CT theory

if one side is grounded and commoned, as the drawings of them show, and the other side is open, does the voltage rise still occur?

Basically yes. CT transformers - like any other transformer - divide or multiply the supply voltage and current into the secondary according to the ratio of the turns. In normal operation you're not bothered about the output voltage so arrange a suitable division of the current to suit the instrumentation - which usually means a 1:rather-a-lot ratio (the straight conductor the CT is wrapped around being a single turn primary) - which divides the current, but at the same time multiples the voltage. However the relatively low impedance of the current measuring instruments on the secondary together with the low currents involved means the actual voltage on the secondary collapses to something quite modest in normal operation. Leave the secondary open circuit and the theoretical full voltage can develop between the ends of the secondary winding.

   - Andy.

that sounds about what i was thinking took place, thanks for clarifying it for me


is it true about them being open circuit damages them (one site said the voltage gets so high it creates plasma discharges!

An open circuited CT will produce a very high voltage, often enough to destroy the transformer, and potentially to kill anyone coming into contact with this high voltage.

Earthing one side of the CT does NOT reduce these risks. The only way to avoid the danger is never to open circuit a CT whilst current flows in the primary.


Earthing CT secondary circuits is good practice for another reason, to avoid accidents if any inadvertent connection should occur between the CT circuit and the voltage circuit to say a KWH meter or other instrument.

By convention or custom, such earth connections are often white, rather than green/yellow.

It rather depends on the core size and the winding materials. The other problem is that in the CT the current is more or less fixed by the load side, and as many volts per turn are available from the supply to 'make it so'. In a voltage transformer the voltage is fixed, and the current varies with load.

As current flows in the 1 turn primary, unless and equal and opposite number of amp-turns flow in the secondary, the magnetic field builds up much like a choke ~(which is like a transformer with an unloaded secondary) At some point the core will saturate, when all the magnetic domains (imagine little molecular magnets) have rotated to align. At this point the effective inductance (and so the voltage drop per amp) drops. This results in a clipping of the induced voltage. However, to give good linearity and to fit round the bus bar,  CT cores are oversized, and the  turns ratio can be very high, especially the sort clamped to 100 amp bus bars. The pracitical up shot is that a turns ratio of 1000 to 1 or even 10,000:1 is realistic, so when the 1 turn measuring current is only dropping maybe a tenth of a volt to 1V , the secondary side may be enough to do serious injury.

In normal operation the primary is dropping a few millivolts.

As the metering wiring is intended  to handle a few tens of volts at most, it may well fail. It would be possible to wind a CT with suitable HV winding like a car ignition coil so that it did not flash over, but it is far easier to leave something connected to limit the voltage.


Personally I use CTs in a pulse power world, and there I like to see anti-series zener diodes or gas discharge devices, directly on the winding, or cascades of two transformer cores e.g.  two of 33:1 rather than 1 of 1000:1 with limiters on the transfer winding.

Mike.

As Mike has said, it depends on the core size and material whether you are talking hundreds ov volts, or thousands of volts. With a decent amount of magnetic material in the core, the secondary voltage will be limited by the core saturating, or something flashing over. The latter may be within the CT secondary winding itself, or within the equipment / cable connected to it. 


We had an ammeter go open circuit on an 11kV circuit during a fault last year. The arc filled the case of the instrument, totally destroying the coil and covering the inside of the instrument in carbon. I will post pictures as an example when I find them. 


To sum up, an open circuit CT secondary is a very bad and dangerous thing to have on a power system.

thanks, the explanations are great, kinda confirmed what I had read up about, was just finding it a bit hard to confirm.


the CTs we have are split-core 1a or 5a on 400v up to 600a, just for some context.