Fusible or Non fusible Isolator

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Sajid khan

Sajid khan

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Pakistan
In my project there is a MCC feeding different mechanical loads and near to the load is Isolator for equipment. What is the better choice, to use Fusible isolator or non fusible isolator though i have protection in MCCs for the load.

It will be very helpful if answer is provided with NEC code reference.


Regards,
Sajid Khan
 
If your circuits have a high available fault current you will need to use fused switches. As far as I know non-fused disconnect switches are only suitable for use on circuits where the available fault current is 10,000 amps or less.
 
don_resqcapt19 said:
If your circuits have a high available fault current you will need to use fused switches. As far as I know non-fused disconnect switches are only suitable for use on circuits where the available fault current is 10,000 amps or less.
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For the most part, you are correct.
However some of the non-fused versions have series combination ratings with upstream devices, but usually not the small "IEC style" disconnects.
 
jim dungar said:
For the most part, you are correct.
However some of the non-fused versions have series combination ratings with upstream devices, but usually not the small "IEC style" disconnects.
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Jim,
That is good information. I was under the impression that series combination ratings only applied to OCPDs in series.
 
don_resqcapt19 said:
Jim,
That is good information. I was under the impression that series combination ratings only applied to OCPDs in series.
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I want to think there is a little bit of issue with word choice, possibly something like:
Protective devices are series rated.
Non protective devices are series combination rated.

For the Square D non-fused safety switch product line, there is a chart in the Digest showing ratings when the upstream device is a specific circuit breaker (e.g. same rating) or a type of fuse.
Other manufacturers publish similar information.
 
The marking requirements in 110.22 use the terms:
CAUTION ? ENGINEERED SERIES COMBINATION SYSTEM RATED ...
or
CAUTION ? SERIES COMBINATION SYSTEM RATED ...
 
Also keep in mind that the available fault current at the load end of the circuit is going to be less then it is at the MCC. If the circuit is pretty short in length it may not be reduced enough though.
 
Like Don, I thought the series rating was in terms of the overcurrent device (fuse/circuit breaker).


I wonder if the manufacturers have since been able to get the non-fused disconnects listed as part of a Series Rated assembly? Or something like that.

I for one believe we are going to see more and more Series Rated installations as those designing them become more comfortable with putting their names on them.
 
Pierre C Belarge said:
I wonder if the manufacturers have since been able to get the non-fused disconnects listed as part of a Series Rated assembly? Or something like that.
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I am most familiar with Square D products, here is some information directly from product catalog - this come from the heavy duty 600 volt non fused safety switch section of catalog.



Note table 3.12 indicates a general SCCR of 10kA, but if the switch is protected by a class J or R fuse it can be used up to 200kA.

Have to dig deeper but I also believe it doesn't matter if the protection is before or after the switch - which allows you to use them on service conductors ahead of the service disconnecting means like when you need a disconnect for a utility company meter.

Oops, table 3.12 is for fusible switches - but if using class J or R fuses you still can use with available current of 200kA, just have to look at the other table. And that makes sense to me - they use the same switch in both units, they just don't put fuse holders in the non fusible switches.
 
Pierre C Belarge said:
I wonder if the manufacturers have since been able to get the non-fused disconnects listed as part of a Series Rated assembly? Or something like that.

I for one believe we are going to see more and more Series Rated installations as those designing them become more comfortable with putting their names on them.
Click to expand...

The Square D has listed series combination ratings for non-fused safety switches since at least 1992. I am sure other manufacturers have also been doing so for +20yrs.

Other product lines that routinely use, or have, series combination ratings include: Multi-meter centers, Motor Control Centers, and Motor starters. A common theme seems to be a protective device in series with components that are connected by 'wire'.
 
If it meets requirment

If it meets requirment

If it meets fault requirment use non fused, less troubleshooting and problems due to one fuse here blown and one fuse there blown.
 
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