Question On MCCB Tripping

OK

1. how about starting with a clip-on ammeter on each phase, to see what is really going on?
2. Or how about installing current transformers and a power cube on the inlet to this panel so that the incoming current can be monitored?
3. How about looking for other systems tied on to the upstream breaker that the Client hasn't mentioned?
4. In an environement where there is no discrimination, why does it matter which breaker trips, so long as one of them does?
5. If both breakers are the same rating, why are two of them needed?

You are correct as MCB in each phase can draw 250 amps load. You can just replace MCB in electric room from where you have taken power and show it to consultant there would not be any tripping.

The caus of tripping in MCB at electric room could be different and it has so many reasons one is that is has dust accumalated inside and second its thermal O/L charaectristic is having problem. So simply you can replace it and charge cost from client.

C.S. PANT

_{"my client consultant says that my panel is overloaded .... he says that as my panel is taking around 120a in each phase ..... the total 3 phase current is around 360a... 120+128+135 =383a.... and his breaker capacity is only 250a........ he says this is the reson why his mccb in the electrical room is tripping........."}

Wow!

Your client has completely wasted his money if he paid for that consultant! That is something that a complete novice (or Mechanical Engineer) would say, not an Electrical Engineer.

You are absolutely correct; a 250A circuit breaker means that EACH POLE can take 250A. He is a moron. In reality though, if you are in the US you can only legally load that MCCB to 80% of that rating, but that is still 200A; far more than your measured load. Where does it say that? Well, in ANY Electrical Engineering text that is describing circuit breakers!

Here are more reasonable things to check:

1. What is the load profile? Is it motor starters and do they start Across-the-Line (DOL)? If so, what is the Instantaneous (Magnetic) Trip setting of each of those MCCBs? If they are not adjustable, it could very well be that the upstream MCCB has a lower factory setting than yours, so it is tripping on starting current.
2. What is the history on that upstream MCCB? If it is old and has tripped many times before, it can build up what is called a "thermal memory" and will trip easier and easier every time until such time as you can no longer get it to hold in at all.
3. Are you sure the cables going from the upstream MCCB to your panel have been properly checked out? If so, what method did they use? If it was not a megger (which charges the circuit to 500 or 1000VDC and measures leakage to ground over time), then the testing may have not picked up damaged insulation or a bad connection somewhere. A quality electrical installer would have kept testing records after the initial installation.
4. Does the upstream MCCB have ground fault (earth leakage) protection built-in whereas yours does not? If so, does it use the Residual Current sensing method? That 12% current imbalance you indicated may be causing it to trip if it is set lower than that, regardless of what the thermal trip setting is.

There are a lot of things it could be, but for sure one thing it is not is a 383 Amp overload!

Dear all

This issue has been raised because India doesn't have and doesn't Believe in PE Concept any one can practice any branch of engineering as a consultant with out formal engineering qualifications .still Out Dated BS standards are referred hardly IEC or IEEE standards are referred for design existing Indian standards are based on good old British standards which Briton itself has discarded.you would be surprised switch Gare's manufactured for cu Cables are modified for AL cables by adding an extension box at site and installed

crm

THANKS FOR THE ANSWER ....

1) the load used is heaters and heat exchangers... and there are some 2hp motors with some light load....... about the trip setting ,i ve also asked the consultant to check the same ....as the electrical room was locked at the time of my inspection i was not able to see the type of breaker............. but he is still clinging upon the fact that the total current in the 3 PHASE MCB calculation is sum of all the 3 phases.... i was really stunned to hear that.... and that too from a well know consultant in uae "DRAKE AND SKULL" the guy is not an indian but soemwhere from egypt or lebenon....

2)the mccb is not an old one .. and it jhas jus recently developed the habbit of tripping

3)i havent yet checke dthe cables

4)i ll check if there is any ELR connected to the upsteram MCCB

THANKS A LOT FOR THE RESPONSE

In that case first find out what make of MCCB ,Model,type contact Manufacturers application manual if possible ask for tripping character curve from manufacturer which would help you in trouble shooting and then you can confront the consultant and save your skin.Thats what i do here in India you need to maintain data base on all components you use in your product.most of the information is available on web sites of component manufacturer like ABB,GE etc.good luck

crm

can i know rated current of motor as this is a major reason for overload.To protect the motor against any overloads it is necessary to adjust L function of your circuit breaker to a current higher than or equal to the rated current of motor i.e I1 ≥ Ie.

Adjust this L function of your breaker, i think the motor overload problem will get solved,

This is Pawan Jha