# Close and Latch Rating



## Lone Crapshooter

What is the Close and Latch Rating refer to on MV switchgear. Saw that term for the first time on a switchgear data plate today. The rating is in amps.
Thanks 
LC


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## AU Facilities

Breaker Open to Close = Close & Latch

Close & Latch is basically the maximum current that the switch can close on.


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## frenchelectrican

Basically it means it will close and lock in postion on max rating of current.

Breakers and knife switch have simauir ratings. 

But there is other rating is opening rating which it is backward of close and latching.,, it mean it can open at max amps ( most case oil circuit breakers is common on this apps .,, but conventail air circuit breakers can do same but have arc flash chute. but some knife switch are not made to open under load )


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## Bird dog

frenchelectrican said:


> Basically it means it will close and lock in postion on max rating of current.
> 
> Breakers and knife switch have simauir ratings.
> 
> But there is other rating is opening rating which it is backward of close and latching.,, it mean it can open at max amps ( most case oil circuit breakers is common on this apps .,, but conventail air circuit breakers can do same but have arc flash chute. but some knife switch are not made to open under load )


I think I've seen one of those. It was marked "Isolation Switch / Do Not Open Under Load".


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## JRaef

Bird dog said:


> I think I've seen one of those. It was marked "Isolation Switch / Do Not Open Under Load".


Similar theoretical issues behind it, but different.

Close and Latch ratings have to do with the fact that the breaker / switch is transitioning from Open to Closed and you are closing it into a fault condition. So if the rating is 65kA for example, you can close that breaker into a fault that is going to draw 65,000 amps the instant that breaker closes, and it's OK with that. But if the fault current is 85,000A, what might happen is that the magnetic forces involved in that instantaneous current might cause the contacts to force open, so the breaker will not "latch" closed. That then means the closing mechanism might not cease trying to make it close, and it chatters (briefly) which can make it blow up, literally. In switches, this is also referred to as "Fault Make" rating. 

The Interrupt Rating of the breaker has to do with the breaker being ALREADY closed when the fault takes place, and when the breaker is going to open the circuit (Closed to Open transition), it will not become a bomb up to that rating. What you typically see in MV gear is that the Close and Latch rating is a lower value than the Interrupt rating, so that is the rating you should consider when looking at your available fault current. 

An "Isolation Switch" is a switch that has neither, because it is NEVER going to be put into that kind of situation. So the only place you can use that is if, for example on a motor starter, where there is a contactor that is going to do the opening and closing of the circuit. So that Isolation Switch will have an Aux contact added to the mechanism that always opens BEFORE the main contacts open, and that Aux contact is put into the coil circuit of the contactor. So as you start to move the disconnect handle, the Aux contact opens first, the contactor opens,THEN the Isolation Switch opens AFTER the circuit is already dead.


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## Lone Crapshooter

Thanks for the quick response now I know what it means. Like I mentioned earlier I had never seen that term until today. Live and learn. 
Thanks again 

LC


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## MechanicalDVR

JRaef said:


> Similar theoretical issues behind it, but different.
> 
> Close and Latch ratings have to do with the fact that the breaker / switch is transitioning from Open to Closed and you are closing it into a fault condition. So if the rating is 65kA for example, you can close that breaker into a fault that is going to draw 65,000 amps the instant that breaker closes, and it's OK with that. But if the fault current is 85,000A, what might happen is that the magnetic forces involved in that instantaneous current might cause the contacts to force open, so the breaker will not "latch" closed. That then means the closing mechanism might not cease trying to make it close, and it chatters (briefly) which can make it blow up, literally. In switches, this is also referred to as "Fault Make" rating.
> 
> The Interrupt Rating of the breaker has to do with the breaker being ALREADY closed when the fault takes place, and when the breaker is going to open the circuit (Closed to Open transition), it will not become a bomb up to that rating. What you typically see in MV gear is that the Close and Latch rating is a lower value than the Interrupt rating, so that is the rating you should consider when looking at your available fault current.
> 
> An "Isolation Switch" is a switch that has neither, because it is NEVER going to be put into that kind of situation. So the only place you can use that is if, for example on a motor starter, where there is a contactor that is going to do the opening and closing of the circuit. So that Isolation Switch will have an Aux contact added to the mechanism that always opens BEFORE the main contacts open, and that Aux contact is put into the coil circuit of the contactor. So as you start to move the disconnect handle, the Aux contact opens first, the contactor opens,THEN the Isolation Switch opens AFTER the circuit is already dead.



Great info as usual very informative! :thumbsup:


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