Disconnect Fuse / CB

[Rson]

For those of you that design / build panels:

Do you typically provide a fuse / circuit breaker on your main disconnect inside the panel?

We always have, but I've seen some panels we purchased from other vendors and not one of them has a main fuse. The motors / transformers are all protected, but there is only a disconnect on the main - no protection.

Looking through the code, it appears it is not required. Is this typically something the customer / building would state? I would assume it is better to have than not - but it is also more expensive.

Thoughts?

 

[the_msp]

Bear in mind I'm in a different country with different regulations.

Also, note most of my panels are low power consuming (e.g. the main MCB would be 6A B).

So for me, no.

The wires enter the panel and are protected by the MCB in the distribution board. They go to incoming terminals which then take them to the isolator. Out of the isolator to another set of terminals, generally jumpered, and then off to their protection as required, be it fuse or MCB.

 

[ASF]

Bear in mind I'm in a different country with different regulations.

Also, note most of my panels are low power consuming (e.g. the main MCB would be 6A B).

So for me, no.

The wires enter the panel and are protected by the MCB in the distribution board. They go to incoming terminals which then take them to the isolator. Out of the isolator to another set of terminals, generally jumpered, and then off to their protection as required, be it fuse or MCB.

Same here, with the same disclaimer.

The main disconnect is an isolation point for working on the panel or it's equipment. Circuit protection is provided by the fuse or circuit breaker in the DB supplying our panel, as long as both the main switch and any wiring between the main switch and the downstream protective devices are rated for the upstream circuit protection, there is no requirement to do so (in Australia, anyway).

Occasionally we'll use a circuit breaker as the main switch, but this is only in rare cases where a specific quirk of the application requires it, or in even rarer cases where the customer requires it.

Edit to add: in the occasional cases where we come across a panel that does have a fused disconnect (usually imported from our friends in the states), my experience has been that they're pointless anyway. Proper fault discrimination is never taken into account when a supplier ships a generic panel with a fused disconnect, so invariably you'll wind up with the upstream breaker tripping first regardless.

 

[JesperMP]

One reason for having a fused disconnect or MCCB at the panel entry could be for lowering the short circuit current after the disconnect, thus allowing components and wires with the lower SCCR rating in the panel.

That being said, what is mostly done is to not have such a fused disconnect or circuit breaker, instead merely specify the SCCR rating of the panel and the customer must then make sure that his distribution board limits the short circuit current at the panel.

I d not know if there are other regulations in the US maybe relating to arc flash protection that could mean you must have a fused disconnect or circuit breaker at the panel entry.

 

[rupej]

I almost always fuse the disconnect, but that's more for SCCR and CYA. It's redundant if you think about it.

 

[Damaged]

Why would you add a fused disconnect for an SCCR rating, isn't the SCCR rating scored by the lowest rated component in the enclosure. So a 5KAIC rated breaker down stream of a fused disconnect rated at 65KAIC would still result in a 5KAIC score on the name plate, isnt this correct?

 

[Rson]

Why would you add a fused disconnect for an SCCR rating, isn't the SCCR rating scored by the lowest rated component in the enclosure. So a 5KAIC rated breaker down stream of a fused disconnect rated at 65KAIC would still result in a 5KAIC score on the name plate, isnt this correct?

I'm trying to wrap my head around that as well.

The only thing the fused disconnect would do is protect the wires from the disconnect to the other fuses or circuit breakers.

 

[JesperMP]

Why would you add a fused disconnect for an SCCR rating, isn't the SCCR rating scored by the lowest rated component in the enclosure. So a 5KAIC rated breaker down stream of a fused disconnect rated at 65KAIC would still result in a 5KAIC score on the name plate, isnt this correct?

