CT Failure In A OCB Panel

What sort of ballast are you using in the system?

Is it from a locomotive?

Sorry could not resist!

It could be too many amps.

It could be too many volts.

It could be too much heat dissipation nearby.

It could be vandalism.

It could be military action.

It could be some of the above.

It could be something else.

Only, I can't see it. If repeated replacement of the CT is giving the same result every time, then it cannot be the solution, can it?

The locomotive went to fast should be on the list! Must be an Express CT!

Just use this OCB: OCB

You're not telling us what you have already investigated:

Is the CT burning out at a very exact timetable, such as when a specific action or event happens? What else is happening in the panel and system when the CT fails? Are there any warning signs? Is the 20 days exact, or meant to say roughly after 3 weeks?

Is the failure internal to the CT (windings burn) or external (damage to the case & insulation)? Have you checked the external circuit for insulation resistance and continuity? If there is something that opens the CT circuit, which can cause the CT to fail catastrophically (my suspicion!)? Have you checked and tightened all connections in the CT circuit? Is there anywhere that the CT circuit wiring has been spliced which could cause an intermittent open circuit?

What is in the CT circuit? Protective relays? Meters? Other instruments? If it is part of a circuit from 3 phase CTs to a single phase meter with a selector switch, is the switch the proper type to "make before break" so that the CT circuit won't be opened?

The only way you will find the cause is by doing some detective work. We can't see your equipment from here.

THANKS FOR REPLY SIR,THE CT BURNING IS NOT EXACT 20 DAYS ,YES BUT AROUND 2-3 WEEKS.the ct is damaged windings,yes and you may be right,there can be an open circuit but i think it will cause high voltage to the secondary side of ct,will it get burn by this? there is protection relay and metearing section both ,both have different secondary output at ct.

If the path to the load on the CT, e.g., the meter, is ever broken, even for an instant, the CT windings will be fried. That's because it's actually a step-up transformer, with one turn for the primary, and a high step-up ratio, with the primary fed by your entire ac line voltage. Sheesh! We're talking tens of thousands of volts! The destruction will occur, it has to occur, even if the measured current is quite low when the open-circuit occurs, and even if the time is short, even under a millisecond.

Personally, I'm a big promoter for protection for the CT right at its output winding, as an integral part of the CT. It's ridiculously easy to provide: Just add a pair of back-to-back diodes in parallel, if the CT metering drops less than 200mV, or a pair of back-to-back zener diodes (1N5341, 5W, 6.2V, $0.50), if more than 200mV is required. Properly chosen, these diodes will have no effect on the measurement, but they can protect a CT from damage, no matter what its mistreatment.

With a pair of paralleled diodes soldered in place, a defective wiring event simply results in an improper zero current reading at the meter, which can be debugged and fixed, without any damage to the CT while this is going on.

A few minutes poking around in Google Images reveals dozens of pictures of expensive ruined CT units, all totally unnecessary if the CTs were protected in this way.

So, is the CT on the input of the transformer? Is the CT rated for "11 kv"? What is the CT current rating? What type of transformer are we talking about? Has all of the equipment been properly derated?

The CT fails on one phase, but is it the same phase each time?

If that's the case, and now that we are dealing with HV, how is your bonding for the CT That can be critical.

If you are measuring with test equipment and find that the current is hardly 4.5A, then leave your test equipment in place and record the readings over the 20-day period, or up until the point of failure. The higher the resolution of your recordings the better, I would say at least 1 second averages (considering curve times).

Normally, I'd say, the CT would be placed on the output of a transformer, anyway.

Is your meter set to kW/in² or kW/m²?

It's all in the units.

How does heat density apply in this situation, specifically?

The OP was measuring current.

IF U ARE NOT TECHNICAL,PLEASE BARE US

No Pressure please, no pressure. Calm down man.

What rating is the CT? From your sparse figures I would guess 40/5A.
What PF is the furnace running at? They can be pretty abysmal.

Without some meaningful figures we're wasting our time.

As to why it only fails on one phase, most induction furnaces only utilise two phases. Are CT's fitted to all three phases or just one?

Connect a power analyser on 11kV side(3phases)and record voltage,current,pf,harmonics etc for a month and check the graphs of those components once it burns,it should give you a clue.Also check PFC if connected as well as inductors.

thanks peter,the tinkerer,tonys,pnaban.....pf factor of induction furnance is corrected by capacitor at the secondary side of transformer,is it ok with it?

yes it is 40/5 A,

yes the ct fails in only one phase,rest of the two are running ok,

and it is a normal step down transformer.

please help.

thanks for all ....please ideasmith..i am not giving pressure but someone posted kw/in½ and kw/m½ ..what kind of answer it is..some other is telling i am not techinical?ok..u are...the ballast answer is merging with this question ....if u google for indian locomotive class WAG-9,a 6500 HP locomotive..it has ballast to control the current to the converter of traction motor.

So you found out what the ballast is for. Happy days!

And btw I find the unit question is a very important one. Never underestimate the impact of wrongly used units.

Using units in EE lets say Volt should be a big V. Ampere as you used it correctly is A.

In a good engineering environment quality unit usage is essential for quality work.