CTs may be considered as opposites to voltage or potential transformers in some electrical characteristics. Where loads or “burdens” for potential transformers are almost always placed in parallel [shunted] with PT secondaries—loads or “burdens” for CTs are almost always connected in series [looped.]

Just as it’s problematic to short out an energized PT secondary, forcing damaging overcurrent through its windings, one should not open-circuit an energized current-transformer secondary—for this effectively forces ‘excess voltage’ on CT windings. Also, though not an immediate hazard, it is poor practice to leave in-place CTs with open-circuited secondaries, if they later can be energized in this condition.

Another thing to avoid is fuses in CT circuits, for that may easily create an undesirable open-circuit condition in the CT loop. It is good practice to fuse PT circuits on their primaries and in a number of cases, on secondary sides. {added paragraph}

In general, one consideration is that as the mass of copper and steel in a CT increase, the hazardous open-circuit potential typically increases. For CTs and PTs, increased size and electrical ratings usually dictate increased hazard an likely decreased tolerance to forgetful practices.

Besides the practice of not open-circuiting CT secondaries with any primary current present, it’s wise to also not leave grounding jumpers off of CT secondaries. [Note that the ground-jumpering need holds true for PTs, too.] Floating secondaries can develop high electrostatically induced voltages to ground, presenting hazards to personnel and equipment.

Noise to watch out for working around CTs is “singing” or a higher-pitched squeal, that can be caused by open-circuited CT cores operating in saturation. Also, a sputtering or crackling noise can originate from intermittent CT-secondary connections. Because winding voltages are typically very low in normally operating CT loops, voltage measurements on secondary wiring will not be effective for determining if primary circuits are safely deenergized. If a screw on a terminal block for secondary wiring is loosened, and any sputtering or crackling sound is heard, suspect current flow and the hazards that may grow if ‘loosening’ is further attempted.

Open-circuit conditions in CT-secondary wiring can be limited through the use of shorting-style terminal blocks. Typical components are the “SC”-series blocks at www.marathonsp.com/PDF%20Files/HDTB%20-%201500%20Series.pdf
[Linked Image from 6l6.net]

A stodgy old {but detailed and free!} publication on instrument-transformer characteristics is www.geindustrial.com/products/applications/GET-97D.pdf [~6MB]




[This message has been edited by Bjarney (edited 06-07-2004).]