Metering Cores of a large number of Indoor C.Ts up to 33 kV voltage class are usually connected to an Ammeter and a WATT / VAR meter. On some rare occasions an Energy meter and current coil of a P.F. indicator may also be connected across the metering core. For C.Ts of higher voltage class, many more meters may be connected across the secondary of the C.T.

With the improvement in the technology subsequent to the publication of Is-4201/1983 - "Application Guide for Current Transformers" - the burden of individual instruments has reduced considerably. It is suggested that the burden indicated in Table 1 may be considered for determining the total burden on the metering core.

Description of the Load Burden in VA
Ammeter 1.0
Current coil of Watt/VAR meter 1.5
Current coil of Energy meter 2.0
Current coil of P.F. Indicator 2.5
Current coil of Trivector meter 3.0
Leads between C.T. & meters 2.0
  • Core 1 - Metering
  • Core 2 - Over current and Earth fault protection.
  • Core 3 - Transformer differential protection.
  • Core 4 - Bus - differential protection.
  • Core 5 - Distance protection.Al
It will be seen from the above that for majority of applications of Indoor C.Ts, the burden on the metering core is about 5 VA an din any of the cases, it is unlikely to exceed 10 VA. It is therefore suggested that the rating burden for the metering core of C.Ts be a standard value and as close to the connected burden as possible. The Standard specification IS-2705/1981 lists standard values of rated output as 2.5, 5.0, 7.5, 10, 15 and 30 VA. It is accepted that for E.H.V. C.Ts, the rated burden for the metering core may be higher - say of the order of 15 VA particularly because the length of leads will correspond toi about 0.8 ohms. This would mean a burden of 20 VA for rated secondary current of 1 Amp).

Instrument Security Factor (I.S.F.) for the metering core is often specified to be less than 5. Whilst the IS Specification as also other international specifications indicate that I.S.F. shall be as low as practicable, a point is many time stretched by specifying I.S.F. to be less than 5 for two or three ratio C.Ts., where the change of ratio is obtained by taps on the secondary winding. It is apparent that in such cases I.S.F. cannot be maintained to be less than 5 for all ratios as, if it is less than for the lowest ratio, it shall be around 10 for the next higher ratio and may be around 20 for the highest ratio (Ratios assumed to be in the proportion 1:2:4).

Besides the above, it is understood that the purpose of specifying I.S.F. to be less than 5 is to prevent excessive currents from flowing through the connected meters. From the specification of indicating instruments, it is seen that the current circuits of all indicating instruments can carry 10 times the rated current for 5 seconds. If it is assumed that the fault on the system may be cleared within one second which means that specified burden and class of accuracy, to economies on the cost of inputs.
It may be mentioned that the specification of a single rated primary current helps in keeping the value of I.S.F. low.

It has been confirmed by instruments manufacturers that all current meters and current coils of power meters have been tested to withstand 10 times the rated current for 5 seconds.
Protection Core
It is suggested that the ohmic value of the burden of various relays proposed to be connected across the protection core be determined and may be added together to obtain the total ohmic impedance connected across the C.T. core. For satisfactory operation of the protection scheme, it shall be necessary to ensure that the C.T. is capable of developing voltage sufficient to pass required current through the impedance. (The required current is invariably a multiple of the relay setting and not a multiple of the rated current). The requirement of this voltage shall thus be the product of the current and the impedance (the lead impedance, if significant, may be added to the value relay impedance referred above). If the value of voltage is say V, this may be broken up in to Burden and Accuracy Limit Factor by using the formula.
V = Burden x A.L.F. / Rated Current .
As the rated current is known , any standard value of Burden and A.L.F. may be selected to satisfy the above equation . The I.S.S. lists following standard values of Burden and A.L.F.
Burden in VA : 2.5 , 5.0 , 7.5 , 10 , 15 and 30 .
A . L . F : 5 , 10 , 15 , 20 and 30 .