OT: Tell me whats wrong with this situation...

[JesperMP]

OK. It is partially a trick question.
The primary side of a CT is the small piece of cable or busbar that goes thru the center of the CT. The voltage over the primary side on the CT would have to be the voltage difference over the 2-3 centimetres of cable or busbar. Without getting out my calculator, I can see that it is not the same as the general supply voltage.

In that link I posted, read Phil Corso's comments. He makes a pretty educated explanation as to what the secondary voltage will be.

 

[drspanda]

When we open ckt CT the below happens:

Primay current does not care that secondary is opened, so it continues to flow and due to this exitation current will be equal to secondary current (As there is no burden or load) and this causes more iron losses and core will be heated up.

Due to the temperature and hence insulation fails and causes CT to blast.

 

[JesperMP]

drspanda.

I dont know for sure if that is exactly what happens, but your argumentation looks correct to me.

If the posters that argue that open circuit on the secondary will result in an extreme voltage were right, then the CT would selfdestruct immediately.

If drspanda is correct, then the CT will slowly degrade until it bursts into flames.

 

[osiberis]

So which situation would it be or is it just kind of depends on which way the wind blows?
Also as long as where in the engineering subject at what final reading on a megger would you consider a motor bad?

 

[Andybr]

drspanda.

I dont know for sure if that is exactly what happens, but your argumentation looks correct to me.

If the posters that argue that open circuit on the secondary will result in an extreme voltage were right, then the CT would selfdestruct immediately.

If drspanda is correct, then the CT will slowly degrade until it bursts into flames.

Jesper.
These two conditionns are not mutually exclusive. Either one of them could occur if the conditions are right at the time. The potential for damage to the CT, however, has got to be a minor issue compared to the potential danger presented to the person who is working on the circuit. Even if the extreme voltages required to break down the insulation are not reached there is still the possibilty of serious injury.
Andybr

 

[JesperMP]

Andy.
Your post indicates that you believe that opening the secondary while power is off will result in extreme voltage when power is on.
I have not yet seen anyone provide a good explanation why there should be an extreme voltage under these conditions. Can you ?

 

[Andybr]

Jesper.

I do not believe that energising the primary WILL definitely result in a high voltage. I do, however, know that a high voltage COULD occur under transient overload conditions if the secondary is open circuit. The transient condition could be due to motor starting current, transformer magnetising current or a fault for example.
Andybr

 

[Leadfoot]

Jesper.

You are right on both methods of CT's failing. SOme just explode due to the high voltage breaking down the insulation causing high amperage and excessive heat. Some just sizzle.

I witnessed testing of ct's once. These were 600:1 used on generator controls. SOme detonated immediately and looked like a pealed banana. Some just sizzled and caught fire. All of them were made on the same machine by the same operator.

Pictures were taken and passed around so there was positive proof you did not unburden a CT.

BTW, the primary voltage on the CT is the rated LINE voltage. That is what the engineer that was designing them told me.

 

[JesperMP]

Because this is a safety related topic, I went and tried to find some good documentation about the matter.
Phil Corso´s comment that I linked to looked persuading enough, but further searching about the matter resulted in many texts that consistently warn about high voltages when open the secondary on a CT. Therefore I now think that Phil Corso is wrong.

I still havent found a good precise explanation why this is so.
(For sure it has nothing to do with the load supply voltage, but everything to do with the current flowing in the primary circuit).

This is the best explanation, despite not completely satisfactory from http://www.toroid.com/knowledge_base/AC_current_transformers.htm:
"A current transformer should never be open-circuited while main current is passing through the primary winding. If the load is removed from the secondary winding while the main circuit current is flowing, most of the primary winding current becomes magnetizing current, but the vector angles change in such a way as to keep the total current in the primary the same as before. Because the main circuit is now mostly magnetizing current, the flux in the core shoots up to a high level and a very high voltage appears across the secondary. Due to the high turn ratio usually found in these transformers, the voltage in this condition can reach a dangerously high level, which can break down the insulation."

To the original question: I would still do the test while taking all possible safety measures. The risk would be that the meter or the CT gets toasted.

 

[TConnolly]

Thats not the only risk. The voltage can exceed the insulation rating of the test meter leads.

 

[Thomas]

Instrument removal

If I remember correctly, the general rule for removing metering from an active system is:

short a current transformer

open a potential transformer

This minimizes the danger from inductive kick on the current trasformer and letting the smoke out of the potential transformer.

Regards,

Thomas

 

[BudW]

Just some more reading material from Simpson (Amp Meter manufacture) to add fuel to the fire.


http://www.simpsonelectric.com/uploads/File/186_manual.pdf
http://www.simpsonelectric.com/uploads/File/datasheets/donuttransformer_datasheet.pdf