Adjustable Magnetic Thermal Trip Units.

[bphgravity]

I have a Siemens 1000-amp molded case breaker with adjustable magnetic trip units on the front. They scale from 5000 to 10000. How does changing this setting effect the trip function of the breaker? What does the 5000 to 10000 range signify?

 

[Ed MacLaren]

Re: Adjustable Magnetic Thermal Trip Units.

That sounds like the short-time (instantaneous) pick-up setting which is usually set at 6 to 10 times the trip unit's rated current.

If this is the Siemens "Sentron" series breaker, the adjustments are explained here.
http://www.enm.com/training/siemenscourses/mccb_3.pdf

Ed

 

[ron]

Re: Adjustable Magnetic Thermal Trip Units.

If the magnetic trip unit has an adjustment, it is typically only the instantaneous region of the trip curve that is adjustable (long and short time is fixed). The instantaneous portion of the trip curve is the part that protects against short circuits from continuing.

 

[templdl]

Re: Adjustable Magnetic Thermal Trip Units.

Magnetic or instantaneous pickups are common on breaker with frames of 225a and above. They are generally adjustable from 5 to 10 times the continuous rating of the breaker with a calibration allowance of +-20%. The is no delay when the current reaches the pickup setting and the breaker will trip immediately. It is not a short time delay.
There is no code that tells you when and how to set this adjustment. However, there is and outside chance that you may coordinate an instantaneous adjustment with breakers upstream or downstream in order for one to pickup before the other but this is a big time long shot. The magnitude of the fault combined with the dynamics of the distribution system makes it potluck anyway.
Setting it in the lowest position may allow you to pick up and arcing fault quicker.
A short time delay is available on an electronic trip unit. Short time delay works in conjuction with a short time pickup setting such values similar to an instantaneous pickup. When the pickup setting is reached the breaker will delay tripping for the short time delay value set which quite commonly is .1-.5 seconds. When set properly with down stream devices this delay can provide enough of a hickup to allow the downstream device time enough to trip and clear the fault.