550 ELECTRICAL EQUIPMENT 



for this capacity. When, with a certain capacity of current 

 transformer determined by the load of the circuit, the scale of the 

 instrument would be too large to allow a good reading at light loads, 

 4-ampere windings may be used, the scale then being about 80 

 per cent of that corresponding to that used with the 5-ampere 

 winding. Secondary potential coils for all instruments except 

 voltmeters are ordinarily wound for 110 volts, the voltage of the 

 secondary side of standard potential transformers. 



Instruments may be operated from the same current trans- 

 formers which are used with the oil circuit-breaker trip coils or 

 relays, providing the volt-ampere load is such that the accuracy 

 of the instrument and transformer combination comes within 

 certain set limits. Wattmeters, however, should not be connected 

 to the same current transformers which are used with differential 

 or reverse power relays or with compensated voltmeters (indi- 

 cating or contact-making) or line-drop compensators. 



The same potential transformers can be used for operating 

 instruments and potential coils of relays, low-voltage release or 

 other apparatus as long as the rated secondary volt-ampere load of 

 the transformer is not exceeded. This load and its power-factor 

 must be clearly distinguished from the load and power-factor of 

 the main circuit which are measured by the measuring outfit 

 of which the instrument transformer is a part. 



The term " equivalent secondary connected load " is used in 

 connection with a circuit to denote the volt-ampere load carried 

 by the secondary of an instrument transformer when this load 

 differs from the result of combining the volt-amperes of the separate 

 devices in series or in multiple because the secondary is inter- 

 connected with other instrument transformer secondaries. The 

 power-factor of the equivalent secondary load of a current trans- 

 former under these conditions is also affected by the intercon- 

 nection. 



The volt-ampere of the various secondary devices, such as 

 indicating instruments, meters, relays, etc., varies considerably 

 and should be obtained from the manufacturer. 



The secondaries and cases or frames of current transformers 

 should be grounded whenever possible. The switchboard wiring 

 should be carefully considered to see if this can be done without 

 interfering with the proper operation of the instruments connected 

 to the transformers. The grounding of the cases serves the double 



