Thanks for all those responses. As far as I can tell, there was unanimous agreement, the ct can't increase external current. Except Dandel made a good point about intermittent open may have some unexpected results.
First and foremost, I was interested in the general question. My reason - at our plant for certain critical problems, our problem solving process is called a fault tree. You get together in a conference room with 10 other people and you list every possible cause and then you rule them out by logic if possible and then you develop a plan to troubleshoot the remaining (not ruled out) items in a prioritized manner based on probability of being the cause and based on ease of tests to rule a given item out.
Over the years, we have had various motor trips on a variety of occasions with a variety of relays - electromechanical (GE) and electronic (ITE Circuit Shield) relays providing functions of instantaneous overcurrent , time overcurrent, and high-dropout overcurrent. I would like to have justificaiton to remove the CT from consideration in these scenario's without having to test the CT.
There is one issue that we were fighting yesterday. It is now a closed issue (unless the problem recurs).
It is a long convoluted story. Since you guys asked even after I warned you ....(electricpete /Electrical / 1 Jul 05 8:00 wrote: "btw, I know someone out there is itching to ask about the trip or give me suggestions for testing the ct... that is not my interest at this point... just want to focus on understanding possible failures of CT")...., since you asked I'll give some details.
I'm pretty sure there is not much help you can provide on this long convoluted ill-posed question, but if you are a glutton for punishment and want to read the whole thing, be my guest. I welcome any thoughts on the question at the end. Once again it was not my choice to describe our scenario but you guys asked for it
This is an ITE electronic Circuit Shield 50/51 relay. It protects an 4kv 800hp horizontal sleeve bearing induction motor driving a between-bearings double-suction centrifugal pump through a Thomas Shim Pack coupling. We have two of these relays - one for trip and higher setpoint and one for alarm at lower setpoint. We got a pump trip 13 hours after pump start. At the time of pump trip we happened to be reducing flow in the system. Pump curve shows for this pump that equates to reducing the current in the motor. Flow was at a level where we expect current somwhere in the neighborhood of 70% FLA. Alarm relay is 1.2*FLA and Trip relay is 1.3*FLA. We got the time overcurrent flag for on the trip relay but not the alarm relay (and no instantaneous on either relay). This in itself was a very strong indicator of relay problem since alarm relay should have flagged before the trip rleay. But not exactly able to confirm any problem on the trip relay or the alarm relay. Alarm relay which did not trip is in series on the same secondary CT circuit which would seem to rule out CT altogether in this case, except maybe for DanDel's scenario.... but at this point in time that doesn't seem too probable (although interested to hear comments).
Cable was meggered sat, motor meggered and bridged sat. Our ace relay technician did report some kind of a trip during testing of the alarm relay that was not reproducible, but the story of the circumstances surrounding that trip are completely a mystery to me considering it is a complex test procedure on a relay I am not familiar with, and the story of the trip during testing was relayed from nightshift through several intermediate people. The trip did not occur when they were testing the trip function (all trip tests were sat). I think maybe the trip occured when they were removing testjacks.
It seems pretty close to a "case-closed" scenario that we have a faulty trip relay based on:
1 - the alarm relay didn't trip (lower setpoint) but the trip relay did trip (higher setpoit)
2 - nothing found wrong in the motor or cable.
3 - some anomaly identified during testing of the relay.
4 - Winding and bearing temperatures were normal and rock steady up to the time of the trip.
btw - we don't have any record of current. Wish we had them SEL relays!
The one thing that is bugging me a little bit is that the trip occurred exactly at the time we were manually reducing flow in the system (a very infrequent occurence).... 30 seconds after they began reducing flow. Seems too much a coincidence to be unrelated. I generally don't believe in coincidences. But we don't see any possibility for emi/rfi conducted or radiated interference between the motor operated valve used to control flow and the relay dc power (the only power provided is dc voltage... same dc voltage that is used to sense the contact position). Double checked that no operators were in the switchgear room where handheld radio's could have affected the circuit.
For all useful portions of this pump curve, reducing flow causes reduced BHP (and therefore reduced current). At the extreme right of the curve (very high flows), the bhp vs flow curve starts to turn in the opposite direction, but I am very confident that we were not at that region of the curve (don't want to talk about that either... take my word for it). I am confident we were in the region of the curve where reducing flow reduces BHP.
I have a theory on how reducing flow might possibly cause a trip, but most people would say it is way way way way way out there. I happen to think it is halfway credible (if the trip recurs , it will be my #1 theory). It is based on some additional info which I haven't given you, which most people would consider irrelevant. I'm not going to give that additional info to you because #1 - I feel a little embarassed about the theory, #2 - I don't want to predjudice you. Maybe I'll give you some additional info or give you my theory after you've had awhile to think about it. For now, can you guys think of any possible way that reducing flow can cause an overcurrent trip (again, my rules are that you have to take my word on the flow and shape of the bhp vs current curve - reducing flow reduces bhp).
Sorry to be so convoluted in my discussion. I did warn you that there is probably no useful input you can provide. We will attempt to recreate the flow conditions and take some current measurements on all three phases... will let you know any more info.
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