2. What are "Front" and "Back" contacts of a relay?

3. What is a registration device as applicable to relay?

4. What is meant by registration code of a relay?

5. What are the different types of contacts in signaling relays?

6. What is usually meant by the term "life" of a relay?

7. What are the common voltages for the DC miniature line relays?

8. What is the typical contact resistance for the DC miniature relay?

9. What are the tests performed for quality assurance and approval?

2. ' Front ' contact is a contact which is made when the relay is energised , while ' Back ' contact is a contact which is made when the relay is de-energised.

3. Registration device is that part which is provided in a relay to prevent an incorrectly coded relay from being plugged into a particular plugboard. The part is usually protected from being manipulated from outside unless a seal is broken and sturdy enough to withstand the wear and tear due to plugging and un-plugging.

4. Registration Code is a code which is based on the operating voltage , contact arrangement of the relay. The Registration code thus identifies a relay according to its most significant specification namely its operating voltage and contact arrangement thereby preventing incidence of wrong voltage / contact arrangement being applied to a relay.

5. Contacts can be classified into two categories i) Metal to Metal & ii) Metal to Carbon (non-fusible).The former has the advantage of low contact resistance but can fuse with high current flowing through the contact whereas the latter usually has relatively higher contact resistance but do not fuse due to choice of contact material for one of the contacts

6. 'Life' of a relay is usually taken as one million cycles of operation with one cycle comprising of energising resulting in picking up of the relay followed by de-energising causing release of the same. The relay shall be considered as having a shelf-life of atleast 3 years prior to being put into operation and during use can be considered to have about 250 cycles of operation per day.

7. The common voltages of use for DC miniature line relays are 50 volt , 24 volt and 12 volt.

8. The contact resistance of a metal to metal contact shall be better than 0.05 ohms ( measured with 100ma dc flowing through the contact ) and for metal to carbon contact shall be better than 0.2 ohms measured with similar current passing through the contacts.

9. Relays are subjected to the following tests ;

Type tests: This includes a set of tests and measurements typical for the type of relay for which type approval is sought. The type tests are done after development ( first time approval ) or after the expiry of 'First Time Approval' as 'Maintenance Type Approval'. The duration of the type test approval usually varies between 36 months to three years.

Type tests include the following ;

i) Ability of the relay to withstand 10 million operation ( pickup followed by release ).

ii) Ability to withstand shocks ( checked by dropping from a height of one inch over a concrete surface )

iii) Ability of the relay to withstand sympathetic vibration (resonance) . Both front contacts are checked under operated condition with simulated vibration ( 40-100 cps, .005 inches in amplitude )

iv) Ability of the relay to retain its physical characteristics after climatic variations ( ambient temperature varied between -40 deg centigrade and 55 deg centigrade )

v) Ability of the relay to remain within the temperature limit of the insulation class after continuous operation at 120% of nominal voltage at an ambient temperature of 60 deg centigrade.

Routine Inspection tests : This included a set of tests and measurement , individual relays are subjected to for acceptance.

Routine inspection tests include the following ;

i) visual inspection of the entire relay

ii) Plating thickness test confirming minimum plating thickness of vital part such as armature.

iii) cover annealing test

iv) Contact pressure test for both front and back contacts

v) Contact resistance test with current hrogh th econtact as 100 mA DC.

vi) Confirming specified maximum voltage across winding for full operate and minimum voltage for full release.

vii) Insulation test by applying 1000 VAC (rms) between two metallic parts insulated from each other

viii) Non-bridging test on the contact springs to ensure adequate physical gap between them. All front and back contacts are included and the test voltage used is 500 VDC.

For slow release relays the following tests are additionally performed ;

viii) Ability of the relay to remain energised at 80% of rated voltage with interruptions of 250ms at 1 second interval for 10 times consequitively.

ix) Ability to remain de-energised after sudden application of 120 volts at 50 Hz.