Solving SCCR deficiency by lowering available fault current

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rplecha

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Question: Why not use current limiting fuses at the installation to lower available fault current to a panel? This is more of an NEC question than a UL question.

If the fused disconnect is not part of the control panel in question, why couldn't one just use the manufacturers data on the current limitiing fuses and consider the fused disconnect a new power source for the control panel. This way the available fault current to the control panel is determined by what is truly available from the load side of the fuses rather than that useless SB2.2 table. My whole angle on this is that adding the fuses outside means that it is not part of the control panel and maybe is not strictly bound by the infamous UL508, so then the manufacturers data for say J fuses could be used, easily obtaining an available fault current to the panel <10ka. Any thoughts on this?
 
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rplecha said:
Question: Why not use current limiting fuses at the installation to lower available fault current to a panel? This is more of an NEC question than a UL question.

If the fused disconnect is not part of the control panel in question, why couldn't one just use the manufacturers data on the current limitiing fuses and consider the fused disconnect a new power source for the control panel. This way the available fault current to the control panel is determined by what is truly available from the load side of the fuses rather than that useless SB2.2 table. My whole angle on this is that adding the fuses outside means that it is not part of the control panel and maybe is not strictly bound by the infamous UL508, so then the manufacturers data for say J fuses could be used, easily obtaining an available fault current to the panel <10ka. Any thoughts on this?
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One reason, is that fuses do not always clear faults faster than other overcurrent protective devices may begin to operate. This is why most devices, with automatic contacts, must be a "tested combination" when used in series with fuses.
 
Let's say the feeder and branch circuits in the control panel are all fused and properly selectively coordinated. Let's then say that the branch circuits all contain one contactor protected by branch fuses. The contactor has no combination rating with the fuses so it assumes an SCCR of 5KA thereby making the SCCR of the entire panel 5KA. Here's the fusing: branch HSJ20, feeder HSJ40. Now, in the field, the panel is to be installed where the available fault current is 50K RMSSA. The peak let thru plot for the Ferraz HSJ shows that the load side of a HSJ80 has an available fault current of 2500 RMS symmetrical amps when the line side has 50K RMS sym amps available. So...does anyone know if this would be rejected and why.
 
rplecha said:
Let's say the feeder and branch circuits in the control panel are all fused and properly selectively coordinated. Let's then say that the branch circuits all contain one contactor protected by branch fuses. The contactor has no combination rating with the fuses so it assumes an SCCR of 5KA thereby making the SCCR of the entire panel 5KA. Here's the fusing: branch HSJ20, feeder HSJ40. Now, in the field, the panel is to be installed where the available fault current is 50K RMSSA. The peak let thru plot for the Ferraz HSJ shows that the load side of a HSJ80 has an available fault current of 2500 RMS symmetrical amps when the line side has 50K RMS sym amps available. So...does anyone know if this would be rejected and why.
Click to expand...

The contactor has contacts that may be opening or closing during a fault. If this happens the arc/contact resistance may impact the clearing time and resultant let through current of the fuse. No one can say without testing.

Where did you find a contactor that does not have a series combination with a Class J fuse?

Where is would your control panel be that it would have 50kA available at the end of a 80A feeder circuit?
 
Presently the only contactors which have listed combinations with j fuses or fuses period are Allen Bradley. Every other combination listed by UL is for a motor starter and includes the overload.

My panel is in a location where the service has a fault current of 50ka available.

What I said was...I should have 2.5ka available at the end of the 80amp feeder per the fuse manufacturers data. Therefore the 5ka panel would be protected.
 
Like I've been trying to tell Jim though, is that the contactor is a passive component and it would not be opening due to an overcurrent like a motor starter with an overload would be.
 
rplecha,

Per NEC 110.10

Listed products applied in accordance with their listing shall be considered

to meet the requirements of this section.
 
rplecha said:
Like I've been trying to tell Jim though, is that the contactor is a passive component and it would not be opening due to an overcurrent like a motor starter with an overload would be.
Click to expand...

Are non-latching contactors considered passive devices? What happens if the contactor closes onto a fault? Will the contacts develop enough pressure to appear closed or will they bounce and create an arc?

The problem with motor starters and SCCR is not that the OL relay will cause the contactor to try to open but rather that the OLR element itself will fail.
 
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