Certain circuit breakers have current limiting abilities. It means that they disconnect so rapidly that they interrupt the current before it reaches its peak value. So downstream of the circuit breaker the SCCR rating can be lower.
Fuses does the same thing.
The name plate of the panel is for the supply cable feeding the panel.
Generally for smaller currents, circuit breakers works best, and may be easier to coordinate. For larger currents fuses works better.
This is not trivial matter. I know the principles of it, but I am not up to actually dimensioning a control cabinet and its SCCR rating.
Also, I dont know if there are other regulations in the US.

edit: For completeness sake.
yet another way to reduce the SCCR rating in the panel is to use a transformer or power supply.
This is very common, and one of the reasons for using a transformer for control voltage in the first place. Think about this, you have some huge copper bus-bars from the main power supply. From these bus-bars you split into many consumers including the controls. Do you think that the thin wires of the controls can survive just as long as the thick bus-bars in case there is a short circuit close to where the control voltage is split from the main power and therefore experience the full short circuit current ?

 

[JesperMP]

The only thing the fused disconnect would do is protect the wires from the disconnect to the other fuses or circuit breakers.

I have experienced once that it was necessary to have an intermediary common circuit breaker to reduce the short circuit current before the normal circuit breakers further downstream. The smaller circuit breakers simply did not have a sufficient rating to match the SCCR that the customer demanded because of limitations on his side.
A fused disconnect could do the same thing as the higher rated circuit breaker.

 

[bill4807]

One reason for having a fused disconnect or MCCB at the panel entry could be for lowering the short circuit current after the disconnect, thus allowing components and wires with the lower SCCR rating in the panel.

That being said, what is mostly done is to not have such a fused disconnect or circuit breaker, instead merely specify the SCCR rating of the panel and the customer must then make sure that his distribution board limits the short circuit current at the panel.

I d not know if there are other regulations in the US maybe relating to arc flash protection that could mean you must have a fused disconnect or circuit breaker at the panel entry.

I almost always use fused disconnects or MCCB. Jespers reason here explains why.
If my panel is rated at 30KA and the service feed can supply 65KA...
Some customers know what their service cababilities are alot of others don't. I found if I stay above 65KA that surpasses most of our customer ratings. I just add it, yes it is a little extra but it beats scrambling in a case where everybody is onsite and the electrician cannot hook it up, maybe because the inspector wont let him.
It all depends on what is worked out between your company and the customer.

 

[bill4807]

Isolation transformer can bring it down too,
As Jesper mentioned later on.

 

[Damaged]

Bill,

That what my question is about. Why would you add a fused disconnect or MCCB for an SCCR rating, isn't the SCCR rating scored by the lowest rated component in the enclosure. So a 5KAIC rated Branch breaker down stream of a fused disconnect rated at 65KAIC would still result in a 5KAIC score on the name plate, isnt this correct?

 

[bill4807]

Bill,

That what my question is about. Why would you add a fused disconnect or MCCB for an SCCR rating, isn't the SCCR rating scored by the lowest rated component in the enclosure. So a 5KAIC rated Branch breaker down stream of a fused disconnect rated at 65KAIC would still result in a 5KAIC score on the name plate, isnt this correct?

Hi Damaged,
From what i understand,
Yes it is scored by the lowest component if you are not using combination components or any of the exceptions where you can increase the SCCR value of the downstream circuits.
Transformer and current limiting fuses are two of those, to determine peak let through.

If you find any document that says otherwise please let me know.

 

[rupej]

Bill,

That what my question is about. Why would you add a fused disconnect or MCCB for an SCCR rating, isn't the SCCR rating scored by the lowest rated component in the enclosure. So a 5KAIC rated Branch breaker down stream of a fused disconnect rated at 65KAIC would still result in a 5KAIC score on the name plate, isnt this correct?

Only if you are not using current limiting fuses or CBs. Most designers I see DO use current limiting devices.

 

[ndzied1]

Sometimes a device will have a note like this “ Short-circuit rating achieved when used with respective fuse type and maximum fuse rating”

This is for an Eaton R9 type rotary disconnect switch. The 30A disconnect has a 100kA Short circuit rating with class J 30A fuses. Without it, you get the default rating for a switch which is 5 or 10 kA…