Instantaneous Trip tolerances for field testing of circuit breakers

Status
Not open for further replies.

awg

Member
Table 100.8 of NETA ATS-2007 shows the tolerances of nonadjustable breakers at +25% and -25%.
Right below the table it states " For circuit breakers with nonadjustable instantaneous trips, tolerances apply to the manufacuturer's published trip range i.e., + 40% on the high side, -30% on the low side."
Does anyone know if this has been updated? If it hasn't, which is the the correct tolerance?

Thanks
 

zog

Senior Member
Location
Charlotte, NC
Updated to show correct tollerances from 2009 NEMA AB 4. Should be +25% -25% (They also added tollerances for electronic trip units).
 

templdl

Senior Member
Location
Wisconsin
If you are testing the magnetic calibration of a thermal mag breaker I believe they are calibrated +-20%.
If you are attempting to test a mag only MCP DON'T DO IT unless you have a suitable testing station.

One of my customers supplied some MCPs with their equipment to Cape Kennedy 20 years ago and in Cape Kennedy's infinite wisdom they thought they would check the mag calibration of the MCPs that were sent to them.
And then came the complaint that the breakers didn't work and sent them back to my customer. Upon visiting the customer I picked an MCP up and smelled it and it was fried which indicated that the MCP was burned up. Took the cover off and showed them the burnt up coil.
The problem with attempting to test the magnetic trip setting of a mag only breaker it that it has no thermal protection. When one attempts to "ramp up" the current to find a trip point the breaker is stressed thermally and it on gives a breaker multiple progressive shots the breaker can be stressed thermally which more often than not will result in the magnetic element being damaged.
Thermal magnetic breaker have a very simple magnetic element, that is it is not very complex, that when combined with the thermal element the breaker is not likely to be damaged if one attempts to test the magnetic pick up.
MCPs up to 150a have an actual solenoid structure consisting of a coil of a coil of wire, the gage of the wire is smaller for the lower rated MCPs and larger for the higher MCPs.
When one attempts to test a MCP there is a great possibility that the coil will be severely damaged by overcorrect.

The question them becomes what is the objective of testing the magnetic calibration. I've was a circuit breaker application engineer for 18 years and I had always been intrigued by those who didn't understand that it takes very special equipment to test magnetic calibrations, equipment that is very expensive. And that fault currents are instantaneous and often time don't ramp up, that all breakers in series with the fault will see the same fault, that the fault current most likely be within the trip calibration if each breaker no matter what rating breaker it is and as such there will be a race between the breakers of which one will trip. In other words unless there is an adjustable time delay which is only available in larger breaker there basically will be no practical coordination.
Then getting back to the reason for trying to test a breaker's magnetic calibration, what is the practicality of it?
 

awg

Member
I appreciate the responses. These are molded case thermal magnetic breakers used for distribution on a data floor. We have been having some problems with breakers failing on the instantaneous test. Some of them would pass if the 40 - 30% was used but not the 25 - 25% tolerance. The question was to clear up what I perceive as a contradiction in the stated information. Some information that I have read on one of the manufacturere's web sites that stated the most important thing, under an instantaneous situation, was for the breaker to trip as testing equipment and techniques can create different results. That probably is true on the high side but becomes a little more problematic on the low side, where most of our breakers are failing. There is the fact that there is not a large startup surge with this equipment so they may be OK, but that doesn't change the fact that with written industry standards of testing in place, that if something did happen to cause a nuisance trip, management would not be very happy with us relying on our own opinon as to what is acceptable. Another problem we are having is breakers that trip without the handle moving to the trip position. Some of the data equipment does not alarm if one of the power sources goes down. This creates a problem with dual corded equipment when we de-energize a panel to install new whips. We now have to physically check the breakers of the alternate source panel to make sure none have tripped without us knowing. This is a little unnerving as some of the breaker handles have a very light touch. We are considering going to a different brand breaker, on our new distribution equipment, along with branch circuit monitoring.

Thanks again
 

zog

Senior Member
Location
Charlotte, NC
What is your test set up like? There are specific cable size and length requirements for different size breakers, some of the set ups I see out there are pretty scary and the techs have no idea about the set up requirements.
 
Status
Not open for further replies.
Top