CB & Fuse Calibration Chart

[Dennis Alwon]

I found this on an old pdf online and thought it was interesting. Does this seem accurate to you guys. I assume this means a 15 amp cb is tested with 45 amps for 50 sec maximum time to open. Wow-- what gets me is the 225 amp breaker can last almost 4 minutes with 675 amps

 

[jim dungar]

I found this on an old pdf online and thought it was interesting. Does this seem accurate to you guys. I assume this means a 15 amp cb is tested with 45 amps for 50 sec maximum time to open. Wow-- what gets me is the 225 amp breaker can last almost 4 minutes with 675 amps

Definitely seems reasonable.

It is one of our industry's persistent myths, that protective devices operate instantly at 100.0001% of rated current.
People are consistently amazed that devices can carry 200% for many minutes.

Conductor overheating protection is primarily from proper selection based on connected Load. Protective devices are pretty much there just for when catastrophe starts.

 

[Dennis Alwon]

Thanks for the feedback... I had known that the breakers could go awhile without tripping but I was surprised at how long esp for the larger sizes.

 

[zog]

That is similar to the NEMA AB4 standard (See below, sorry about the format, tables never look good when you copy/paste them in here)

These times are "typical" for a 300% test for MCCB's, however the times may vary so it is better (And often required) to use the actual TCC for that specific breaker.

Range of Rated Continuous Current (Amperes) Maximum Trip Time in Seconds
For Each Maximum Frame Ratinga
< 250 V 251 ? 600V
0-30 50 70
31-50 80 100
51-100 140 160
101-150 200 250
151-225 230 275
226-400 300 350
401-600 - - - - - 450
601-800 - - - - - 500
801-1000 - - - - - 600
1001 ? 1200 - - - - - 700
1201-1600 - - - - - 775
1601-2000 - - - - - 800
2001-2500 - - - - - 850
2501-5000 - - - - - 900
6000 - - - - - 1000

 

[steve66]

Seems possible - circuit breakers are short circuit and ground fault protection devices.

They are not overload protection devices.

 

[kingpb]

Seems possible - circuit breakers are short circuit and ground fault protection devices.

They are not overload protection devices.

A typical MCCB provides protection through thermal magnetic tripping, meaning temperature and electromagnetic sensitive to mechanically trip the device.

This translates to overloads over time and instantaneous for short circuits, which include ground faults.

 

[jim dungar]

Seems possible - circuit breakers are short circuit and ground fault protection devices.

They are not overload protection devices.

According to that chart, neither are fuses.

 

[UK Des]

A circuit breaker sized correctly for the cable it is protecting, it can carry currents exceeding that rating for periods that can be quite surprising

Looking at our own time/current graphs I would expect a semi enclosed 100 amp Fuse should carry a current of say 110 amp almost continuously
I don't know about the US but we refer to our time current graphs in our books for these ranges of values of our devices,we refer to them for our 0.1 and 5 sec disconnection times

 

[steve66]

According to that chart, neither are fuses.

I'd agree, at least for the fuses they tested.

If we could really trust circuit breakers and fuses to protect conductors and devices from overloads, I think the NEC would be about half its current size.

For example, there are a bunch of rules mostly intended to limit the load on receptacle circuits. If a 20A breaker actually tripped at 20.1 amps, we wouldn't really need any of those rules. People would learn not to overload receptacle circuits because they would get tired of resetting breakers.

Just as an example, consider the rule requiring 2 Small appliance circuits. That could go away if we could trust breakers.

People who don't cook might get by with one circuit. But people who need 5 crock pots at once while running the microwave and coffee pot would probably have more circuits installed, or find other ways to cope.

 

[don_resqcapt19]

....

Just as an example, consider the rule requiring 2 Small appliance circuits. That could go away if we could trust breakers.

...

If they tripped at 20 amps at time zero, I think we would need a lot more than 2 small appliance branch circuits.

 

[don_resqcapt19]

It would have been nice if the would have used the same percentage of current for both fuses and breakers.