Current Transformer

Please see this discussion, for example.

My sincere apologies. i rarely post before reading all other posts. This is what happens when i don't. GA to you.

nothing will happen to it.

Bravo88 "nothing will happen to it."

Other than the minor problems of it burning out or catching fire. But otherwise no problem!

A CT left open circuit, will only have higher terminal voltage, perhaps the waveform will be distorted. It definitely will NOT burn out or catch fire, as there is no current flow ,no power dissipated.

Even though the voltage isn't "going anywhere," it can result in insulation breakdown, creating a short circuit. If/when that happens, the voltage does go somewhere. Please retract your statement.

Here is an excellent explanation : http://www.control.com/thread/1026220105

Responding to Curt's Mar12, 12:00pm and Meir's Mar 12, 12:02pm comments:

A) Turns-ratio is not responsible for overvoltage. Even CTs with lower ratios, like 5:1 or 1:1, have the cautionary notice (at least in the USA!) Furthermore primary voltage is a fraction of Volts. So, using turns-ratio as the primary to secondary multiplier, secondary volts would be in the order of volts to tens of volts.

B) Overvoltage occurs when the secondary open-circuits and the demagnetizing effect of the secondary emf is lost. Flux density quickly increases, limited only by core saturation.C) As core saturation progresses, waveshape changes. The usual sinewave changes to a peaked wave. Such a waveshape has an extremely high dV/dT characteristic. It is very likely that insulation flashover and/or fire results from dV/dT... not from a high amplitude sine wave!

Regards,

Phil Corso, PE {Boca Raton, FL, USA}

Thanks to this potentially lethal post by you bravo, i found this gem. i also used to think that the high voltage is naturally due to turns ratio.

i hope you will also be benefited by this one.

Thanks, Mr Corso.

oop! I guess I was too fast to jump in. I admit my experience with CT was with small one, 10A type and with much higher turn ratio (500 to 1000) than that mentioned by the OP. The open cct terminal voltage of these small CT , although , much higher but it is not in kV range , perhaps in the 10 ~20 volts. I agree, for 200A/5A CT, the Open cct terminal would be much higher, as to whether it will reach kV range, only actual measurement can tell. If I can find a similar CT, I will check it out .

Due to core saturation , it will not be a full wave waveform , but spiky voltage at beginning of every half cycle of the AC current cycle.

has any one ever actually seen a CT that has been left with an open circuited secondary? after it has got burned?

are any pics available as web resources?

Yes.. Me!! Not a pretty sight.... and the smell.....

Sorry looked for photos.. can't find them, will keep looking

I have seen this happen in 3 ph LV circuits generally due to a defective ammeter-selector switch. The switch fails to short circuit the two unselected CT secondaries and the CT gets damaged. This may happen over a period. I have noticed charred insulation but fortunately there was no fire.

Fire, explosion, electric shock...

Is current flowing in the primary when the secondary is kept open circuited?