THE POST OFFICE 3000 TYPE RELAY 



a small screw and spring. A residual stud or screw prevents the armature 

 sticking to the coil face. The contact springs are mounted above the 

 armature, and are operated by it through projecting pins : lugs on some of 

 the springs engage on a moulded buffer block. The relay is best mounted 

 on its side so that accumulated dust may fall clear of the contacts. 



The coil unit 



The coil assembly is held in place by a single nut and may be removed 

 even when the relay is mounted. The windings terminate in tags on the 

 rear bakelite cheek; up to five tags may be provided. When the coil is 

 viewed from the rear with the tags at the bottom, reading from left to right 

 the tag positions are designated a, b, c, d, e. Details of the windings printed 

 on the coil are correspondingly lettered. Tags connected to the inner end 

 of a winding are usually coloured red ; it is customary to connect them to 

 the earthy end of the supply. 



Single and twin coil relays are common; triple and quadruple coils are 

 available for special purposes. Solid copper 'slugs' of three sizes — I, 1 and 

 1| in. — may be provided at either the armature end or the heel end of the 

 coil; the use of slugged relays is discussed in a later section. 



The maximum power dissipation of a relay coil is 6 W. The insulation 

 of a coil is rated to withstand 50 V to earth, but this rating is so conservative 

 that 250 V may be applied with little likelihood of failure. Furthermore, 

 the relay is usually insulated from its mounting plate. It is, of course, good 

 practice to arrange the coil in the earthy end of its operating circuit. 



Contact springs and buffer block 



The contact springs of a 3000 type relay are nickel silver and usually 

 0-014 in. thick. For special applications 0-012 in. springs can be obtained. 

 Four types of contact are available, as shown in Figure 34.5. Contacts are 

 assembled into groups, each of which can contain a maximum of nine 

 springs. Thus each group can provide up to four makes or breaks or three 

 change-overs. Two such groups are mounted on each relay; these groups 

 are 'handed' so that a left-hand assembly cannot be used on the right-hand 

 side of a relay. 



Various contact materials are available. For currents up to 300 niA silver 

 is used, and twin contacts (Figure 34.6) to reduce open-circuit failures are 

 standard. Platinum contacts of similar construction can be used up to 1 

 amp; such contacts are identified by a notch in the tip of the spring. The 

 rated voltage for contacts of these types is 50 V, but no trouble is likely to be 

 encountered in operating them up to 250 V. For heavy currents silver-nickel 

 (or, rarely, tungsten) cylindrical contacts are used. These are rated at 250 V, 

 8 amps. 



All the foregoing contact assemblies are insulated with bakelite spacers. 

 For very high voltages pillar-type insulators are available, with contacts 

 rated at 1 kV 2-5 amps or 2 kV 1-25 amps. 



When contacts with low contact pressure are used on very low voltages, 

 for example at the input of an amplifier, it is found that the oxide film on 

 the contacts does not always break down: the resistance between the con- 

 tacts will then be high. To avoid this effect it is sometimes possible to arrange 



513 



