Main CT - Minimum burden?

[derek22]

In designing an interposing CT circuit, should a known resistive element be designed into the secondary of the main CT to reduce error? It seems that shorting the main CT secondary through the interposing CT with only the resistance of the wire might result it maximum error from the main CT.

The interposing CT is being used as a sacrificial part and the main CT is extremely difficult to access for replacement if necessary. That being the case, one of the main design features is that the interposing CT can be easily replaced without disturbing the main CT secondary, so the interposing CT would not factor into the burden calc.

 

[LiteYear]

I know rules of thumb are generally used in these cases but I'd be tempted to just simulate it. The main CT will actually see the burden of the interposing CT, reflected through the turns ratio of the interposing CT, so the main CT wont actually be shorted unless the interposing CT gets shorted somehow. Whether this is sufficient depends on the turns ratios, the current ratings and the burden.

Fundamentally, I don't think you need the interposing burden for accuracy sake. I suppose if you really want to protect the main CT though, it might be cheap insurance.

 

[davidbeach]

The main CT will be shorted if and only if the main CT is shorted. The aux CT will be shorted if and only if the aux CT is shorted. Leave the aux CT open circuited and the main CT won't give a whit if it isn't open circuited.

 

[randyman]

derek22 -

A CT works best with minimal burden, therefore a short circuit provides its best accuracy performance. I am confused as to why the aux CT is there, is it not connected to anything other than the main CT secondary?? And you mentioned easy removal without disturbing the main CT - you still have to short the main CT secondary if you are going to remove its connection from the aux CT.

If the aux CT is stepping down the current, reflected burden into the main CT secondary will be low. Conversely, if the ratio is being stepped up then the reflected burden to the main CT secondary will increase by the square of the turns.

Whatever the case, the main CT secondary should never be open circuited, even when replacing the aux CT. And I would short the secondary of the aux CT if it is not connected to anything else.

 

[derek22]

The aux CT will be connected to the relay, and Liteyear is correct, the design is essentially for "cheap insurance" to the customer in the case that the CT secondary is opened accidentally (response to multiple previous expereinces). I seemed to incorrectly assume that the CT would have the greatest error under low burden conditions.

 

[randyman]

derek22 -

"...in case that the CT secondary is opened accidentally". Are you refering to the aux CT's secondary circuit here??