Dear All

There is a very curious case that occurred at our site against which I am sharing details. Will appreciate if others can pitch in if they have had similar experiences.

We have a few IC60N 2A C Type MCB breakers (manufactured by Schneider Electric) installed at a critical PLC panel installed at our site. Recently we faced an issue of tripping of one of these breakers. During investigation it was observed that 2.4 Amps were flowing through the breaker however what was concerning was that breaker has been in service for the past 4 years approximately without tripping since. We have checked and load has been constant throughout. We have the following 3 queries for which we need support from this forum members.

1. What is the tolerance margin available for these breakers
2. Can voltage rating of application affect the operation of these breakers? Breaker is rated for 440VAC whereas it had been installed on a 24VDC circuit.
3. Do these breakers have ambient temperature compensation in them? We carried out thermography after taking load back in service and maximum observed temperature rise on the breaker was found to be 38 degrees Celsius. On the breaker datasheet it states that the reference temperature of the breaker tripping curve is 50 degrees Celsius. Is it possible that due to low ambient around the breaker, the heat generated by the flowing current was less to prevent the bi-metallic element initiate tripping of the breaker?
4. Is it possible that due to continuous operation near the rated condition the Bi-metallic strip has faced wear and tear overtime and ultimately resulted in nuisance tripping.

We carried out bench test of a new breaker of similar rating and this breaker and the results of bench test are also attached with this post. Can anyone shed some light on the above queries.

 

I'm a little mystified why you are doing any tests at all.
Your tabulated results are exactly what I would expect from your experiments, with the thermal effects affecting the older MCB.

You can repeat the tests with a 2amp fuse - except the higher overload currents will break faster.

The tolerance is as the 'spread' on the 'C' curve graph
Voltage is of no consequence unless you are breaking supplies above 440vAC (note no DC rating at these high voltages)
Ambient temperature can make a difference - some enclosures I worked with reach 45Deg C.


Is it possible that due to continuous operation near the rated condition the Bi-metallic strip has faced wear and tear overtime and ultimately resulted in nuisance tripping.
No - not even in elevated temperatures as above and rated for the circuit.

 

Rafay Usman...
1) What is the Voltage-drop measurement, Vd, across the contacts of a closed breaker in service?
2) What is the Resistance measurement, Rc, across the contacts of a closed breaker not in service?
Regards, Phil Corso

 

Initially I carried out thermography with the breaker installed within the cabinet with only the front of the breaker in view. Temperature was only 38 Degrees C as mentioned above. However during bench testing of the breaker with the breaker sides also available for trending the temperature recorded was 80 Degrees C throughout the testing for both New and Old breaker. This led to me question the affect of prolonged heat on Bimetallic strip health.

Do you think this could be a probable cause for wear and tear?

 

Hi Phil,

I have not yet measured these parameters but will measure and let you guys know