Burden on Each Secondary Winding

It is recommended that the burden on each secondary winding be determined as accurately as practicable. It is appreciated that one set of P.Ts may be supplying to the number of instruments on a panel board. It is, however, concluded from experience that the burden on P.Ts. is specified in an adhoc manner. The following table indicates the burden of various instruments/meters imposed on the P.T.

Description of Load Burden in VA
Voltmeter 10.VA
Pressure coil of Watt/VAR meter 1.5 VA
Pressure coil of Energy meter 2.5 VA
Pressure coil of P.F. meter 2.5 VA
Pressure coil of Trivector meter 5.0 VA

It is possible that in certain cases long length of leads may be used. It is however, incorrect to calculate the lead burden as in the case of Current Transformer. in the case of P.Ts. it is the voltage drop at the leads which is required to be considered. This will be apparent from the enclosed figure 1. Even if we were to consider the P.T. with

110   V  secondary with accuracy class


0.5, it will be seen that with a two way lead resistance of 0.5 ohms, for a rated burden of 100 VA connected at the end of the leads, the drop at the lead shall be
100 x 0.5 = 0.79 which when<

    110 Ö3  

compared to the rated secondary voltage is 1.24%. Thus, even if the P.T. delivers the rated burden with 0.5% accuracy, the voltage available at the end of the leads is substantially lower. It is therefore recommended that the lead resistance be kept as low as is practicable by reducing the length of leads and by using leads of larger cross section. This problem may not become apparent in a large number of cases due to the following reasons.

  • The lead length is less, therefore the lead resistance is negligible and/or
  • The actual burden connected across the P.T. is substantially lower than the specified burden.
In the latter case it may be more economical to specify lower rated burden for the P.T.

The drop in the leads has, however, been a matter of concern when large burden was required to be delivered over long length of leads. The problem was reportedly overcome by placing the metering equipment close to the P.T., thereby shortening the length of leads.

It is further recommended that the continuous and non-continuous nature of the total burden be examined. This burden may be supplied from the same P.T. either from two different secondary windings or separate runs of the leads. It may be noted that it is obligatory for the manufacturer to deliver the rated P.T. burden with specified accuracy at the P.T. terminals and not at the end of certain length of leads.

Referring to the open-delta winding provided on some P.Ts., it may be seen that this winding feeds the burden only in the event of a fault on the system. Under normal operating conditions the burden on this winding is near zero and as such this burden does not need to be taken into account when the accuracy of the main metering is determined.