VCB Tripping

breakers fail too

What make is the VCB?

Is it not giving any indication as to why it trips?

I've just looked at http://cr4.globalspec.com/thread/96373 Employ an engineer because you clearly don't know what you are doing.

The circuit breaker may not be be correct for the application or the trip settings are not correct.

Transformers and motors require a longer trip time setting to allow for the higher inrush current during energization.

You need to collect all of the information on the transformer nameplate then contact the breaker manufacturer service department and ask if you have the correct breaker.

If the breaker is rated for your application, the breaker manufacturer can determine the correct trip setting for your application.

Keep in mind that most companies have system protection setting standards and it would be a good idea to make sure the settings are in compliance with policy.

Why don't you rack out the VBC to the maintenance position and test your VCB again. If the VCB does not trip that mean your VCB to your transformer OC or EF setting values are not correct.

VCB trips due to inrush current . Inrush current will be approximately 10 times full load current and this will be for 100 milli sec.The over current relay provided in the VCB should be appropriately set

Hello Sir, OP said transformer is disconnected and still trips...So where is this Inrush current from ?

Please read the sentence carefully.The output from the transformer is disconnected.This means secondary side of the transformer is disconnected.The inrush current will prevail irrespective of load on the transformer.

Yes,..You are correct. OP has to check the relay setting and take the tripping data from the Relay if possible. Normally Trasformer trips during the first energization and it will be Ok for second try. However, check the settings for the short time, long time and earth fault. Inrush current will creat a current unbalance and Relay will sense the unbalalnce current as earthfault. Hence it trips.It happends in some relays where there is no inrush current protection.

The VCBs in the outgoings of RMUs come with self powered protection relays. It seems the relay is not set correctly.

It appears the protection is operating on transformer inrush and causing the trip.

What is the make of RMU and type and model of the relay!

Fine, some have responded with remedies. But it is always better to mention makes of the machines and also the indications if any the system provides. Tripping of VCB could be due to many reasons and shortlisting of faults without all those details is a super expert's judgement I feel. feed a bit more for analysis... ALos provide your analysis if done with views or say could not analyse because....reasons.