Dissimilar metals

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ggonza

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Is it acceptable to use regular steel bolts to connect lugs to bus bar. It seems that manufacturers mostly use zinc-plated steel bolts for all of their elctrical connections. Any facts about this would be appreciated
 

SG-1

Senior Member
Yellow Zinc-chromium hardware is used on bus connections, so we can tell the bolts have been plated. Clear zinc is fine, but you could end up with unplated bolts & corrosion. Grade 5 is recommended. Look for 3 lines on the bolt head.
 

ZinskI/E

Member
Location
New Orleans, LA
I have seen many different bolt materials used for bus connections. As SG-1 said, the zinc-chromium bolts seem to be favored by manufacturers when making a bus-bus connection; field installed hardware varies greatly. In order from most to least common (in my experience): zinc finish, stainless steel, silicon bronze, and copper.

The corrosion-resistance properties of each material is different, and the selection is made with regard to the environment where the connection is made.

The concern with dissimilar metals in close contact is galvanic action/corrosion which occurs naturally when metals of different potential are in close contact with the presence of an electrolyte. Optimally, electrical connections are kept dry, thus minimizing the presence of an electrolyte and preventing galvanic action.

The fasteners should ensure good electrical connection between the lug and the bus, not carry current as part of the circuit. Though I can't say for sure, intuitively, current through the fastener would promote galvanic action. This is why we shouldn't use washers as spacers for a lug-bus connection as seen in Brian John's post http://forums.mikeholt.com/showthread.php/136026-it-s-Friday-I-m-Bored.
 

MichaelGP3

Senior Member
Location
San Francisco bay area
Occupation
Fire Alarm Technician
Nuts and bolts

Nuts and bolts

I have seen some that were copper plated steel that had another plating or coating over the copper that had the flat appearance of lead. I never tested it for lead.....maybe it was gray phosphate? The surface was soft.....easily scratched off with a screwdriver tip exposing the harder copper. They came with various 12 volt 18 & 26 amp hour batteries. The lug terminals these were to connect to were tin plated copper. Is this flavor of electrical fastener seen elsewhere?
 

jeremysterling

Senior Member
Location
Austin, TX
I think ZinskI/E made a good post to answer the OP. His point that the connecting hardware should carry no load, IMO means we can use any fastener we want as long as it is suitable for the purpose and properly torqued.

The brian john picture of the stacked washers is imminent failure of the connection!
 

ggonza

Member
So what is the most accurate answer? Should we always steer clear of unplated bolts? Or is it ok to use them as long as they're not being primarily used to carry current? It would be nice if the NEC was more spefic about this. I think we all know how hard it is to get an employer to fork out more money for something that they've always done one way.
 

Finite10

Senior Member
Location
Great NW
Here's a LINK:
Ttitle; ELECTRICAL CONNECTIONS FOR POWER CIRCUITS
3.3.1 has a table listing anode and cathode organized by range of disimilarity.
The internet version is dated 2000.

Good question ggonza, it would be useful to see some sort of ANSI reference or something. We always treated bus connecting bolts and washers like gold so we didn't lose them. An electrical contractor should really want those bolts to last a lifetime. You probably don't need a machine screw shop to find them, we have Tacoma Screw around here.
 
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jeremysterling

Senior Member
Location
Austin, TX
Here's a LINK:
Ttitle; ELECTRICAL CONNECTIONS FOR POWER CIRCUITS
3.3.1 has a table listing anode and cathode organized by range of disimilarity.
The internet version is dated 2000.

Good question ggonza, it would be useful to see some sort of ANSI reference or something. We always treated bus connecting bolts and washers like gold so we didn't lose them. An electrical contractor should really want those bolts to last a lifetime. You probably don't need a machine screw shop to find them, we have Tacoma Screw around here.

Thanks for the great link. After reading, it seems dissimilar metal expansion rates is a more important consideration than dissimilar metal corrosion effects.

The link states Bellville washers should always be used with steel bolts on copper bus because of differences in thermal expansion.
 
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