Breaker Not Tripping

[JimmyJunior]

Our campus type facility is supplied from a 15kV dual lineup circuit breaker arrangement. Each lineup is fed from it own 20MVA transformer. The two lineups are tied through a NO circuit breaker. Each line up has 15kV, 1200A circuit breakers protected by the usual 50/51 overcurrent protection scheme. We are using workhorse electromechanical relays (IAC).

Recently the plant engineering group was notified that a phase relay had a target flag showing (time delay flag) but the associated breaker did not trip. Our electrical engineer PE became alarmed that such an event had occurred. He was of the opinion that we should obtain the services of a competent service company to test the relays and breaker in question as soon as possible, if not sooner. Other engineers and experienced electrcians took an opposite point of view. They said that it was probably due to vibration. The PE said that this was a possibility but the possible consequences in case of a fault that is not cleared in time or at all, compelled us to take action.

The Plant Engineer ruled in favor of waiting until a convenient time could be set aside to test the breaker/relays; "Since nothing had happened...nothing probably would..."

It may be worth noting that the PE has over 20 years of experience in power systems analysis, runs our short circuit and protective devices studies and previously worked for a couple of utilities doing the same kind of work.

 

[gobblerhuntr]

PE or not, the guy has the right idea. I am not a PE but have the 20 years in power systems and I can tell you if I see a target down on one of our electro-mech relays I want to know what caused it. I have had the same senario happen and chose to test and found nothing wrong. It is whatever makes you fell good, I myself want to know what caused it due to the fact if you don't take some action when it burns down your superiors will investigate and you better have tried everything possible or here comes the chopping block.

In reality, can you feed the load on one breaker and isolate the potential trouble one long enought for test?? If so get a good test person or company to look at it, it may not be anything to it. I have seen ones that you couldn't explain and are still in service today.

Good luck

 

[davidbeach]

So, what has to blow up before you are willing to do some testing? There should be at least an attempt to understand every relay target. Yes, IAC relays can be sensitive to vibration, but if vibration is going to cause an operation it will cause a complete operation, trip contacts closing and a target. Target without trip is a cause for concern; you either have a malfunctioning relay or a malfunctioning breaker. It may not be something repeatable, but do some testing, fix any problems found and if nothing is found keep a close eye on that relay/breaker combination.

 

[DanDel]

It's only obvious to maintenance/engineering people in a facility that regularly tests and maintains their equipment that testing must be done in this instance. Others will look for any reason not to shutdown for testing no matter what problems occur.

Not only must the relays and C/Bs be tested individually (immediately), but each relay circuit must be trip tested to verify that they will actually trip the C/B when called upon to do so. This will probably not tell you what caused your flag, but it will tell you whether or not your protection system will operate, which is something that you do not know right now.

By the way, this is normal maintenance, typically performed on a three year cycle.

 

[oldfieldguy]

I'm going to concur with everyone else's comments that testing should be done immediately.

Further, I will add that I do NOT operate under the assumption that this equipment was tested properly on installation. I have seen numerous installations where protective relays and circuit breaker controls would not operate after being in service for many years, and the faults discovered indicated that it had NEVER been fully tested from the time it was installed. And NEVER accept "we tested it at the factory".

I've found relays incorrectly set, CT's with shorting links in place, controls incorrectly configured, etc., etc., ad nauseum.

And finding these problems has made for a fairly profitable career, especially enjoyable when the fault was left by one of my competitors.

In short, TEST IT! Before you find out it doesn't work as you investigate a really BAD problem...

old field guy

 

[Zogzog]

Test it! A relay failing to trip a breaker is a huge potential problem. You can find someone local at

http://www.netaworld.org

 

[alehman]

"Since nothing had happened...nothing probably would..." This is such a common attitude. Yes, that 'probably' true, but what if it is not? You have a known problem. What is the risk of not investigating? How will you explain if the improbable occurs and someone is hurt or the plant burns down?

As a commissioning engineer I hear that attitude over and over. As others have observed, we find a lot of wiring problems in switchboards that were supposedly tested. My opinion is that testing frequently doesn't happen unless someone is hired to witness it.

 

[gobblerhuntr]

Full functional test, run current through the bushings and watch what happens. This will tell all.

 

[lz5pl]

"Since nothing had happened...nothing probably would..." - In general I am agree with this point, because if it has been a real short circuit, at least the next breaker on the line should be tripped. Most probably mechanical flag have felt from vibration.
But one could never rely on such assumptions.
I also have seen protection panels with factory test reports from a famous company (4 from a batch of 12 identical busbar protection panels) with identical mistake in the wiring, compromizing the zones selection! Factory test reports were looking great, including not really tested items!
After almost 20 years as a commissioning engineer my slogan is : "Don't trust any authority, including yourself! Just check everything twice."
Several postings before gobblerhuntr pointed out very exact reaction of your superiors if something wrong happen in the future. I would add only: even if the eventual future problem will not be connected really with this now, they can try to "hang" it on you. This is type tested case!
So, check the problem and only after that sleep well!

 

[swn1]

Soundslike these may be Magneblast breakers? They are very prone to having the trip mechanism stick and burning out the trip coil, the seal-in coil on the relay, or both.

It is also possible that the target was left down after a scheduled test of the relay. Has it recently been tested? When what the last time you can be certain that the target was not showing?

Finally, the IAC relays target/seal-in unit (and the instantaneous unit, which shared the same design) is a "high seismic" design that has very good vibration immunity. It takes a very significant vertical accelleration to reset it falsely and a very sharp vertical, horizontal, or tortional blow to cause it to set falsely.

You could remove the relay and evaluate the mechanical integrity of the TSI unit. Try to set/reset it with moderate shock/shaking. While you have it out carefully inspect the seal-in coil and contacts.

Unlike the instantaneous unit, the TSI is operated by current through the trip circuit. If it was operated electrically and the breaker did not trip, you absolutely must have that breaker competently serviced.

The instantaneous unit is operated by secondary current and, depending on how it is adjusted, it is possible to have it drop a target without quite closing the trip circuit.

Is the "closed" indicator light on? This is *usually* wired to indicate continuity in the trip circuit, but not always.

 

[DTR2011]

I am going to have to completely agree with swn1 here. Test it!. At a minimum, the relay in question can be removed (while the system is in service) and checked for obvious defects. The TSI test, should reveal this. What the owner should be prepared for is what if the relay operated correctly and there is a 'stuck magne-blast'?

Its a bit funny how some companies can spend thousands of $$$ on NFP 70 compliance (by putting stickers on swgr), but a much more serious problem is ignored.

 

[pwrengrds]

swn1 has the right idea, the first thing I would do is check the closed light. If it is on then your trip coil is most likely good for the breaker. The way these are typically wired is the close light is powered through the trip coil. If the light is on, check the schematic to see how it is wired, if thats ok then check the relay. If it is off check the voltage on the back of the socket to see if you have power. If you don't have power to the light bulb then the trip coil is probabley open and you have no protection reguardless of your relays.