UL891 question

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Savred22

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Hi. this is my first time posting on this website. I am trying to get clarification on couple of different topics regarding the UL891 requirements 12th edition.
1. In order to certify a 480V switchboard to UL891, does the bolts that hold the bus bars and joints need to be SAE Grade 5 minimum? the only section in the UL891 that discusses Grade 5 bolt is under appendix G at the end of the document. There is not a whole lot of other explanation within the body of the document.
2. Table 23 covers the ampacity at the clamp joints. The rating ampacity of copper at the clamp joint is 200 amperes/in^2(up to 3000amps). let's say we have two
1/4x4 inch bus bar risers that are bolted together 90 degrees with two other runback bus bars with identical size. the question is when calculating the total surface area at the joint, the cross sectional area of the bar that are touching the adjacent bar on both side, do I added it up twice or only one time?

does anyone know of good reading material or reference books on bus bar sizing and calculation?

thank you
 
That's not one of the UL standards I have access to, but I do have UL 845 for MCCs, which was originally an offshoot of UL 891 switchgear. But in that spec, and from my observation of UL specs in general, they rarely get into the nitty gritty details of HOW a manufacturer is to construct something, they really care more about it passing their test requirements. So on gear with bus bars, that will include a heat cycling test, followed by the short circuit testing, with no damage or loosening of components, then followed by a dielectric voltage withstand test afterward. These tests are WAY more expensive than the added cost of grade 5 bolts for as long as you make the gear, so most reasonable people would just not bother taking the chance of failing and use them rather than risk having to spend hundreds of thousands to re-test if the cheap bolts failed.

Secondarily, most specifications put out by Engineers will require grade 5 bolts anyway, UL or not, so again, why bother not using them to pass UL?
 
1. UL does not spec common hardware. Grade 5, 8, sometimes metric, stainless, and siliconized bronze are all used.
2. Bus bars are engineered systems. You just took a simple text book joint and complicated it. Only way to know for sure is build several and using a circuit breaker tester like a 4000B to do heat runs. Bus bar design is not just ampacity either. Short circuit forces are part of the design.
3. You can’t use bulk numbers and rules of thumb out of context. Alpha spots are your only contact giving rise to constriction resistance. Any good contact theory book explains this. Plus with complex joint geometry you are creating complex magnetic fields.
4. Contact Storm Copper or the Copper Research Association (?) they are the experts.
 
Jraef said:
That's not one of the UL standards I have access to, ...
Click to expand...
Actually you do, but it is not very user friendly.

UL uses the same method to provide free access to their standards as does the NFPA. Just like with the NFPA, all you have to do is to register with an email address, then go to the standard and select "digital view". However, there is no search or copy function, and you need to go through the document page by page.
 
I haven't looked at it real closely but there are some bus bar standards in the back of UL 508A that you can use to make your own bus bars for UL 508A systems. I haven't looked at it real close cuz it's not anything we do but you got me wondering if they specify the hardware to be used.
 
petersonra said:
I haven't looked at it real closely but there are some bus bar standards in the back of UL 508A that you can use to make your own bus bars for UL 508A systems. I haven't looked at it real close cuz it's not anything we do but you got me wondering if they specify the hardware to be used.
Click to expand...
If you have a PDF version and do a search on the word "bolt", you will not see any reference to the "grade", only that if used, they be protected from corrosion and that they not be used for BOTH mounting AND current carrying connections (which is interesting because I swear I have seen that many times). The wording for all of those uses in UL508A is exactly the same as is used in UL 845, so I imagine it's the same everywhere.

Now in UL845, they do mention the requirement for spring loaded washers on bus connections. From a mechanical standpoint, the washer itself might require a hardened bolt to get the right amount of torque to compress it, I don't know. But it's not specifically called out in the standards.
 
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