The liability is on the Asset Owner and Ultimately the Electrical Worker. So What should be the strategy?
Summary: Circuit Breakers are designed to a UL489 UL1077 & IEC60947-2 and the UL489 standard states a Short Circuit Test of 5kA or less should clear a fault 1 to 3 times and may not be operational on the third clearance.
Motor Starters as stated in the Sprecher & Schuh article are design to a UL508 standard and "Note
that there is no promise that any of these devices are re-useable
following a short circuit.
Sprecher & Schuh state that Circuit breakers design standard is UL489 while Motor Starters are covered under UL508.
So effectively to the standard Motor Starters are a single shot device which will be a surprise to many Electrical Workers, while Molded Case Circuit Breakers are good for 3 clearances.
Here's what Sprecher & Schuh say:
We often get questions concerning what damage may occur to our contactors, overload relays or manual motor starter and controllers during or following a short-circuit. In response to these questions we would like to start by clarifying that passing a UL short-circuit test for any of the above mentioned devices means "The current carrying parts may not move" when interrupting a short-circuit fault. Note that there is no promise that any of these devices are re-useable following a short circuit. This should not come as a surprise to most electrical personnel concerning contactors or overloads, but some people assume that a manual motor starter and controller is a circuit breaker and this assumption is in error. Some industrial controls manufacturers’ literature may reference manual motor starters and controllers as a circuit breaker, and that may be true in Europe under IEC rules, but manual motor starters are NOT circuit breakers under UL and NEC or CSA standards. We hope the following will help users understand the proper application of Manual Motor Starters as compared to Self-protected Combination Motor Controllers.
What The Tests Mean Many electrical personnel assume that a manual motor starter and controller is a circuit breaker because some of them have thermal and magnetic response mechanisms. A manual motor starter and controller is tested and approved under UL508 Part III, Sections 72-75 entitled "Manual Motor Controllers". A molded case circuit breaker is tested and approved under UL489. Clearly, there are different tests required for each and they do NOT perform the same functions.
UL standards and NEC code covering molded case circuit breakers suggests that ALL circuit breakers should be inspected following interruption of a short-circuit to verify that the circuit breaker is operating properly. Note no one promised that a UL489 600 V class circuit breaker is reusable after a short circuit, but many people assume that to be fact. We get the impression that a circuit breaker is useable after a short-circuit because when we work on the toaster at home while it is plugged-in and trip the breaker; then we can simply go to the power panel and reset the breaker. Residential breakers are not subject to high fault currents (KA) like industrial facilities are exposed to. If a 25 KAIC breakers experiences a short-circuit fault on a power system capable of producing 24 KA then that breaker will most likely be damaged.
There is a BIG difference between the tests required for a "Manual Motor Starter and Controller" and a "Self-protected Combination Motor Controller" although they may look quite similar. A "Self-protected Combination Motor Controller" is tested and approved under UL508 Part IV and some have passed both Part III and Part IV which is to say they have mixed (or dual) approvals. A Self-protected Combination Motor Controller (sometimes referred to as Type E Combo) has to pass a "life after short-circuit test" that even Molded Case Circuit Breakers do not have to pass. Type E Combo controllers promise life after short-circuit while contactors, overloads, manual motor starters and even molded case circuit breakers DO NOT promise they are useable after interrupting a short-circuit. Further, a molded case circuit breaker can be used as a branch-circuit protection device (BCPD) for many types of loads including lighting, heaters, control circuit transformers, power supplies and motor loads under NEC 430-52C Option 1. Alternatively, a Self-protected Combination Motor Controller can be used for MOTOR loads under NEC430-52C Option 6. A manual motor starter and controller can NOT be used as a branch circuit protection device (BCPD) at any time. END QUOTE
The Short Circuit Test under UL 489
Many Electrical Workers may not even think the Protection devices are compromised when a Motor fails and just concentrate on getting the Motor replaced.
So What Should You Do?
As stated in the above article an inspection is necessary, which means what? This is a very subjective statement.
When looking at the common failure modes of protection devices a Mechanical operation should be conducted. Does the device Isolate when open and when reset or closed provide voltage?
As Many of these devices are sealed e.g. Molded Circuit Breakers inspection is difficult. What is recommended is to conduct a resistance test of each phase across the closed device, with a Meter in the MilliOhm range.
The component which has been stressed when a short circuit occurs is the contacts being in the Circuit Breaker or Starter.
These resistances should be balanced within 3% as this affects voltage drop to the Motor and hence reliability & Energy Efficiency.
While these tests are not a guarantee of Reliability they do cover the main failure modes and if documented are a mitigation factor if something seriously goes wrong. Having the attitude of She'll be right really doesn't look good if your facility burns down.
3Phi Reliability recommend that Circuit Breakers and Starters be tested initially to find the ones that have had multiple clearances in the past and have a replacement program.
A log should be made of resets and Once three clearances have occurred, conduct testing and replacement. This means control over ALL PERSONNEL resetting protection devices.
If a Dead Short is suspected and not an Overload then Testing should be mandatory. Some training to Non Electrical Personnel should be made of the risks. Resetting protection devices in these situations risk an ARC FLASH event and the Asset Owner has safety liability to their personnel and contractors.
Hopefully this Article has highlighted that Protection Devices have a Service Life much shorter than most people realize, and a Preventative Maintenance Strategy is essential.
3Phi Reliability regularly test Contactors, Circuit Breakers, and Isolators and find even New Components have defects.
The acceptable resistance across a set of is approximately 30mOhm, this means a 20 Amp Circuit Breaker will dissipate approximately 12 Watts at this resistance. A MMC DIN Rail row of devices can easily add up to several hundred Watts and above this resistance becomes unacceptable.
Experience shows many New Isolators, Contactors and Circuit Breakers can have large imbalances eg 30 mOhms to 180 milliOhms when comparing the three phases. So Acceptance Testing should be conducted and the defects feed back to vendors. This practice ensures continuous improvement in Reliability and Energy Savings.
Mark Gurney Motor Analyst
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