time delay setting of branch feeder 3

[SELEC]

I am doing a relay setting for a plant. I found that the settings for MCC feeders should be set at short time delay and long delay without any instantanneous settings. Otherwise it would be hard to corrdinate with down stream circuit breakers. Is is a common pratice here in North America?

 

[davidbeach]

That is certainly a common way of solving the time coordination issue if the system has adequate withstand ratings so that everything can wait if necessary. What it does though, is greatly increase the incident energy available when doing arc flash calculations.

 

[dpc]

It's fairly common and the only way to achieve good selectivity with downstream molded case circuit breakers.

Arc-flash levels will be higher than on instantaneous trip, as mentioned. But if short time delay is kept at minimum, the downstream arc energy is generally not too high.

 

[rcw retired EE]

Using a Zone interlocking between the feeder breakers and the switchgear main breakers can reduce the clearing time for switchgear faults and possibly reduce the arc flash level. The Main breaker is set on instantaneous trip unless it recives a signal from one of the feeder breakers that the fault is downstream.

This does not help the arc flash level at the motor control centers or otehr loads.

What you have found is very common in low votlage equipment.

 

[alehman]

I agree with rcwilson. ZSI can be extended to create muliple zones where multiple breakers are in a circuit. If there are only a few starters in your MCC it may be practical to do something like that.

One note - Switchboards built to UL 891 standards (most common in the U.S.), instantaneous trip function is required. Only ANSI design power circuit breakers in ANSI standard switchgear may be configured without instantaneous trip.

 

[SELEC]

GE company has a new product(I forgot the name) special intelligent switchgear with ZS device) to deal with instantanneous setting cordinations between MCC. But that would be too expensive.I believe it is accetable pratice to disable the instantaneous function to achieve good selectivity with downstream molded case circuit breakers, so long as it acts fast enough to restrict the arc flash level.

 

[davidbeach]

One note - Switchboards built to UL 891 standards (most common in the U.S.), instantaneous trip function is required. Only ANSI design power circuit breakers in ANSI standard switchgear may be configured without instantaneous trip.

True, but it is possible to get equipment with that instantaneous trip level above maximum prospective fault current. In that case the functional result is a situation where only Long and Short times are used.

It is also possible to get UL 891 devices where the instantaneous is a high current override that does not show in the adjustable settings of the breaker, but needs to be included in the trip curves. Typically these will be used where the maximum fault current is well below the override value.

 

[RalphChristie]

Can somebody define short time and long time for me? Is it certain trip-curves? I am not familiar with these terms.

Thanks

 

[RonShap]

Ralph,
Look at page 3 of the following website.

http://www.squared.com/us/products/...DB0105.pdf#search='time current coordination'

 

[alehman]

David,
I agree. Setting the inst. pickup above the available fault current is the only way to assure coordination of devices with insta. trip at high fault current. Instantaneous override sometimes confuses people (even those sell circuit breakers) into thinking there is no instantaneous trip function because there is no dial to adjust.

 

[RalphChristie]

Thanks Ron

 

[SELEC]

To be accurate, the insta. setting od upstream setting should be higher than available downstream max. fault current.