Thermal-magnetic circuit breaker question

Dubledox

Member
Location
Iowa City, Iowa
Greetings, all. Does anyone know why a thermal-magnetic tripping unit (an Eaton KT3400T hooked up to an Eaton HFD 3400F MCCB), would have three adjustable settings? I always thought they generally had one setting for instantaneous tripping, which is often 5-10 x I_n. My thinking is this is because there are three phases, so there is one instantaneous setting per phase. But I can't see why this would be.

If you are interested in more details, here is the system: A three-phase, 480V panel is being fed from a low voltage switchgear bus. The main breaker is a 400 A (I_n = 400 A) molded case circuit breaker. The panel feeds a couple of control panels and a few VFD's for pumps. Currently, the settings are maxed out at 10x In (4000 amps). I calculated the minimum sized fault on the panel to be just over 8 kA, so in addition I do not even see how adjusting these settings can realistically affect my system.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
From the labeling it sure looks like the dials are magnetic trip as a multiple of the nominal trip for each of the three phases.
Possibly because they did not want to put in a three winding potentiometer to adjust all three at the same time?
or because there might be known single phase loads with different characteristics (large transformer on one phase, for example.)
 

big john

Senior Member
Location
Portland, ME
Each phase has an independent instantaneous pickup adjustment.

No idea why it was designed that way; I assume it's simply an obsolete technology because I don't ever come across it in modern trip units but it's not an uncommon feature to see in older molded case breakers.
 
The Westinghouse/Cutller Hammer KT3400T Trip unit for the KD and HKD frame has the 3 dials so you can set the instantainous trip for each phase. We always set the settings at 10 (high) This is 10 times the thermal rating (4000 Amps) Adjusting these settings will not affect the thermal triping at all.


Sincerely, joe
 
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iwire

Moderator
Staff member
Location
Massachusetts
When instantaneous are set to HI or MAX, that is generally the setting for a breaker that has no adjustments. It should be safe UON
Should be, but not my call to make.

I don't touch breaker settings, OL settings or fuse sizes without being directed too.

In general no good can come from it.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
It should be safe UON
How do you define safe?
Higher Inst settings usually result in significantly higher levels of Arc Flash Incident Energy. When performing Coordination and Arc Flash studies, it is extremely common to make recommendations that involve turning down an Inst setting. There is a balance that needs to be reached, one-size does not fit all.
 

ron

Senior Member
How do you define safe?
Higher Inst settings usually result in significantly higher levels of Arc Flash Incident Energy. When performing Coordination and Arc Flash studies, it is extremely common to make recommendations that involve turning down an Inst setting. There is a balance that needs to be reached, one-size does not fit all.
Agreed. If an arc flash and / or coordination study is done, there are very specific settings that MUST be implemented. I was talking about the other 90 - 95% of the jobs where that doesn't happen.
 
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