Summation current transformers are used to add up secondary currents of multiple main current transformers in order to measure with one instrument only.
The output signal is again a standardised signal which is received by adding the input currents and divide the sum by the quantity of inputs. But is has to be differentiated between summation current transformers for equal or unequal main current transformers.
The ratios of the main unequal CTs have always been advised together with the order as the primary input current are always been accordingly weighted. The ratio of the primary currents should not be more than 10 : 1 (biggest to smallest primary current).
Unused primary terminals have to remain open and shall never be short-circuited in contrary to the secondary terminals.
The currents are always vectorially added under consideration of the amount and phase disposal of the current. The sum of the currents can add up to Zero e.g. in case of a differential current measurement. The standard application is the addition of currents of the same phase. With a reverse connection of the main current transformers it is as well possible to subtract currents.
For equal main current transformers it is independent where the input currents are connected. But in case of unequal main CTs each has its definded input slot.