RTU SCCR (again)

[jay j]

I ran across this white paper from Johnson Controls printed in 2020, link below.

Johnson Controls White Paper

From the paper (page 2)

"The electrical system designer has the following options when coordinating equipment SCCR and power panel fault currents:"

"Add a true current limiting device (fuse) in the feeder circuit to the equipment. The current limited fuse with a higher SCCR interrupts the current during a fault condition thereby protecting the associated equipment. The fuse’s interrupting current must be sufficient to disconnect the power at the equipment. The current limiting device allows the equipment to be connect to the high fault current power panel because the fuse’s higher SCCR value is used instead of the equipment’s SCCR."

I know this topic has come up here many times, below is a couple of threads from the forum:

example1

example2

So the question of the day, what's your take on the White Paper from Johnson Controls?

I'm more inline with the previous threads.

This is not a problem on project, just something we ran across.

Thoughts?

 

[texie]

The paper is wrong.

 

[jim dungar]

The paper contains just enough truth to be misleading.

Yes you can probably add current limiting fuses, but the fuse size and type need to come from the manufacturer, no one else can choose them.

 

[petersonra]

The paper contains just enough truth to be misleading.

Yes you can probably add current limiting fuses, but the fuse size and type need to come from the manufacturer, no one else can choose them.

you can mark a UL listed control panel to require external CL fuses to get a higher SCCR.

why more of the hvac people do not do this escapes me since it costs them almost nothing other than the label they have to add.

 

[MyCleveland]

you can mark a UL listed control panel to require external CL fuses to get a higher SCCR.

why more of the hvac people do not do this escapes me since it costs them almost nothing other than the label they have to add.

Peter
”you can mark….”.
Is this not true ONLY if all of the internal components have been tested to show compliance with the CL fuse?

 

[petersonra]

Peter
”you can mark….”.
Is this not true ONLY if all of the internal components have been tested to show compliance with the CL fuse?

Not entirely. There is a part of the ul sccr procedure that allows you to take credit for the reduced letthru of the cl fuse in at least some cases. Depending on the fuse rating it might or might not be helpful.

 

[MyCleveland]

Not entirely. There is a part of the ul sccr procedure that allows you to take credit for the reduced letthru of the cl fuse in at least some cases. Depending on the fuse rating it might or might not be helpful.

"allows you...."
Is this the "under engineering supervision" or is it something UL grants to someone via a training class.

 

[petersonra]

"allows you...."
Is this the "under engineering supervision" or is it something UL grants to someone via a training class.

It is part of the UL 508a standard. You can only use it if you are listing a control panel to the standard.

Take a look at UL508a SB4.3.3 for when you can use a CL fuse let thru value as the short circuit current.

SB4.3.4 allows for the CL fuse to be outside the panel if marked.

SB4.3.4 The specified circuit breaker marked “current limiting” or current-limiting Class of fuse supplied
in the feeder circuit that limits the peak let-through current available in accordance with SB4.3.2 and
SB4.3.3 is able to be provided in the field when the panel is marked in accordance with SB5.1.3.

 

[MyCleveland]

It is part of the UL 508a standard. You can only use it if you are listing a control panel to the standard.

Take a look at UL508a SB4.3.3 for when you can use a CL fuse let thru value as the short circuit current.

SB4.3.4 allows for the CL fuse to be outside the panel if marked.

This is allowed by UL without checking internal cabinet components SCC ratings are less than let-through value of the CL fuse for a known SCC at the panel?

 

[petersonra]

This is allowed by UL without checking internal cabinet components SCC ratings are less than let-through value of the CL fuse for a known SCC at the panel?

The SCCR of the component has to be higher than the let thru rating of the fuse. Presumably you would check to make sure, but a lot of things have a default rating that exceeds the let thru of small CL fuses.