Protection Secondary against Short-Circuits for a PT

[Crisitan De la Ossa]

Hi everybody, there's someone who who has dimensioned the MCB on the secondary the PT on based to the Short-Circuit current, I had read different books but I didn't find a standard to size it. Anyone knows a standard or a methodology to do this. I attach the information about the PT.

Best Regard
Thanks in advance

 

[RRaghunath]

There is no overload protection required for VT.
VT secondary load is pre-determined and there is no possibility of overloading.
Having said that any MCB rating that is above the rating of the VT secondary should do.
In your case, each secondary is rated 10VA, 66.4V. Thus, the rated current comes to 0.15A. So, an MCB of 0.5A rating should be adequate.
MCB type-C is preferred and allows 5-10 times inrush in the secondary circuit without causing undue trip of MCB.

Rompicherla Raghunath

 

[Tahir28]

The nominal rating of the MCB is to be decided based on the rated thermal burden (Rated output) of the VT.
In your case, it is 1000 VA.
The Secondary Fuse or MCB Rating should be 1000VA / 115/sqrt 3
1000 / 66.4 = 15A

Additionally, you may also need to consider how the thermal burden (Rated output) is distributed between the windings.

Using the rated accuracy burden might still work but the breaker might be undersized in this case and may not withstand the secondary burden inrush currents.

I am not sure about the short time ratings of the VT and also regarding the short circuit protection selection and curious to know.

What does the VT factory say about it?

Thanks