Circuit Breaker Settings
- Thread starter 1965mt
- Start date
- Location
- Illinois
- Occupation
- retired electrician
That is typically specified by the design engineer.
In a perfect world yes
What I am asking is what tools does the design engineer use? I know the connections and all other pertinent information.
The reality that I am finding is that the design engineer tells the electrical contractor. Then someone adjusts the settings until it quits tripping.
Not looking to get it designed here, just looking for information.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Those tools would be the manufacturer's Time Current Curves for that breaker and any other breakers or fuses it needs to coordinate with. Many design professionals use the curves found in software packages, but coordination can be done manually using printed curves.
Most importantly, someone also needs to make the decision on what level of selective coordination is required and at what current levels.
I have the curves for the breaker. I have also reached out to the manufacturer.
What is the thought process regarding how to set? I know all the connected loads, FLA, etc... I also know all the wire sizes, and lengths.
There has to be a rational behind where to set the times. Like augie47 wrote, my experience is to set until it quits tripping. I have also reached out to the specific designers and others I have worked with in the past. There does not seem to be a solid method.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Coordination is an art and a science.
If you google protective device coordination, you will find many references that provide the basic understanding. This is a specialty, and not just something that they teach in a class.
Many of us learned over years and being taught by senior engineers.
The IEEE Buff Book (IEEE 242-2001) has been around for a long time, but the cost turns people away. Maybe you can find other free stuff by googling around
If you google protective device coordination, you will find many references that provide the basic understanding. This is a specialty, and not just something that they teach in a class.
Many of us learned over years and being taught by senior engineers.
The IEEE Buff Book (IEEE 242-2001) has been around for a long time, but the cost turns people away. Maybe you can find other free stuff by googling around
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yep, system coordination is the issue at hand. The basic concept is that if there is a fault, you want the fault to be cleared at a point as close as possible to the source of the fault, so as to minimized the impact on the entire facility. So how you set this breaker is based on how the breaker (or fuse) is designed to react. You want this one to act SOONER than the one upstream, but without nuisance tripping either.
As an example: I was at a sawmill once where we installed a 600HP motor starter (soft start) that had a 1200A breaker on it. I asked the end user how they wanted me to set the trips on the breaker, they told me to just set them at maximum on everything so that it wouldn’t nuisance trip. I warned them against that, they insisted. Sure enough, on startup the motor was bad, which shorted some SCRs, which single phased the motor and instead of this local breaker tripping, the main breaker for the entire sawmill tripped! Cost them an entire day of production to get everything cleaned out and readjusted.
As an example: I was at a sawmill once where we installed a 600HP motor starter (soft start) that had a 1200A breaker on it. I asked the end user how they wanted me to set the trips on the breaker, they told me to just set them at maximum on everything so that it wouldn’t nuisance trip. I warned them against that, they insisted. Sure enough, on startup the motor was bad, which shorted some SCRs, which single phased the motor and instead of this local breaker tripping, the main breaker for the entire sawmill tripped! Cost them an entire day of production to get everything cleaned out and readjusted.
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
It depends where in the system this breaker is. Going with what Ron said, one of my text books was called The Art and Science of Protective Relaying.
Is it the last breaker feeding an across the line motor starter with overloads?
Is it a breaker feeding an automatic transfer switch. what happens if this breaker trips and a transfer occurs?
How big of an area is affected by this breaker not being coordinated with its upstream device? Is any part a critical load?
There are no fixed guidelines. What is more expensive, having a breaker trip or having to replace burned conductors and equipment?
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in the swgr there is a 100a breaker that feeds this panel. the feed is directly into the above mentioned breaker that also serves to lockout the panel. this breaker then feeds a bus. off the bus are motor cb’s/contactors with overloads and fuses. not much exciting.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
With two protective devices in series, without other information, I would make their setting as similar as I could.
steve.bell@cleantechcontr
New User
- Location
- Sydney
- Occupation
- Electrical Engineer
You would normally do a discrimination study using electrical software to see if the curves are overlapping. If they are overlapping it means that the upstream breaker may trip before the downstream one. Also normally thermomagentic breakers can only be wound down to 0.8 of their nominal rating and electronic to 0.4 so you might have some problems getting to the current levels you are looking for here. At 65kA all the breakers will need to be MCCBs so the switchboard will be very heavy duty for a 250A board. Check out this article here: Discrimination Studies
Steve
thank you, this is a good read.