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Low-Voltage Power Distribution and Installation Technology

MCCB SELECTION FOR TRANSFORMER PROTECTION

Thread Starter: jayanta   Started: 7/18/2010 10:03 PM   Replies: 10

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  7/18/2010, 10:03 PM
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Dear Experts,

can tell me if MCCB (cat no. - 3VT1708-26A36-0AAa0) is suitable for protection of transformer? If yes, can explane me selection prosidute.
Because the above mention MCCB (80A) is  tripping in inrush current of a 37kva (415/240V)
transformer.

Kindly give us reply.

Thanks in advance.....
 



=== Edited by jayanta @ 7/18/2010 10:04 PM [GMT ] ===


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  7/19/2010, 8:20 AM
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Transformer primary is 2~ or 3~?. How & what type of load is connected to sencondary?. Thru contactor, relay or directly. How much is seondary fuse or MCCB rating?
DEEP. 

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  7/19/2010, 5:02 PM
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Transformer's primary & sencondary is 3~. Sencondary open. But actually it will be connected with thyristor circuit.

Jayanta

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  7/20/2010, 6:33 AM
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Hello Jayanta,
The specified MCCB is of general purpose MCCB & not of dedicated MCCB for only LV transformer protection. The 80 Amp range selection is correct one but the breaking capacity 25 KA is not adequate for industrial or your application. Just try MCCB of amp >70 A rating with 50 KA breaking capacity.
Regards,
DEEP. 

Tell me and I will forget. Show me and I will remember. Involve me and I will understand. -Confucius
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  7/20/2010, 7:42 PM
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Mr. Deep,

We also tested this Transformer by useing 63A, 25KA, Cat No. - 3VT1706-2DC36-0AA0 MCCB and no any problem arrive.
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  7/21/2010, 7:41 AM
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Mr. Jayanta,
With 63 A, 25 KA MCCB, have you checked the transformer on load or only transformer is checked? Moreover, if on load you have not faced any problem with 63A 25 KA rating MCCB &  on same transformer, 80A 25 KA MCCB is tripping, then in that case your 80A, 25 KA rating MCCB may be faulty.
Please check.
DEEP.

Tell me and I will forget. Show me and I will remember. Involve me and I will understand. -Confucius
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  7/21/2010, 7:41 AM
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Mr. Jayanta,
With 63 A, 25 KA MCCB, have you checked the transformer on load or only transformer is checked? Moreover, if on load you have not faced any problem with 63A 25 KA rating MCCB &  on same transformer, 80A 25 KA MCCB is tripping, then in that case your 80A, 25 KA rating MCCB must be faulty.
Please check.
DEEP.

Tell me and I will forget. Show me and I will remember. Involve me and I will understand. -Confucius
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  8/27/2010, 8:03 PM
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Dear All / Mr. Deep

I like to inform you that the said 63A MCCB is Siemens make Characteristic „D“, & 80A MCCB is Characteristic „L“. I think (after watching MCCB catalogue) these two types of Characteristic MCCB is totally different. I also like to inform you that we tested the said transformer using lots 80A (same type) MCCB. So, please inform/guide me I/C MCCB selection calculation to avoid MCCB tripping at switching inrush. it would be better if anyone can give me supporting document also.


For any information anyone can also mail me (jayanta.santra1@gmail.com).

Please guide me with supporting document.

Jayanta




=== Edited by jayanta @ 8/28/2010 5:20 PM [GMT ] ===


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  2/11/2011, 4:24 PM
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Hello Jayanta,
As I understand problem is line-side protection of a transformer?

We have to see that an inrush current of 20...30 x In (rated current) is realistic.

Instantaneous trip (Ii) of a MCCB is about 10...13 x In. Therefore the circuit-breaker must be operated in its lower setting range.
Example: Inrush current for a transformer In = 500A may be higher than 10000 A.
Selection: Breaker In = 1000A, setting of In = 500 A, Ii = 11 x In11000 A (22 x setting of In).
It must not be forgotten that the circuit breaker has to trip on the minimum short-circuit Ikmin. So the breaker should provide short time delayed short-circuit protection (Isd), which normally can be set to 2...10 x In and a time delay of a few hundred milliseconds.
Please be aware not to use a breaker for motor protection. Phase-failure sensitivity may lead to unintentional tripping behaviour.

Regards,

Sentronix

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  5/9/2011, 6:49 PM
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Hello Colleagues

When switching on low-voltage AC transformers, the extremely high inrush current peaks place special demands on the trip unit or on the making capacity of the circuit breakers if these are also used to switch the transformer.
For most applications, an inrush current of 20 to 30 times the rated operating current is expected in practice and must be taken into account when selecting the circuit breakers. The maximum short-circuit current Ik of the 3VL circuit breakers is 11 x Ie (rated operating current). A circuit breaker in the lower setting range must therefore be operated for primary side transformer protection. Example: A transformer with 500 A rated current; 20 times the inrush currentSelected: ETU with In = 1000 A; setting range 0.4 - 1 x In = 400 A to 1000 A50% of In = 500 A; Ii = 11 x In = 1000 A x 11 = 11000 A = 22 x current setting

Rnf:lblSmileyTooltip_  think 

Have a good day..!

VonSiemens
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