Bus duct

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shockrocker68

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We have a 2500 amp bus duct that feeds several pieces of machinery, The machines have 200a fusible disconnects for overcurrent protection, with time delay dual element 150a fuses, yet every time we have a motor go out , it bypasses the disconnects and takes out the Main on the Switchgear, Why dont the fuses interrupt the fault, The load of the bus duct is about 1800Amps.
 
Re: Bus duct

shockrocker,

As Bennie has stated, the system is not co-ordinated. The feeder breaker likely has "long-time" and "short-time" pickup and delay adjustments on the trip units.

If it does you could try a longer setting. It would help to know whether it is tripping on overload (long-time) or short circuit (short-time).

Ed

[ April 12, 2004, 10:09 PM: Message edited by: Ed MacLaren ]
 
Re: Bus duct

Is there ground fault protection on this main?

If it has electronic trips, you may be able to utilize the I^2t function to better coordinate with the fuses. If it is magnetic trips as bennie indicated, you are in trouble. You would have a real hard time getting that to work.
 
Re: Bus duct

As said above, coordination is the issue.
You didn't say what the voltage was so I will assume it is 480v and, as such, the main breaker almost with all certainty has ground fault.
When motors fail the will most likely fail winding to ground or phase to ground which causes a ground fault.
If you check the ground fault pickup of the main it will be 1000a which means that it will trip instantaneously if it detects a 1000a ground fault unless it has a time delay which is commonly adjustable up to a delay of 500 ms.
With this in mind a fuse won't be even close to coordination with it.
You would have a better chance if you use a combination started that uses a motor circuit protector (MCP) type magnetic only breaker that has an adjustable magnetic trip. The NEC art 430-52 advises their maximum setting. But I always advise clients that they should set them just above the nuisance tripping set point for the closes protection, a set point that is just above the inrush current (not to be confused with LRA) spick when the motor is first energized. Therefore the MCP will allow the motor to start under normal conditions but if the MCP detects a current higher than that is will trip instantaneously, that is it will clear the fault in LESS than a cycle unlike a fuse. Keep in mind that we are talking about low level fault which are often arcing faults and not bolted faults.
 
Re: Bus duct

Hello , Maybe I should mention the main is a Square D Bolt loc switch and it does have a ground fault protection or tripping mechanism, But the system has no adjustment for trips, also the system is 480/277v 3p 4W.
 
Re: Bus duct

All Square D main ground fault protection (GFP) systems have some method of adjustment. They have used at least 4 different models of GFP on their Bolt-Loc equipment over the past 25 years. All of the adjustments are located in the fuse compartment (the main needs to be turned off to gain access).

Before the GFP and the branch fuses can be coordinated, you need to determine what level of damage is acceptable to you should a fault to ground occur. For example a Bussmann LPS-RK150 fuse will allow about 1500A to flow for .1sec and roughly 1000A for .5sec so it is all but impossible to coordinate a standard GFP with the full fault rating of the fuse, however it is very unlikely that you would develop an arcing ground fault of this magnitude. So, chosing a coordination level is important.
 
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