THERMAL TRIP ELEMENT

 THERMAL TRIP ELEMENT

A thermal trip element circuit breaker uses a bimetallic element that is heated by the load current.

The bimetallic element is made from strips of two different metals bonded together. 

The metals expand at different rates as they are heated. 

This causes the bimetallic element to bend as it is heated by the current going to the load.

Above Figure  view A, shows the trip element with normal current. 

The bimetallic element is not heated excessively and does not bend. 

If the current increases (or the temperature around the circuit breaker increases), the bimetallic element bends, pushes against the trip bar, and releases the latch. 

Then, the contacts open, as shown in figure  view B.

The amount of time it takes for the bimetallic element to bend and trip the circuit breaker depends on the amount the element is heated. 

A large overload will heat the element quickly. 

A small overload will require a longer time to trip the circuit breaker.

MAGNETIC TRIP ELEMENT

A magnetic trip element circuit breaker uses an electromagnet in series with the circuit load as in figure. 

With normal current, the electromagnet will not have enough attraction to the trip bar to move it, and the contacts will remain closed as shown in figure  view A. 

The strength of the magnetic field of the electromagnet increases as current through the coil increases. 

As soon as the current in the circuit becomes large enough, the trip bar is pulled toward the magnetic elements.

The amount of current needed to trip the circuit breaker depends on the size of the gap between the trip bar and the magnetic element. 

On some circuit breakers, this gap (and therefore the trip current) is

adjustable.

THERMAL-MAGNETIC TRIP ELEMENT

The thermal trip element circuit breaker, like a delay fuse, will protect a circuit against a small overload that continues for a long time. 

The larger the overload, the faster the circuit breaker will trip.

The thermal element will also protect the circuit against temperature increases. 

A magnetic circuit breaker will trip instantly when the preset current is present. 

In some applications, both types of protection are desired. 

Rather than use two separate circuit breakers, a single trip element combining thermal and magnetic trip elements is used. 

A thermal-magnetic trip element is shown in figure.

In the thermal-magnetic trip element circuit breaker, a magnetic element (electromagnet) is connected in series with the circuit load, and a bimetallic element is heated by the load current. 

With normal circuit current, the bimetallic element does not bend, and the magnetic element does not attract the trip bar, as shown in figure view A.

If the temperature or current increases over a sustained period of time, the bimetallic element will bend, push the trip bar and release the latch. 

The circuit breaker will trip as shown in figure  view B.

If the current suddenly or rapidly increases enough, the magnetic element will attract the trip bar, release the latch, and the circuit breaker will trip, as shown in figure view C. 

(This circuit breaker has tripped even though the thermal element has not had time to react to the increased current.)