Fault current reduction for lighting control

[winnie]

Martin,

Please keep in mind that I am only now wrapping my head around this. I request that David or Ron tell me what I've missed

I agree with you on the following point: If a fault goes through both the fuse and the breaker, it doesn't matter which is 'closer' to the source. The two devices are in series on an AC circuit, and it will be a toss-up as to which opens first.

However a design distinction must be made between the downstream and the upstream device:
A downstream device is not considered to protect an upstream device; because a short circuit might bypass the downstream device.

If the fuse is downstream of the breaker, then the breaker would be _required_ need to be capable of opening the available fault current at the panel, just as if the fuse were simply not there, because the short circuit might be able to bypass the fuse. The downstream fuse can not be expected to protect the upstream breaker.

If the fuse is upstream of the breaker, and the breaker is _fully rated_ for the available fault current, then again there is absolutely no problem. Either the breaker or the fuse will open, but each will be within its rating, so no safety issue.

The issue of 'series ratings' and 'dynamic impedance' only comes into play if the downstream device is not rated for the available fault current. Now we are asking the upstream device to protect the downstream device. The up-over-down method is not a valid technique for determining if the upstream current limiting fuse will protect the downstream breaker. Only a properly tested 'series rating' will provide this information.

However if you have a properly rated circuit breaker protecting a branch circuit, and a fuse in the branch circuit to provide protection for 'static' downstream devices, then you don't have an issue of the fuse acting to protect the circuit breaker. Because of dynamic impedance it is not clear if the fuse or the breaker would trip first in a short circuit condition, but since the dynamic components (the fuse and the breaker) are both applied within their ratings, the dynamic impedance will not present as a safety flaw in the OCPD selection.

-Jon

 

[davidr43229]

I agree with you on the following point: If a fault goes through both the fuse and the breaker, it doesn't matter which is 'closer' to the source. The two devices are in series on an AC circuit, and it will be a toss-up as to which opens first.

It is not a toss up. You can determine this by the opening time of each overcurrent device.

However a design distinction must be made between the downstream and the upstream device:
A downstream device is not considered to protect an upstream device

Correct. It is the job of the closest upstream overcurrent device to handle the fault or overload.

If the fuse is downstream of the breaker, then the breaker would be _required_ need to be capable of opening the available fault current at the panel, just as if the fuse were simply not there, because the short circuit might be able to bypass the fuse. The downstream fuse can not be expected to protect the upstream breaker.

They both can share the fault IF it is a "Series Rated" combination, and a label of Series Rating is posted within the box. This usually is designed by an Engineer.

The issue of 'series ratings' and 'dynamic impedance' only comes into play if the downstream device is not rated for the available fault current

IMO, dynamic impedence only comes into play with breakers and no other devices. I mean to say that while fuses exhibit this, it is not figurable into any math. Dynamic impedence is only figurable when published by the manufacturer and should be taking into consideration if that devices' job is to inturrupt the fault.

The up-over-down method is not a valid technique for determining if the upstream current limiting fuse will protect the downstream breaker

You are correct! unless that breaker is old or have an actuation (opening, not clearing) time within the 1st 1/2 cycle (.008) seconds

However if you have a properly rated circuit breaker protecting a branch circuit, and a fuse in the branch circuit to provide protection for 'static' downstream devices, then you don't have an issue of the fuse acting to protect the circuit breaker

The downstream branch fuse maybe protecting all the other downstream components, and may not play a roll at all in protecting the upstream breaker, unless again, it says series rated on the panel.

Because of dynamic impedance it is not clear if the fuse or the breaker would trip first in a short circuit condition, but since the dynamic components (the fuse and the breaker) are both applied within their ratings, the dynamic impedance will not present as a safety flaw in the OCPD selection.

Even though both Fuse & Breaker are applied within their ratings, it becomes undetermined as to if they coordinate, unless you are an Engineer. I can not agree or disagree with this. It is determined on a case by case basis.
Just my $.02