TRANSFORMERS 393 



obvious that under normal conditions the maximum voltage stress 

 between the high-voltage leads is 1000 volts, and to ground 500 

 volts. If a ground should occur at one of the high-voltage con- 

 nections to the mains, the stress will be 1000 volts. 



In the case of the low-voltage winding, if the two coils are con- 

 nected in series and non-grounded, the stress to ground under 

 normal conditions is 100 volts, which is also the maximum stress if 

 the junction point or neutral is grounded. If not, and with one 

 lead grounded, the stress becomes 200 volts. The stress between 

 the two windings is equal to the high-voltage plus or minus the 

 low-voltage, depending on the arrangement and connections of the 

 coils. 



In order to avoid the danger of excessive voltages being im- 

 pressed on the low-voltage circuits, caused by crosses between 

 the high-voltage and low-voltage lines or windings, grounding of 

 the low-voltage circuit is now generally advocated for all voltages 

 up to 250 volts. No point of the circuit can then, except under 

 unusual conditions, rise above its normal potential, and such 

 grounding, therefore, prevents accidents to persons and damage 

 by fire to property. If the low-voltage side, on the other hand, 

 is not grounded, and the transformer breaks down, the high- 

 voltage may be impressed on the low-voltage circuit, and a per- 

 son touching any bare part of the low-voltage circuit is liable 

 to receive the full shock of the high voltage, if he were grounded 

 by contact with, for example, a gas fixture, etc. Furthermore, 

 if the low-voltage side is not grounded and there is a ground on 

 the high-voltage circuit, the high- voltage impressed on the fittings 

 of the low-voltage circuit might cause a fire. 



For a two-wire 110-volt circuit it is common practice to con- 

 nect the ground to one side, while with a three-wire Edison cir- 

 cuit the neutral wire is grounded, limiting the potential from either 

 outside wire to ground to 110 volts. On a 220- volt single-phase 

 power circuit the middle or neutral point of the transformer wind- 

 ing should be grounded. 



To prevent any increase of the potential stress between ground 

 and either low-voltage wire, the ground should be well made so 

 that it cannot readily be broken. It should not be fused and 

 should consist of a conductor which, without overheating, can 

 carry a current sufficient to blow the main fuses. 



Two-phase. This system practically consists of two separate 



