Will a CT Blow a Fuse?

[Laplacian]

For example:
15:5 doughnut CT with 5A in the primary and 5/3A through the secondary.
If a fuse sized for 1/10th ((5/3)*(1/10)=1/6A)the secondary current was shorted across the secondary terminals, would it blow? I wish I had a test bench to work this out on.

 

[RAMConsult]

Yes it will, the fuse doesn't know what is forcing the current through it. A better question is why do you want to do this? If you try this, make sure that you have a properly sized, non-fused burden resistor across the CT secondary at all times.

 

[scottf]

Laplacian-

I question the reasoning for putting a fuse in a CT secondary circuit. Open circuiting CT secondaries can be very dangerous, as high voltages can appear. The magnatude of the voltage is dependant on the CT core design.

RAMConsult-

I would be wary of putting a resistor in parrallel with the connected burden. This configuration will of course form a current divider. Depending on the application, this will likely cause faulty readings/performance in the connected device.

 

[RAMConsult]

scottf- This was an answer to a specific experimental question, not a general operational one. The resistor is there to prevent an open circuit from occurring when the fuse blows thereby preventing the destruction that will take place. Please note that I also questioned why Laplacian wants to do this experiment, perhaps his answer will guide us in providing a safer means to accomplish his goal.

 

[Laplacian]

This will not be used in a standard CT application.
It will be monitoring a DC circuit with a maximum of 1 amp DC on the primary. (I don't care about steady state current and transients are not an issue) That is all I can say and thanks for your input and concerns.

 

[scottf]

This may be a really stupid question, but how do you propose to transform DC current with a current transformer??

I assume you really a CT won't pass DC?

 

[Laplacian]

I'm interested in the current rise from 0 to 1 amp, not the steady state DC current.

 

[CarlPugh]

Maybe the fuse will blow and maybe it won't.
Tested one fuse and it wouldn't open with 10 volt applied. The same fuse from a different manufacture opened at a much lower voltage.
Test the fuse you are going to use to determine at what voltage it opens.

 

[Bung]

Fuses are current operated. Source voltage has b**r all to do with it - after all they are series connected.


Bung
Life is non-linear...

 

[jbartos]

Suggestion to Laplacian (Electrical)Oct 17, 2003 marked ///\\For example:
15:5 doughnut CT with 5A in the primary and 5/3A through the secondary.
If a fuse sized for 1/10th ((5/3)*(1/10)=1/6A)the secondary current was shorted across the secondary terminals, would it blow?
///Please, would you clarify where that fuse is located with respect to the short on the CT secondary terminals, and VA rating of the CT.\\\

 

[scottf]

Another question...what is the voltage on the line you're trying to measure current in?

 

[Laplacian]

135VDC trip circuit, 1A trip coil.

 

[jbartos]

Suggestion to Laplacian (Electrical) Oct 17, 2003 ///\\For example:
15:5 doughnut CT with 5A in the primary and 5/3A through the secondary.
If a fuse sized for 1/10th ((5/3)*(1/10)=1/6A)the secondary current was shorted across the secondary terminals,
///Please, would you clarify this? If the secondary current is shorted across the secondary terminals of CT, the fuse location would be downstream of the shorted terminals then, or is this a typo?\\ would it blow?
///It depends on the answer above.\\ I wish I had a test bench to work this out on.

 

[scottf]

This may be another stupid question, but why are you using a CT in the circuit? Can you not just take a direct measurement, since the voltage is only 135V DC and the maximum current is 1A?

Also, I still question whether a CT is going to give an accurate output (i.e. secondary modeling primary waveform) for a DC transient.

Additionally, the CT you have is rated 15:5A and you're trying to measure a transient from 0-1A. Assuming the CT will even work, what accuracy are you expecting? You would be better off using a different ratio, such as 1:1A, etc...

 

[jbartos]

Suggestion: The omission of CT VAs and the relatively large 15:5A ratio as addressed in the previous posting are posing a concern about the conceptual approach to the task.

