siemens sentron breaker adjustments

Status
Not open for further replies.

paul hodge

Member
Location
atl
Had a situation where a 175 amp 3 phase breaker that was primary for 112.5ova transformer had overheated on phase b&c. This particular breaker was a siemens sentron series with adjustments for each phase. The sails went from low to high sitting, if I recall it was from 900_ 2000. The settings for the original breaker were at low setting. After installing the new breaker an attempt was made to energize transformer until no load condition, breaker would immediately trip. After several failed attempts, ajustment wrer make on the 3 sails from low to max high. The transformer energized the first time. My questions are as follows when adjusting settings from low to high what setting where being changed and why do you think transformer would not energize on low setting as the old breaker was sit on low position.
 

big john

Senior Member
Location
Portland, ME
Were the breakers and trip units identical? If so, I can't explain why one would hold but the other would trip with the same settings.

What was very likely being adjusted was the Instantaneous pickup. Transformers have a very short duration inrush current before a magnetic circuit has been established in the windings where just about the only thing that opposes current flow is the DC resistance of the conductor. This means you can see an extremely short duration current 15x greater than the FLA of the transformer, and this can be enough to trip a breaker on the instantaneous setting.

It kinda sounds like somebody is just sorta guessing at breaker settings though, which is a bad idea.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
"Sails"?

"Dials" spelled wrong and spell checked maybe?

Yes, typically the only thing adjustable on a Sentron (that is not an Electronic Trip version) will be the instantaneous magnetic trips and big john is dead-on with the inrush current issue (not to mention the comment on SWAC; Setting Without A Clue). Breaker trip settings should be coordinated with the other distribution equipment settings and protection curves.
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
Another item to consider, if a study has been done for arc flash, changing instantaneous settings will most likely change the incident energy level. Usually in a study, the coordination is done and then ways to reduce the incident energy level (arc flash) is looked at. Many times the incident energy level can be substantially reduced by lowering the instantaneous setting.
 
Status
Not open for further replies.
Top