Circuit Breaker Fail Protection.

A circuit breaker will trip just above the set current. Which is waht it is supposed to do. It will clear the overcurrent as long as it is below its sc breaking capacity. If the fault current is ABOVE its breaking capacity, someone has made a bad mistake hasn't he? Any circuit breaker HAS TO have a short-circuit breaking capacity somewhat higher than the maximum short-circuit current that is expected at that point.

As for auto-reclosing, please try this link : autoreclosing

Put another circuit breaker in series with the existing one.

For autoreclosing buy an autoreclose relay.

There is an ISO Standard to which MCCBs are made these days. It might be worth a read, perhaps?

Yes, there is a possibility that the OP is asking whether CBs can 'fail safe'...(i am not sure though).

In LV switchgear though, there are no-close situations like

(a) No incoming voltage ? The undervoltage coil will prevent closing

(b) CB partially withdrawn ? Closing prevented by interlock

(c) Relay taken out for maintenance ? ditto

(d) Closing springs not fully charged ? ditto

(e) Trip command from remote ? ditto

(f) Arc chutes removed for some maintenance ? ditto

some more interlocks may be possible...

So a fair degree of fail-safe is built into modern LVCBs. All CBs are also 'trip-free' meaning that closing and latching onto a system fault is not possible.

For a well designed system, the circuit breaker is the fail safe. It servers to provide protection from someone inadvertently overloading a branch circuit. Devices on the branch circuit should themselves have some type of protection built-in which should act well before the breaker trips. If something is causing a breaker to trip, then you have a serious problem that must be corrected by a trained professional.

That said, on some commercial/industrial systems breakers have a button on them that can be pressed to manually test the breaker. If this is done for preventive maintenance then a breaker failure do to corrosion should be detected before a fault situation occurs. Also, if the current draw is high enough then main breaker will trip, but by this point the damage to the system has already been done.

If you are considering a fail safe like a valve, that when the power fails the valve opens or closed in a safe position, the circuit breaker is not designed that way. If you are worried for the HV breaker, which the control circuit is DC battery voltage, use a DC monitoring circuit to monitor the trip circuit power supply.

If your relay has (like GE UR relays) Breaker Fail monitor (but this function is if the fault occures and the breaker send opens command and still after a few cycles the current flows through the CT, the relays send another command to operate the upstream breaker to cleat the fault.

Under all circumstances that is why relay coordination is important that when the closest to the fault breaker in any reason can not clear the fault the next upstream breaker in line clears the fault.

Simply put:

The circuit Breaker will have a device that will react when the current flowing through its contacts is higher than the set value:

-- Thermal overloads are Bimetallic devices in general that will difform and push a pin (or else) in a way that will release a loaded force(spring) that will make the contacts to trip and open.

-- Magnetic: These are usually used to do the same tripping but when a high current (Short Circuit currents of the order of several 100s or 1000s amps) goes through.

-- Electric/Electronic: Some kind of C.T. will detect the current and use an electronic circuit to energise a Coil that will trigger as explained above, to release the contacts.

-- Maybe there are other methods, but all will finally trigger a kind of reserved force (Usually spring) to trigger the release of the contacts...

Auto-reset: This is a facility (not allways available) that will reset the Overload device to allow someone to re-connect the c.b. manually. If Auto-reclosing is required, then a motorised or powered C.B. is used to attempt to re-close the circuit after a delay. But this is a sophisticated installation and beyond this reply or OP.

Hi.

I was referring to the above figure, where 50BF is cb failure relay, and bfi is breaker failure instantaneous; and if both are present then only after a certain time breaker failure scheme output comes into function. What is BFI here?

sks

Hi,

Here BFI - breaker failure initiation as you indicated is through relay which closes its contacts upon occurence of breaker failure?

sks

Hi,

Would you please elaborate your query a little more...thanks.

Regards,

1. How is breaker initiation time established?

2. I am in need of unit turbine power output, head, weight and cost of a pelton turbine of rating above 10 MW. Can you please help me with this?

Regards,

1. follow the link below to get an idea of setting the 50BF Timer;

http://www05.abb.com/global/scot/scot296.nsf/veritydisplay/c1256d32004634bac1256df000652a99/$File/1MRK505005-BEN_en_Breaker-failure_protection_RAICA.pdf

2. Regrets, I'm not in the field of generation; so, may be one of the members could help you in that respect.

Regards,