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 Post subject: ATS protection and UL
PostPosted: Tue Nov 15, 2016 1:47 pm 
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In looking at the latest edition [7th] of UL 1008 for transfer switches I came across the following:
“The circuit breaker must include an instantaneous trip response and shall not include a sort time trip response” as it relates to switches tested per 9.13.3.10 & 9.1.2.3.
It seems different manufactures have different interpretations. So does this disqualify any breaker with an adjustable STPU / STD setting from protecting a 3-cycle [0.05sec] any breaker rated switch, even if the breaker has an instantaneous override less than the interrupting kA of the switch? It seems counterproductive because if I have a LSI MCCB I can almost always provide better protection in the instantaneous and in the short time than with a standard thermal mag. C/B. For low level arcing / fault currents, that sometimes extend into the LTD portion of the T/M I can adjust the STD down to pick up these currents faster providing better equipment and personal protection? One manufacture I talked to indicated that if the breaker has an adjustable short time ahead of the switch the switch is misapplied, even though the TCC’s show different? Then what about the specific breaker ratings, many breakers on the “Specific Breaker” list can be provided with a myriad of trip units types and styles from your standard TM, to LI, LSI, LSIG. So does this restrict the specific breaker to the T/M? The literature does not differentiate. Just curious if anyone else has run into this and other thought and opinions..


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 Post subject: Re: ATS protection and UL
PostPosted: Wed Nov 16, 2016 9:23 am 
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mike01 wrote:
In looking at the latest edition [7th] of UL 1008 for transfer switches I came across the following:
“The circuit breaker must include an instantaneous trip response and shall not include a sort time trip response” as it relates to switches tested per 9.13.3.10 & 9.1.2.3.
It seems different manufactures have different interpretations. So does this disqualify any breaker with an adjustable STPU / STD setting from protecting a 3-cycle [0.05sec] any breaker rated switch, even if the breaker has an instantaneous override less than the interrupting kA of the switch? It seems counterproductive because if I have a LSI MCCB I can almost always provide better protection in the instantaneous and in the short time than with a standard thermal mag. C/B. For low level arcing / fault currents, that sometimes extend into the LTD portion of the T/M I can adjust the STD down to pick up these currents faster providing better equipment and personal protection? One manufacture I talked to indicated that if the breaker has an adjustable short time ahead of the switch the switch is misapplied, even though the TCC’s show different? Then what about the specific breaker ratings, many breakers on the “Specific Breaker” list can be provided with a myriad of trip units types and styles from your standard TM, to LI, LSI, LSIG. So does this restrict the specific breaker to the T/M? The literature does not differentiate. Just curious if anyone else has run into this and other thought and opinions..


You only quoted one line so it's hard to figure out what the context is here, and I don't have a copy of that standard.

There are two different requirements in UL standards. The first is requirements on an assembly, something that will be sold with a UL stamp on it. For example UL rated control panels that fall under UL 809A must be built to 60 degree rise wiring specifications. Outside of the panel almost everyone uses 90 degree or 75 degree wiring but I rarely see 60 degree wiring used. This does not mean that as the installer I can't wire up the panel following NEC with 90 degree wiring...but as the panel shop I have to build it to the 60 degree standard or else I can't put a UL 809A label on it.

The second set of requirements are installation requirements. For instance terminals are Listed for landing only a single wire in the terminal unless it is specifically designed for multiple wires (such as with wire nuts). Or for example many VFD's don't include any protection internally and require the installer to use breakers or fuses with a specific (very fast) rating on it for protection. UL (the 3rd party testing lab, not the standards agency) in turn tests the equipment following the manufacturer's requirements so that the 3rd party test is valid.

In the case of the former unless you are building equipment that will be Listed (in which case you must follow the UL requirement), you follow the manufacturer's requirements for installation.

The installation requirements may not be spelled out this way but will basically boil down to an overload requirement for long term tripping (a thermal limitation) and various short-circuit withstand type of ratings which boil down to the mechanical design of the equipment in terms of it's withstand rating as well as an opening and closing maximum current rating, the same ratings you would get for any other type of power switching equipment. If the current exceeds the withstand rating, the equipment can fly apart under magnetic force. If it exceeds the opening/closing rating and is triggered to open or close, the arc can destroy it when it attempts to close/open. Generally except for special circumstances the lowest of the three ratings is a design maximum for short circuit purposes.

Often I see strange specifications (specific fuses or breakers) especially for solid state power components because the manufacturer is attempting to "dumb down" the specifications from an engineering specification to something that a commercial/residential installer can easily understand. This of course drives industrial plants crazy because they are used to buying their favorite brand X and don't want to have to buy yet another special part from brand Y or Z for a single special application. This is where the real manufacturer (not the private label on the equipment) can come in very helpful.


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 Post subject: Re: ATS protection and UL
PostPosted: Thu Nov 17, 2016 7:39 am 
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Joined: Mon Nov 05, 2007 10:10 am
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Location: Mid-West
For example see attached image from a transfer switch label note the verbiage “the circuit breaker must include and instantaneous trip response and shall not include a short-time response” the manufacturer indicated that using a breaker with and adjustable STPU/STD is not acceptable [based on the UL snippet above, this is the only part of the standard I have currently], and in turn this seems odd to me as I stated above because when set properly the STPU/STD can provide enhanced equipment and personal protection. It was noted that if an adjustable STPU/STD was used that a short time rated switch should be applied…….


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