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 Post subject: Validating Combination Starter kAIC ratings
PostPosted: Thu May 06, 2021 6:18 pm 

Joined: Tue Feb 11, 2020 8:28 am
Posts: 1
Can an MCC combination starter bucket be rated to handle fault currents that exceed individual components, even if a fault event were to occur directly within the bucket, between under rated protective devices (ex: load side of a breaker or fuse that is not rated to individually handle the fault current) and any down stream starter contactors and/or OL relays inside the starter bucket?

I recognize that there can be series rated combination values; however, I am concerned about the unique situation of a fault event within the bucket itself, while a worker is has the doors open, rather than simply regarding events upstream or downstream of the starter bucket.

For those who like to read, further details about the question are provided below:
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This is a little long winded, but may help clarify my train of thought regarding the above question.

During a recent Arc Flash study using SKM's Power Tools, Version 9.0.x.x, we decided to run the Equipment Evaluation feature, not so much for feedback on which devices might fail under calculated short circuit fault currents, but rather, to provide a sort of component list of protective devices modeled within the study. However, as a result of the Equipment Evaluation, it came to light that most (if not all) of the protective devices (MCCBs with Instantaneous Only), as installed in multiple MCC combination starter buckets throughout the project, were not marked to the same kAIC ratings as the MCC in which it was installed (ex: MCC and associated bus bracing rated for 65 kAIC, Combination Starter buckets rated for 65 kAIC, MCCBs installed within starter buckets rated for 25 kAIC...).

When I pressed the manufacturer for clarification on how there could be markings on individual starter buckets and the MCC as a whole suggesting a rating for 65 kAIC, when there are protective devices within the system that could not handle the events per their individual component ratings, the manufacturer replied with the following:

Quote:
Combination Starter units within (manufacturer) Motor Control Centers (MCC) have many short-circuit current ratings which are higher than the short-circuit interrupting rating of the Branch Circuit Protective Device (BCPD) provided in the unit. These Combinations Starter units have been evaluated by design tests in accordance with the requirements of UL 845 and are documented in the (manufacturer) UL Report for our MCC’s.

Excerpt from UL 845 Fifth Edition:
    5.4.2 Combination Motor Control Units
    A combination motor control unit, for other than variable speed drives, may have a short-circuit current rating greater than the short-circuit current rating of any individual component if evaluated by design tests.

The UL excerpt above does not specify what conditions must be evaluated in the "design tests", and the manufacturer declined to release their UL Report records to me, so I could evaluate the conditions and/or limitations of the tests performed. Knowing that the manufacturer's headquarters was in Europe, I dove a little bit into the IEC standard for motor starters and contactors, 947-4-1, which defined two potential levels of protection/coordination for motor starters (contactors and overload relays) under short circuit conditions.

    Type 1 Coordination
    Under short circuit conditions, the contactor or starter shall cause no danger to persons of installations and may not be suitable for further service without repair and replacement of parts.
    Type 2 Coordination
    Under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and shall be suitable for further use. The risk of contact welding is recognized, in which case the manufacturer shall indicate the measures to be taken in regards to equipment maintenance.

The above is supposedly only in the European standards, and reportedly has no direct equivalent in a US standard, which leads me to wonder what method the manufacturer used to test the bucket, allowing it to have a higher rating than the individual components. They do not specify whether the installed starter is a Type 1 or 2 Coordination device per the IEC, and neither do they clarify whether the tests they performed assumed the buckets are closed or open during an elevated fault current event.

To add even more concern, there was a North American standard, UL 508, Section 58.56 (not yet sure what version, or if this standard has been superseded) "Controllers Intended for Use on Circuits Capable of Delivering High Fault Currents", which suggested the following:
    (a) There shall be no discharge of parts, (however) welding or complete disintegration of contactor contacts and burnout of the current element of the OL relay is acceptable.
    (b) The door or cover should not be blown open (still possible to open the door or cover), (however) deformation of the enclosure was acceptable.

There may be more to available standards, but from what I am reading:
    [1] European standards allow for either a Type 1 Coordination (may be damaged) or Type 2 Coordination (able to be reused, possible maintenance still required) combination starters.
    [2] North American UL standards allow combination starters to be rated for higher short circuit currents than individual components are rated to handle, provided they are evaluated by design tests; however, there are acceptable levels of disintegration and/or deformation to the starters.

Without the manufacturer specifically stating what criteria they used, and any applicable limitations that were applied to their design tests, I do not know whether the combination starter and MCC ratings have assumed individual buckets are closed and/or secured during a fault event.

In the unique situation of an employee interacting with a bucket with it open, and a fault event occurring within the combination starter, I am concerned that the stated kAIC rating of the bucket is not valid or applicable, that the protective devices may fail and expose the worker to an extended duration of elevated fault currents and/or arc flash risks not adequately accounted for in the study.

Is this something that I should be concerned about?
Or am I simply thinking to much into this, and can actually ignore the individual ratings of protective devices within a starter bucket, thus blindly trusting the manufacturer must know what they were doing with their supposedly UL certified design tests?


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 Post subject: Re: Validating Combination Starter kAIC ratings
PostPosted: Mon May 10, 2021 12:33 pm 
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Joined: Mon Sep 17, 2007 5:00 pm
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Location: Scottsdale, Arizona
From my equipment manufacturer days (a lifetime ago) the combination starters where tested as an assembly to obtain the ratings. I recall overloads had very low ratings but when it was all tested together, the actual short circuit rating was greater i.e. 30 kA, 65 kA etc.


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