Discrimination and cascading

[Designmen90]

Well, I'm a little bit confused
I have two circuit breakers, upstream CB is NSX630F (36kA Icu at 380/415 VAC 50/60), while downstream is IC60N (10kA Icu 220...240 VAC 50/60). They are in total dicrimination, BUT - IC60N cannot operete in cascading mode with NSX630F. WTF?

 

[pc1966]

When you say "cannot operate in cascading mode" do you mean that when cascaded you fail to meet the main supply PFC break requirements that you expected from the NSX630F?

When you have two OCPD in cascade the break limit is based on which one has to actually interrupt the supply, and perversely when you have "total selectivity" that becomes the smaller one!

 

[Designmen90]

When you say "cannot operate in cascading mode" do you mean that when cascaded you fail to meet the main supply PFC break requirements that you expected from the NSX630F?

When you have two OCPD in cascade the break limit is based on which one has to actually interrupt the supply, and perversely when you have "total selectivity" that becomes the smaller one!

No idea, ECOStructure told me that.

 

[pc1966]

Well the simple answer is if your DB PFC is below 10kA for the iC60N then you are OK, as it can break the load worst-case.

 

[pc1966]

Just to add - your worst-case PFC should be below the 10kA for the iC60N

 

[Lucien Nunes]

Cascading requires some action by the upstream breaker to limit the current that the downstream one would have to break, whether it is a momentary contact separation or actual triggering. Therefore any current >Icu at the downstream breaker needs to trigger that action. A breaker pairing that is not valid for cascade protection could allow a fault to occur that does not trigger current limitation upstream but cannot be broken downstream.

 

[pc1966]

whether it is a momentary contact separation or actual triggering.

Which models do the momentary separation trick?

 

[Designmen90]

If I have for example upstream CB from ABB, and dowstream from Siemens, how to know are they compatabile for cascade protection?

 

[pc1966]

If I have for example upstream CB from ABB, and downstream from Siemens, how to know are they compatible for cascade protection?

I expect you would have to ask them, as it depends very much on the trip characteristics (i.e. if the upstream breaker is going to current-limit the fault with the downstream one).

It is usually easier with upstream BS88 (or equivalent) fuses - most companies give to the cascading / selectivity figures for that arrangement.