Circuit Breaker Protection

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Circuit Breaker Protection

circuit breakers are designed (and correctly sized up according to use) to protect not only the conductors but as well as itself..it has a thermal element inside that expands when it experiences overcurrent..pushing some mechanisms that would eventually cause the unit to trip...try dissecting a beaker and you may discover what is inside of it..

Low voltage CB's by definition have internal provisions for monitoring and tripping....that's one feature. Here is a good ABB document on them.

Over-current protection is pretty much the definition of a circuit breaker...it's an over-current protection device.

A circuit breaker must be rated at the current for which it is designed to trip. That's kind of the whole point...the CB must be rated for it's trip value.

From the NEC: 490.21(4)(1) The continuous current rating of a circuit breaker shall not be less than the maximum continuous current through the circuit breaker.

Vandie,

The protection CBs for LV are selected based on the cable size, and the cable size is selected based on the total load supplied . Your CB will trip if there is an overload ,a fault in the cable ,equipment failure, or an earth leakage. No relay is used for coordination. The CB senses the fault and automatically trips. However, for industrial LV applications such as motor protection, there are relays used.

how will it trip without having a relay to signal it?

Using bimetal? but then how will it differentiate between an overload, an instantaneous, or a short-time short circuit current?

The current flows thru the bimetal element and heats it.

This heating will make it bend. If the current is high enough, it will be enough to mechanically trip the C.B. after a time delay that depends on the current intensity.

An Overload: The % overload will determine the time delay before tripping.

A short-time short circuit: If the current is high enough, it will trip (Heats quickly) quickly after a short delay (you might be looking at milliseconds).

Some of them will have a special coil that will magnetically trip the C.B. in addition to the bimetallic element. This is calibrated for short-circuits.

Go the sites that have been pointed to you in previous replies. You will get more detailed info.

CB has several characteristics/ratings with regard to voltage,rated current,interrupting capacity,making capacity etc When you specify a CB you should specify AF/AT rating-F means frame size and T means trip current range/trip. For a given frame you can change trip range as well as fixed or variable trip. Depending on cable size,load characteristic, load current etc CB should be chosen.

a low voltage CB means, if u r talking about Miniature circuit breaker (MCB), yes a MCB will trip instantly without any external relay, because MCB has inbuilt thermal magnetic bimetalic realy.

if u r taking about 440V conventional ACB's/OCB's it won't trip without protective relays&control wiring(like UV,ST & external thermal magnetic relays). even though now a days without connecting any kind of extranal protective relyas, the inbiult microprocessor based / controlled ACB's are available in market

hope answered right

Of coarse they will trip, that's what they are designed for. There is more than one type. Consider them an electronic fuse. Most can be reset.

I not only trip, I fall apart when I'm overloaded. -- Cirtuit Breaker

In low voltage circuit breakers, the protection elements (both overcurrent & short circuit) are in-built and they are called releases. They are direct acting and do not need a shunt trip coil.

For MCBs the tripping time depends upon its type(B,C,D etc) and the type of load on the MCB