Dear all,

I want to investigate a curious finding that happened at our site few days back.

We have a PLC Rack which is supplied through a standard 2 Pole, 2A, Schneider Electric IC60N MCB. The breaker tripped and the PLC shutdown which impacted a critical process.

When we were checking why the breaker tripped, we couldn't find any fault. However, we noticed that when we turned on the breaker and monitored the amps drawn through a meter, the current was 2.4A(The breaker is rated at 2A). We checked the load after various intervals and it was 2.4A consistently.

The breaker had been installed 5 years back and we think since then it has been running at 2.4A and not tripped. We did an experiment where we installed an identical new IC60N breaker and this one, with equal load and noticed behavior. Both kept on running on 2.4A for 5 hours. When we applied heat on them through a temperature gun, the old breaker tripped, while the new one remained on.

It is also worth noting that the breaker is AC whereas the circuit it is installed is on 24VDC.

I want to know why a breaker which was running on 2.4A since 5 years suddenly trip now? What things I should investigate more?

Looking forward to your insights.

Regards,
Mutahir

 

MRasool...
Arcing is always expected as the contacts part and in many situations it can be quite apparent. Contact-life is a function of the arc-magnitude and arc-duration.
When used on AC service, magnitude and duration vary... from max to zero. And, of course' current is zero every half cycle.
When used on DC, magnitude and duration are always maximum ! Thereby shortening contact life expectancy.
Phil Corso

 

What things I should investigate more?
I'd be more concerned why a circuit is taking 2.4Amps through a 2Amp MCB....

Which is wrong - exessive current or underated MCB.

 

MRasool...
Arcing is always expected as the contacts part and in many situations it can be quite apparent. Contact-life is a function of the arc-magnitude and arc-duration.
When used on AC service, magnitude and duration vary... from max to zero. And, of course' current is zero every half cycle.
When used on DC, magnitude and duration are always maximum ! Thereby shortening contact life expectancy.
Phil Corso

Thanks Phil.

But since the breaker has been ON since 5 years, as the PLC supplies a critical service(which we can't shutdown) there is no 'contacts parting' experienced on the MCB.

Regards,
Mutahir

 

What things I should investigate more?
I'd be more concerned why a circuit is taking 2.4Amps through a 2Amp MCB....

Which is wrong - exessive current or underated MCB.

That's a whole new debate. The design team looks after sizing. As a maintenance guy, I know the load is consistent at 2.4A and the 2A breaker is supplying that load since some years without any issue.

However, we have installed a 4A breaker now.

Regards,
MRasool

 

MRasool...
Open the subject MCB and reveal the condition of its contacts !
Regards,
Phil Corso