CT Rated burden

[kaspor]

Hello

With CT rated burdens, do you need to factor in the resistance of the CT itself into the load burden calculation?

If not, can am I correct with the below calculation?

If we assume the CT is 1000/1A 5P10, assume secondary cable impedance is 0.1ohms, relay burden = 0.1VA

My understanding is in order to gauruntee performance of 5 percent error at 10x rated current (i.e. 10kA), then CT rated burden must be greater than load burden at nominal current (1A).

Total CT burden = Secondary cable burden + Relay burden

Total cable burden = I^2 x Cable Impedance = 1^2 x 0.1^2 = 0.01VA
Relay burden = 0.1VA

Total CT burden = 0.1 + 0.01 = 0.11VA

Hence CT must have a rated burden of >0.11VA? We do not include the burden of the CT itself (i.e. due to CT resistance)?

 

[RRaghunath]

When we say CT burden, it is lead burden + relay burden. You are right.
We use CT burden (or CT secondary resistance) in the formula for calculating effective ALF while confirming that the CT saturation current is well above the estimated short circuit current at that location.

 

[kaspor]

Thanks for the response. Do we not include the CT secondary resistance in the burden calculation to ensure burden is below rated CT burden?

 

[RRaghunath]

No, not when your intention is to specify rated burden of the CT.

 

[stevenal]

Is this an IEC difference? ANSI/IEEE procedure is to include CT secondary resistance.

 

[kaspor]

Yeah that's what has thrown me!

 

[sasalu]

Hi Raghu,

what should be the fault current considered for the CT sizing ie system design fault current or max fault current seen by the CT.

 

[RRaghunath]

sasalu,
It is always recommended to size the CTs to the design fault level in order to future-proof them.