overload relays

Thread Starter


Joined May 22, 2013
I was reading basic electricity by valkenburgh and on page 5_68, he talks about overload srelay. The part i dont understand is when he says " in many circuits you will see overload relay contacts in series with the line. These contacts are normally closed, but of the temperature of the device becomes excessive, the thermal relay will close causing the overload relay to become energized and opening the line." The part i dont understand is" the thermal relay will close, causing the overload relay to become energized and opening. The line." It seems as if the thermal relay and overload relay are two separate relays, with the thermal relay closing to provide a switch to activate the elecyromechanical relay to open the overload contacts.


Joined Jun 22, 2012
He will be talking about a Circuit breaker, there are several types and ratings depending on your Country, here is a picture of the inside of one.. Breaker

The relay is item 7, and contacts item 3, the relay is in series and is Current activated which trips the lever item 1 and disconnects the contacts item 3.
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maybe "thermal relay" means the instance where the relay is at "this" point in time (thermal or hot) since excessive temperature is said. then goes to the next point in time, "overload relay" where the components melt due to high temperature and produces an open..

this is as far as i understood it..^_^


panic mode

Joined Oct 10, 2011
no, there is no melting involved, think bi-metal. melting is irreversible, while bimetal returns to initial position after cooling off.

circuit breakers can open by both magnetic and thermal trip. cheap products that do not have both tripping mechanisms do no qualify as circuit breakers and can be only used as a supplementary protection.

overload relays work like circuit breakers but have different physical form and electrical characteristic (circuit breakers are designed to trip as fast as possible when overload condition is encountered, overload relays trip after overload condition lasted for specific time, this delay and sensitivity are often user adjustable). usually overloads are designed to mount directly (piggyback) onto contactor (contactors are type of large relay). another difference is type of reset (overloads can have manual, auto or both).

the wires sticking out on the top side go into terminals of contactor of proper size:


here is an assembled version (contactor + overload). overload is the part on the bottom:


Joined Jul 18, 2013
One typical application is the O/L's fitted on the output side of 3ph motor contactors etc.
At one time, these were commonly thermal in nature and different element sizes could be mounted in order to change a O/L value.
Now, typically they are electronic in nature and when an over load occurs, a N.C. auxilliary contact that is wired in series with the contactor coil circuit, opens, thereby dropping the contactor out of circuit, another contact typically available is a N.O. contact which can be used at the installers discretion for alarm or indication usage.
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