I seem to have problems figuring out when shorting pins need to be in place on shorting blocks fed by CTs. One site needs pins on the unused leads, another site doesn't. reasons given for needing them is that when the current flows, it will seek a ground. If the shorting pin is not in place, it will still try to seek that ground connection, jumping over if necessary.

These are for high voltage cts, ones reading 69Kv.

Anyone have info to help me?

TIA

Rick Miell

It is my opinion that one should strongly distrust any information source which suggests that current will try to seek ground. Electric current does _not_ 'seek ground'. Instead the current follows any closed circuit back to its _source_.

In the case of current transformers, the source is the transformer winding itself. Open CT windings can develop quite high potential, enough to arc in order to close the circuit. With some of the secondary terminals properly loaded with a low impedance, it will depend upon the specific layout of the transformer to determine if any of the other terminals need to be shorted.

-Jon

CTs do not like to be open circuited. It is bad (high voltage develops across the open circut) when the circuit is not carrying current, but it is extremely dangerous (arcing fault) if it is done under load.

CTs prefer a complete circuit (they have no problem with being shorted out) instead. So use the shorting pins on all leads that are not connected to a load (meter). Also use the pins before removing any existing load lead. When in doubt use the shorting pins, the worst that would happen is the load device would see no current.

Be careful here. If the load device is a protective relay, the relay will never cause the OCPD to open to clear a fault.
Don

Here is a typical illustration of a shorting-block pole. [http://www.marathonsp.com/MSP%20CATALOG/2005%20Catalog%20pdfs/1500%20Series.pdf]

For multi-ratio (tapped) CTs, only two pins need shorting to prevent high open-circuit voltages across an open secondary. Be careful, though, not to lift the ground connections {typically X1} to prevent high electrostatically induced voltage from secondary to ground with energized equipment..

Also, besides no overcurrent-relay operation from left-in-place shorting pins, any sort of comparison-, unbalance- or differential-relay scheme can falsetrip during 'normal' operation.


[This message has been edited by Bjarney (edited 11-23-2005).]