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Current Transformer

01/01/2013 11:14 PM

what will happen if current transformer open in the secondary side?

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#1

Re: Current Transformer

01/01/2013 11:35 PM

When energised,output voltage will increase and damage the insulation as well as give a shock to anyone touches.

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#2

Re: Current Transformer

01/02/2013 12:28 AM

It depends on the current transformer. I have noticed that for open CT's, used in electronic meters and smart meters on grids up to 240 VAC about NOTHING happens.

I have many open CT's that do not even produce 10 Volts without a burden. The burden normally applied has also little load for these.

Regular CT's like 200-5 types I heard making vibrating noise, but I also used them never on wires, carrying more than 20 Amps. Most of these have a warning sticker, saying NOT to open, but to short if no burden is applied.

Again these are home type CT's. To be safe, follow the instructions on the CT.

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#3
In reply to #2

Re: Current Transformer

01/02/2013 2:07 AM

Can you provide brand name and nameplate details of those CTs?.

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#4
In reply to #3

Re: Current Transformer

01/02/2013 11:41 AM

These CT's come with kWh meters and smart systems.

They have no nameplates on it, just loose end connections. The TED's are sold separately and also EKM metering CT's. The feed through holes are somewhat limited to around 25 mm diameter. If you like I can take some pictures. Details you will find on the EKM metering site.

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#5

Re: Current Transformer

01/02/2013 11:56 AM

I think equipment will stop working that is fed by C.T.

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#6
In reply to #5

Re: Current Transformer

01/02/2013 12:53 PM

Funny, but true

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#7

Re: Current Transformer

01/02/2013 1:43 PM

AH, a current transformer is a current transformer. It takes emphasizing, that by design it transform current. The primary is a conductor passing thru (a single turn), the secondary is multiple turns. The turn ratio establishes the main ratio for it. Example: 1000 turn secondary = 1/1000th of the main current out of the meter. Normally, these coils come with a built-in load. That converts the current to a voltage to be displayed by a common voltmeter, and read as current.

But, what happens, when the load is missing? The transformer - forced by the magnetic flux - forces current thru its nonexisting load.

What do you call that? I call it arcing and short circuit along the line. That allows the current to flow.

That is why you never ever use a current transformer without a definite load.

If you say you do, it tells us plenty about competency.

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#8
In reply to #7

Re: Current Transformer

01/02/2013 9:37 PM

Do you have examples of CT's with normally built in load?

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#9

Re: Current Transformer

01/02/2013 10:01 PM

This is about everything to know about CT's

http://powermetrix.com/wordpress/wp-content/uploads/2013/01/GLEMS%202012%20-%20Inst%20Xformers%20Theory%20and%20Testing.pdf

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#10
In reply to #9

Re: Current Transformer

01/02/2013 10:17 PM

DVM. I do not care about the details. You are welcome to them. You intend to disprove this and Newtons? You are free to try.

Will you accomplish any of that? Can try. But succeding, it depends my and your stupidity. Today, it is unlikely. In the future, the chesebrains make unlikely making coherent decisions.

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#11

Re: Current Transformer

01/02/2013 10:21 PM

You really ought to cunsult your textbooks about this and the herm relay otherwise you are unlikely to understand enough to pass your exams.

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#12

Re: Current Transformer

01/02/2013 10:51 PM

Open circuit of a CT secondary while the primary side is carrying current will cause a high voltage to appear across the secondary terminals. This voltage is dangerous and can cause electric shock and even death! (in addition to insulation breakdown and of course equipment not working).

In all installations of CTs the secondary terminal blocks should have a facility to short circuit the CT secondary winding. This allows work on down stream equipment to be done safely without interrupting the primary circuit. In tariff metering installations these shorting facilities would be under the control of the supplier to prevent tampering.

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#13
In reply to #7

Re: Current Transformer

01/02/2013 11:03 PM

well said ,to prove your point I had personally come across 2 persons with burnt hands and fingers. My curiosity proved that both are people involved in the private management of power who always deal hands on with CTs.

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#14

Re: Current Transformer

01/03/2013 3:49 AM

The magnetic circuit of the CT is made and once you start passing current through the cable it is monitoring then something must happen.

Amps Ohms and Volts are all tied together but in this instance you have an open circuit so no Ohms. Like the earlier contributors have said this will cause the voltage to go very high - possibly high enough to destroy the CT and possibly high enough to cause danger through electrocution.

It is usual to put a shorting link across a CT to prevent this happening. The shorting link will protect rather than damage the CT. Never put fuses in the CT circuit.

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#15
In reply to #13

Re: Current Transformer

01/03/2013 9:20 AM

Maybe they need a new approach: "Look Ma, no hands!"

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#16
In reply to #2

Re: Current Transformer

01/03/2013 9:47 AM

GA!

Most low VA ct and standard precision have a low magnetizing inductance that prevents destructive voltage levels. They can still produce shocking levels.

But, large, high precision CT will produce much higher voltages. Beware of those...

An open CT will also produce some voltage drop and distortion in the line but it is often negligible.

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