correction for a pressure reading of the normally closed contacts, this will affect the pressure reading of the contacts that are closed when the coil is energized. Therefore, all contacts should be rechecked whenever an adjustment is made.

Ø  Check the air gap between all normally open contacts and between open contacts when the relay is energized. This air gap should be 0.045” minimum.

Ø  Check the travel gap from the centre of the relay core to the carrier assembly. This travel gap should be 0.038” minimum.

Typical relay adjusting tool

c) Contact surface mismatching-

As we know the resistive value of any conductor is inversely proportional to cross sectional area of the conductor ,so the contact surface must be kept as wide as possible to allow rated current of the circuit to flow though the contact surface without generating heat. If contact surface reduces ,the resistive characteristic of the contacts will be come into play and heat will be generated on contact point resulting pitting or burning of contact surface.

·        Contact surface to be checked minutely for any deformation or pitting mark.

·        Contact surface matching to be checked by using Prussian blue washable color.

·        Ensure more than 90% of contact surface matching.

·        Millivolt drop test should be conducted to check contact resistance on board in running loco and in test room after overhauling.

D)    Formation of non conductive film over contact surface-

The  contacts  are  practically  not  clean  because  the  surfaces  are  covered  by  thin  layers  of  low  conductivity, semiconductor properties or even isolating characteristics.  These layers of oxides, sulphides and other compounds will be formed on the surface of metals by absorption of gas molecules from the ambient atmosphere within a very short time. The growth of these layers will be slowed down and eventually stopped as the layer itself prevents further  chemical reaction. The resistance of these layers increases with their thickness.  To get a reliable electrical contact these layers have to be destroyed. 

·        In general relays are designed to wipe out thin surface of non conductive layers at the time of making contact.

·        If the mv drop test value during test on running locos is found more than the normal value ,the relay to be removed and dismantled for physical check up of contact surface.

·        The of movable contact shunt should also be checked for non integrity and bad soldering.

E)   In-rush current-

When an electro-mechanical relay is de-energized rapidly, the collapsing magnetic field produces a substantial voltage transient in its effort to disperse the stored energy and oppose the sudden change of current flow. This relatively large voltage transient has created EMI, breakdown, and contact tips wear problems. The type of the load and its inrush current characteristics together with the switching frequency are important factors that cause contact welding or burning.

The study of transient voltage and in-rush current reveals following fact with respect to various loads -

·        Incandescent Lamp Load ( Flasher lamp) -I/I_R = 10 to 15 times for 1/3 Sec approx

·        Motor Load (Fuel pump motor and Turbo lube pump motor)-I/I_R = 5 to 10 times for 0.2 to 0.5 Sec approx

·        Solenoid Load- I/I_R = 10 to 20 times for 0.1 Sec approx

·        Capacitive Load- I/I_R= 20 to 40 times for 1/30 Sec approx

                     (I= Normal load current and I_R=In-Rush current )

Suggestion-

·          Use of Transient suppression component or snubber circuit in parallel to load –

For suppression of transient effect based upon the impact on armature motion and optimizing for normally open contacts, the best suppression method is to use a silicon transient suppressor diode. This suppressor will have the least effect on relay dropout dynamics since the relay transient will be allowed to go to a predetermined voltage level and then permit current to flow with a low impedance. This results in the stored energy being quickly dissipated by the suppressor avoiding damage to the relay contact. This technique is already used in ALCO locos where two freewheeling diodes are used in reverse direction as arc suppression rectifier across radiator fan contactors R1 and R2 coil to protect temperature switch contact tips.

2.Operating Coil open circuit failures-

Cause of Operating coil failure are mainly due to open or short circuit of coil. Failures of relay occurred due to failure of soldering joint and melting of coil metal due to excessive heating or insulation damage.

Coil heating-

A negative effect of power consumption is the heating of the coil and, in turn, the entire relay. The coil temperature is a result of:

·        Ambient temperature

·        Self heating (Due to coil Power consumption= V*I)

·        Induced heating (Due to heat generated by contact system)

·        Magnetization losses (Due to eddy currents)

·        Other sources (Due to components in the vicinity of the relay)

Due to coil heating coil resistance increases. Resistance at elevated temperature is expressed by R_t = R₀ [(1+ α(T-23)]

Where  R₀ is the resistance at ambient temperature (23º C), T is the elevated temperature and α is the temperature coefficient on winding wire (Copper).

 The pick-up coil resistance of copper wire increases or decreases by 0.4% per degree C. Due to the increase in coil temperature the coil resistance increases as per the ratio mentioned above.  Hence the pick-up voltage for a hot coil should be higher to

generate required pick-up current.

For example if a copper coil having coil resistance of 400Ω at 20ºC , the resistance value increases to 432 Ω at 40ºC and pick up voltage must be increased to retain the same pick up current.

·        So, measurement of coil resistance at ambient temperature Vs resistance correcting chart to be used for fruitful results.

·        Dry or bad soldering to be checked for if coil resistance value differs from the value mentioned in MI.

·        Temperature rise during continuous operation and drop out to me measured and monitored for and deteriorating trend.

·        Color of the coil to be checked for any sign of overheating.

·        Insulation resistance of the coil to be checked with 500V meggar for deteriorating value. 

·        To obtain initial performance throughout life,  avoid dropping or hitting the relay