THE POST OFFICE 3000 TYPE REILAY 



±30 per cent, will be sufficiently accurate for all practical purposes. If a 

 relay with special adjustments is to be used, or if marginal operation is 

 required, it is essential to test the sensitivity of the relay experimentally. 



For normally adjusted, fully wound 3000 type relays. Figure 34.8 shows 

 the power required for the four cases described above. For thoroughly 

 reliable operation the power read from the graph should be multiplied by 

 the following factors of safety: 



Operate 5 Hold 3 



Non-operate 0-2 Release 0-15 



For coils which are not fully wound the power must be multiplied by 1//, 

 where /is the fraction of the coil occupied by the winding. Strictly, this rule 

 applies only when the winding occupies a length-wise fraction /of the avail- 

 able space; for superimposed windings the inner one has the better power 

 sensitivity, since its resistance per turn is lower than the winding of larger 

 average diameter. Accordingly, for non-fully wound coils the power read 

 from Figure 34.8 should be multiplied by the following factors : 



Twin coil with superimposed equal windings : 



Inner coil 1-1 

 Outer coil 3-5 



Twin coil with balanced windings: 



Either coil 20 



Single coil with \\ in. slug 2-7 



1 in. slug 1-7 



I in. slug 1-3 



Operate and release lags 



The time taken by a relay to operate or release depends on a large number 

 of factors which are best summarized in tabular form {Table 1). 



TABLE 1 



For a 3000 type relay with a full contact load, energized from a low- 

 impedance supply with a power sufficient to ensure an operating factor of 

 safety of 5, the operate and release lags will be between 10 and 30 ms. 

 Operate lags of less than 5 ms can be achieved if the relay is fed with a large 

 current from a high-impedance source, while release lags of the same order 

 of magnitude can be obtained with low operating power and large residual 



gap- 



517 



