600V unfused disconnect switch 35kA

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petersonra

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engineer
Like a bolted pressure switch?

engineering company is demanding I take out a MCCB I put in a system as main disconnect. they want an unfused disconnect switch. they sent me a drawing saying the SCC upstream is just over 22kA so I put in a MCCB since I have not seen an unfused disconnect switch rated that high.

I have never used a bolted pressure switch. Have heard nothing good about them though. Only about 200A.
 

jim dungar

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UL Listed non-fused switches are limited to 10kA fault current, unless protected by a fuse or a specific circuit breaker (effectively a series combination rating).
 

Jraef

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UL Listed non-fused switches are limited to 10kA fault current, unless protected by a fuse or a specific circuit breaker (effectively a series combination rating).
That's correct. So just because your engineer wants it doesn't mean it exists. You need to educate him. Hint: you can get what you want by using a Molded Case Switch, which is s circuit breaker without the trip units. But you ALREADY have the circuit breaker now, so this makes no sense. Is he thinking this somehow messes up his coordination? Because if so, that doesn't make much sense either. You WANT the fault to be interrupted at the lowest level possible.
 

GoldDigger

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That's correct. So just because your engineer wants it doesn't mean it exists. You need to educate him. Hint: you can get what you want by using a Molded Case Switch, which is s circuit breaker without the trip units. But you ALREADY have the circuit breaker now, so this makes no sense. Is he thinking this somehow messes up his coordination? Because if so, that doesn't make much sense either. You WANT the fault to be interrupted at the lowest level possible.

Just a question for you Jraef:

If a Molded Case Switch is just the Molded Case Breaker contacts without the trip unit, are the SCCR and interrupting rating diminished in any way by the fact that during the testing you cannot rely on the trip units to cause the contacts to open?
In other words can the closed contacts tolerate an indefinite period of current at the maximum fault value, since you have no idea when other protection in the system will operate?

Or do you in effect have a series rating situation without any requirement for specific device combinations?
 

Jraef

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Just a question for you Jraef:

If a Molded Case Switch is just the Molded Case Breaker contacts without the trip unit, are the SCCR and interrupting rating diminished in any way by the fact that during the testing you cannot rely on the trip units to cause the contacts to open?
In other words can the closed contacts tolerate an indefinite period of current at the maximum fault value, since you have no idea when other protection in the system will operate?

Or do you in effect have a series rating situation without any requirement for specific device combinations?
No it's not indefinite, it still has to be protected by a SCPD somewhere up stream, and there is still a limit because the SCCR of the MCS is that of the upstream device OR the MCS itself, whichever is LOWER. For example Siemens, whom I am most familiar with having worked there, have a maximum SCCR of 65kA on a 480V MCS (VL Series), OR whatever the up stream SCPD is rated for, not to EXCEED 65kA. So for example if I have a breaker in a switchboard ahead of this MCS, and the rating of that of that breaker is 35kA, I am good to go. If it is 85kA, I'm in trouble.
 

jim dungar

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In other words can the closed contacts tolerate an indefinite period of current at the maximum fault value, since you have no idea when other protection in the system will operate?

Or do you in effect have a series rating situation without any requirement for specific device combinations?

It is common to find Automatic molded cases switches that have an "Instantaneous override" trip point which is the same as their circuit breaker cousins. This is what gives them their high SCCR. This trip point does occasionally give us grief during coordination studies.
 

petersonra

Senior Member
Location
Northern illinois
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engineer
It is not actually installed yet. Just on a drawing. Everytime I send these guys drawings it comes back full of red scribblings, often things that contradict what the red scribblings said on the last go round.

They even complained about the font size used for the panel schedule.
 

Jraef

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It is not actually installed yet. Just on a drawing. Everytime I send these guys drawings it comes back full of red scribblings, often things that contradict what the red scribblings said on the last go round.

They even complained about the font size used for the panel schedule.
I love those guys, they give me something to rant about in the office, keeps my heart rate up so I don't fall asleep and hit my head on the keyboard on slow days...

I just got one yesterday from a Project Manager engineering firm on a job that has been installed and tested, put in full operation a year ago. Now they are demanding that we not only show them UL test reports on EVERY component used inside of a $1million plus MCC project, but they want a list of all testing equipment used in listing it, and calibration reports on all of those instruments. I just had to laugh, it's so ridiculous. We've already been paid so technically, I could just say no and wash my hands of it, but I don't want to leave the poor contractor hanging out to dry for his 5% documentation retention, so today I need to "educate" this engineer with a very long tedious email.
 

Jraef

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Location
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It is common to find Automatic molded cases switches that have an "Instantaneous override" trip point which is the same as their circuit breaker cousins. This is what gives them their high SCCR. This trip point does occasionally give us grief during coordination studies.
Yes, and generally, it is the largest magnetic trip unit available for a given frame size, typically 10X the frame rating. So on a 400AF breaker used as a Molded Case Switch, the instantaneous override will be 4,000A
 
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