TURNING OFF THE MAIN CB INSTANTANEOUS TRIP ON SWITCHBOARDS

engineerintraining12

Senior Member
Location
Saint Louis
I have a question that pertains to selective coordination and turning off the instantaneous function on a switchboards main circuit breaker. For some projects I have turned off the switchboards instantaneous function in order to coordinate with the down stream breakers. I have not thought about this as much until recently, but could this potentially become an issue if the short time delay is set higher than the 3 cycle short time rating of the switchboard? Would the instantaneous override take care of this issue or would this still be a problem?
 

David Castor

Member
Location
Washington, USA
Occupation
Electrical Engineer
If the circuit breakers are molded case circuit breakers built to UL standards, they must have an instantaneous trip. If they are low voltage power circuit breakers, you can get trip units without instantaneous or in some cases turn off the instantaneous.

If the protected equipment had a SC rating based on a 3-cycle fault duration, then technically not having instantaneous trip would be an issue. The other consideration would be arc-flash incident energy. Without the IT, incident energy downstream would likely go up. This is a common tradeoff - coordination vs protection.

Cheers,

Dave
 

engineerintraining12

Senior Member
Location
Saint Louis
If the circuit breakers are molded case circuit breakers built to UL standards, they must have an instantaneous trip. If they are low voltage power circuit breakers, you can get trip units without instantaneous or in some cases turn off the instantaneous.

If the protected equipment had a SC rating based on a 3-cycle fault duration, then technically not having instantaneous trip would be an issue. The other consideration would be arc-flash incident energy. Without the IT, incident energy downstream would likely go up. This is a common tradeoff - coordination vs protection.

Cheers,

Dave

Thanks for the response, yea I'm constantly battling between coordination or protection. Which one would you prioritize? Obviously if it's a hospital, I would want to prioritize coordination, but if you have an industrial facility that doesn't require coordination would you still try and prioritize the coordination?

Also, you can get UL489 breakers, installed in UL891 equipment, which have selectable Instantaneous settings including OFF. When the Inst function is turned off, there will still be an integral 'self-protection', or override, point to comply with the 3 cycle issue.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Thanks for the response, yea I'm constantly battling between coordination or protection. Which one would you prioritize? Obviously if it's a hospital, I would want to prioritize coordination, but if you have an industrial facility that doesn't require coordination would you still try and prioritize the coordination?

Also, you can get UL489 breakers, installed in UL891 equipment, which have selectable Instantaneous settings including OFF. When the Inst function is turned off, there will still be an integral 'self-protection', or override, point to comply with the 3 cycle issue.
The self protection level is often as high as 40kA, so it is not often a problem except when bolted fault current is relatively high, such as at a closely coupled transformer secondary.

IMO it is more important to worry about selective coordination for arcing fault rather than for bolted ones. Bolted faults are usually found during commissioning or power up, while arcing faults are more common during actual use. A risk analysis should be part of the decision process for selectivity.
 

David Castor

Member
Location
Washington, USA
Occupation
Electrical Engineer
Well, the override is more of a semantics game. It's still an instantaneous trip. But if the instantaneous pickup is above the maximum available fault current for the system at that point, then effectively it's out of the picture in terms of protection or coordination (or arc-flash incident energy).

I agree that true bolted faults are not common. But I don't think one can generalize too much about coordination vs protection, especially when arc-flash concerns are factored in. Coordination and protection are generally in direct conflict when choosing settings.
 

topgone

Senior Member
Referring to your TCC, the idea I get is that you are good with allowing the primary protection SMDB-TS taking care of the Instantaneous portion (render the Inst setting of SLA MCB inoperative). They are protecting the same load, i.e. a trip on either of these two breakers will have the same effect: your load losses power!
 

engineerintraining12

Senior Member
Location
Saint Louis
Both SMDB: T-S and SLA MCB are protecting a transformer and also have fixed settings. As you said, a trip on either of these two breakers will have the same effect, which is fine and not an issue. This is just one subfeed coming off the main switchboard that I chose to show because I was trying to find a quick time current curve to provide. There are also 1600 amp and 2000 amp breakers on the main switchboard that feed bus ducts.

I was more concerned with the main breaker (MSB-MCB), which I have set the instantaneous above the max available fault current (turned off the instantaneous) at the main switchboards location. I did this in order to completely coordinate with everything downstream of the main. The short time delay has been set at around 0.4 seconds, which exceeds the 3 cycle rating for switchboards defined under UL 891. I'm concerned that I've sacrificed the switchboards rating for coordination, but as you said, "bolted faults are not common", which I agree with as well.

I did manage to set the short time pickup for MSB-MCB below the worst case arcing fault at the main switchboard, which should help bring down the incident energy a little.
 

paulengr

Senior Member
Instantaneous coordination cannot be predicted but testing results are published as series ratings. That’s in panelboard/switchboard/MCC territory (UL489) where for that reason instantaneous is required. You can also use zone selective interlocking but this is rare. On some switchboards it may be disabled or say that it’s disabled (still with a hidden hardware trip).

With electronic trip units and 3 cycle trip times that are self powered the breaker would require at least 1 cycle to recognize a fault and initiate a trip. The fastest breakers will be 1-2!cycles. There is not enough time to wait for a microprocessor to boot to trip fast enough in an .”instantaneous” situation. This is why the instantaneous trip is typically part of the internal non-user adjustable hardware.

In switchgear disabling instantaneous is common. Coordination is done with TCC curves, not so much series ratings. And switchgear has 30 cycle ratings so plenty of time to coordinate.

Most insulated case and switchgear breakers come with “maintenance switches” for arc flash. Even older gear has this capability in many cases by simply switching the instantaneous trip function on and dialing it down to a setting lower than the arcing current. That is all that most “maintenance switches” actually do anyway.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
If the circuit breakers are molded case circuit breakers built to UL standards, they must have an instantaneous trip. If they are low voltage power circuit breakers, you can get trip units without instantaneous or in some cases turn off the instantaneous.

If the protected equipment had a SC rating based on a 3-cycle fault duration, then technically not having instantaneous trip would be an issue. The other consideration would be arc-flash incident energy. Without the IT, incident energy downstream would likely go up. This is a common tradeoff - coordination vs protection.

Cheers,

Dave
What UL standard requires it?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
But what part? What number? Can you copy and paste a pic of the relavent section?
Have you ever bothered to read UL 489 or even the older NEMA AB1? The NEC is not a detailed/complete product standard.
 

engineerintraining12

Senior Member
Location
Saint Louis
As I understand it a breaker listed to UL489 will protect a switchboard or panelboard listed to UL891.
The main breaker that I am referring to is a Siemens WL circuit breaker. which is performance tested to comply with UL 489. It is protecting a Siemens SB3 switchboard that has been tested to comply with UL 891. Your statement makes sense to me. Otherwise, I can't see the manufacturer intentionally providing an adjustable trip breaker that could potentially disqualify the rating of their equipment.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Like the NEC, I can't remember it all. I frequently ask for code references, articles, ect.
But have you read UL 489?
Probably 90% of your 'application' questions can be found there, especially in regards to breaker performance and construction.

One of the first lessons I learned as an engineer was, knowing where to look up the answer was more important than remembering the answer itself.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
But have you read UL 489?
Probably 90% of your 'application' questions can be found there, especially in regards to breaker performance and construction.

One of the first lessons I learned as an engineer was, knowing where to look up the answer was more important than remembering the answer itself.

Right, but I'm not seeing magnetic tripping in UL 489.
 
Top