 

[Laplacian]

I was hoping I wouldn't have to explain my complete intentions.

This will be used as a trap or tattle-tale on a trip circuit for a 2300V motor feeder circuit breaker. The motor has a history of inadvertently tripping leaving no indication in the control room or switchgear.

There are 9 devices that are paralleled together, in series with the breaker trip coil. My suspicion is that it is one of the two field stop P/B's as they are the only two devices that don't leave any indication.

The control wiring is good (tested, replaced and tested again), the switchgear is new (part of a replacement/upgrade plan), yet every 9mos to 1yr this motor shuts down causing $200K per hour losses while it is down.

I can not install anything that will compromise the reliability of the trip circuit (hence using a CT). I'm not making measurments so accuracy is not a concern. All I want to do is blow the fuse when the trip coil is picked up.

I will install the CT/fuse on each of the 9 parallel legs (be they in the field or switchgear). It must be cheap and easily left installed indefinitely awaiting the next false trip. This motor is projected to run the next 10 years without shutdown. PLC's are not a solution as some tie points for the paralled devices are in the field and some in the switchgear.

I can't run cable/conduit all over the plant on a whimsical fantasy to facilitate future troubleshooting needs. If there are suggestions for something cheaper, less intrusive, and more reliable, let me know. Thanks for all the input.

I don't have a VA rating for the CT (whatever will work is fine). This is not implemented yet which is why I came seeking input on the feasibility of a CT blowing a very low rated fuse.

The 15:5 ratio is the smallest doughnut style CT I can find. Lower ratios are always welcome. I will not use a wound primary type CT or any other style CT that requires adding extra terminations or is otherwise in series with the trip circuit.

 

[johnspark]

Dear Laplacian, please consider the following:
money for testing, if the company can lose $200k/hr for lost production, then they can be kind enough to give you some money for investigations:
I would recommend you obtain some clip on ammeters AC/DC that can measure the small currents you are talking about, buy some simple circuitry that can transform (not Laplace transform) this into a contact, and put all 9 contacts into an intelligent electronic device such as a multifunction relay you have as a spare. Put this ied into a plastic box and have sitting on the floor next to the panels with the 9 inputs and carefully attach wires to 9 inputs of this ied, with the 9 wires going to the simple detection circuitry you have. Carefully place all equipment so it is not in the road because you said it could be here for 9 months. Then set triggers in the ied to operate the event recorder. Whamo! you will find the event time and circuit that has been causing you headaches with a "Gibbs" step rise.

IF your company does'nt own ieds yet, contact a number of relay suppliers, tell them your company has none, tell he / she that you see the successful tracing of this intermittent fault as an opportunity to introduce ieds into the factory, and I think they will be willing to lend you an ied for this ~9 months.

Alternatively, if you cannot borrow one, buy one that has at least 9 inputs to it! Remember, you are trying to save $200k each time this thing trips off!!!

NB: The plastic box should have rugged terminals installed inside of it for connecting the wires to it.

NNB: if you are able to borrow a 3 CT input transformer ied, you will get 9 CT inputs that normally like to see AC. But if you connect your circuits through these 3 + 3 + 3 = 9 CTs, and set the disturbance and event recorder to work off the smell of an oily rag, you will get your detectors neatly packaged within this ied. (It does'nt matter whether it is ac/dc that you wish to measure with because you are only looking at the rising edge of a current pulse). Good advertising to bosses to see that you are utilising time efficiently and are able to generate a tight events report with definite event times within the report and a good snap shot from the disturbance recorder!!! This will further help you obtain better testing equipment to help the company more in the future...

Hope I am clear in my presentation of my thoughts...


cheers

 

[jbartos]

Suggestion: This and similar approaches to safeguard more expensive processes are nothing new.
For some specialized CTs and monitoring equipment, visit

http://www.crmagnetics.com

http://www.crmagnetics.com/newprod/applist.asp

for application notes