CBCT spurious tripping

[samanele]

here is the case
PCC feeding 1 MCC and 1 VFD MCC.
Seq of events
1.MCC motor on DOL running fine.
2.one of the vfd motor start in the VFD mcc.
3.We are facing spurious tripping of few DOL LT motors (conected to MCC )having CBCT wired with 3RB relays.
please help me why is this happening and also please give me solution or this...

 

[RRaghunath]

I tend to believe that the power cable armour is going through the CBCT but is not brought out before connecting to the earth bar.
This is a common problem and worth checking out.

 

[liviu2004]

Tripping is the result of undervoltage. I guess. Any other reasons to foresee?

 

[rcw retired EE]

samanele - Please educate me on the Acronyms you are using.

What is a CBCT? (Circuit Breaker Currnt Transformer?)

Waht is a 3RB relay?

I assume an "LT" motor is a "Low Tension" or low voltage motor and "DOL" is direct across the line motor starting or FVNR (Full Voltage Non Reversing) in NEMA/ANSI world.

With a little more information, we might be able to suggest some issues to look at.

 

[davidbeach]

CBCT is probably a core balance CT, AKA a zero sequence CT.

 

[ScottyUK]

3RB relay is likely a Siemens motor thermal overload relay of some unspecified rating: 3RB are the leading characters of a typically long Siemens part number. Seems a slightly unusual thing to connect to a core balance CT.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[samanele]

yes..scottyuk ur right....3RB12 is a SIRIUS make Solid State Overload Relays. it has earth fault protection feature too...in my case CBCT (CORE Balance CT) is connected to this 3RB12

 

[samanele]

Raghun...We have checked this...the connection is ok...connection is done as shown in figure attached

 

[rbulsara]

Where is the diagram of the relay 3RB12? The pdf is a generic diagram assuming correct type of relay is used.

 

[RRaghunath]

samanele,

What do you find in the relay manual! With some relays, you may have to use custom made CBCTs. See the requirements for CBCT in the relay literature and compare with the name plate of the CBCT that is installed. You may get some clue. It may help if you can post the relay manual on the site so that others can have a look at it too.

 

[burnt2x]

Without a diagram, all will be just guesses! I would suspect the CBCT on the MCC sensing zero sequence from adjacent motor starting on the VFD MCC. Please check grounding system of the VFD side.
The 3RB12 electronic overload relay is suitable for frequencies of 50/60 Hz and their related harmonics. The manual says it's possible to use the 3RB12 on the line side of a frequency converter. Please verify also if the core balance CT you used is the one recommended by 3RB12 manual, as raghun said.