240.85, why ?

augie47

augie47

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Tennessee
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State Electrical Inspector (Retired)
I understand the Code does not allow slash-rated breakers on non-neutral systems. Is the primary reason only that they have not been tested and listed for the higher fault to ground voltages.
It would seem that if the breaker can withstand a phase to phase fault, it would see the same situation in a phase to "ground" short.
 
augie47 said:
I understand the Code does not allow slash-rated breakers on non-neutral systems. Is the primary reason only that they have not been tested and listed for the higher fault to ground voltages.
It would seem that if the breaker can withstand a phase to phase fault, it would see the same situation in a phase to "ground" short.
Click to expand...
In a slash rating, a L-L fault is cleared by 2 poles while still only seeing the lower L-G voltage.
In a full rating, the L-G voltage is higher.
 
I've often wondered about this myself.

Additional thoughts in regard to this is if short conductor distance to the source sometimes the available fault current is higher line to neutral than line to line. That goes away pretty rapidly with conductor length though. And is complicated by the fact there may still be more incident energy in the line to line fault due to the voltage being 1.73 to 2 times higher than line to neutral voltage. Kind of wondering what is actually different inside the breaker. Seems possible there is little or even no difference, just a label they can make more $$ off of by applying it to the unit even though the non labeled ones are exactly the same. I seen QO breaker manufacturing years ago. Other than what component differences is needed to restock the assembly equipment with when making say a 20 vs a 30 amp breaker, I don't see it being worth keeping any other separate parts in inventory if the same part works either way and the added risk of an employee adding the wrong ones to a run of product. In QO line those components are all the same for 20 and 30 amp breaker other then the trip elements themselves. Even those are pretty much same but calibrated differently so still could be easy for an employee to make a mistake just looking at the physical part itself.
 
jim dungar said:
In a slash rating, a L-L fault is cleared by 2 poles while still only seeing the lower L-G voltage.
In a full rating, the L-G voltage is higher.
Click to expand...
I guess that makes sense.

Is still possible to have line to line fault to another circuit on either system and if that other circuit is higher capacity and doesn't open in the event, you still have clearing by one pole. Maybe rare but can happen.

Next question is do they actually use different components, particularly in the smaller frame breakers or do they price gouge your for a label?
 
jim dungar said:
In a slash rating, a L-L fault is cleared by 2 poles while still only seeing the lower L-G voltage.
In a full rating, the L-G voltage is higher.
Click to expand...
Too short of answer.
In a high leg system the L-G voltage can be √3 higher than the tested interrupting voltage. The fault voltage rating is dependent on internal breaker construction involving gasses and arcs.

A full rated breaker breaker is designed to work on ungrounded and/or corner grounded systems where L-L voltage is often interrupted even when only 1 pole is involved.

So we have full rated and slash rated breakers due to manufacturing differences.
 
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