# Steel bolts carrying current



## glen1971

Ideally I'd use Grade 8 bolts, but there's lots with Grade 5 out there.. I use 1/4" x 3/4" stove bolts or 3/8" x 3/4" bolts for motor connections all the time with no issues..


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## farlsincharge

glen1971 said:


> Ideally I'd use Grade 8 bolts, but there's lots with Grade 5 out there.. I use 1/4" x 3/4" stove bolts or 3/8" x 3/4" bolts for motor connections all the time with no issues..


In those cases is the bolt actually carrying current or just providing clamping force?


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## telsa

farlsincharge said:


> Working on connecting a 90kva genset. The parrallel sets of extra flexible wires come out of the load side of the breaker and are terminated with compression rings. [ *lugs ?* ] There are lugs through bolted to an insulated backing board (that red fibreglass type stuff) and then on the backside of the board the ring terminals are slid over the exposed remainder of the bolts and held on with nuts in a stacked arrangement.
> 
> I'm having a hard time being comfortable with 250 amps being carried from the wires to the lugs through a 5/16 plated steel bolt.
> Am I crazy?


Your post makes me think that the conductors are incorrectly landed.

This is the true source of your corn fusion.

If the compression lugs were squeezed from the other side... how would that play out?


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## HackWork

I agree that it sounds like this is not being done correctly. 

Just the 5/16" size bolt alone sounds way wrong.


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## Cow

Pics?


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## canbug

Grade 8 bolts should not be used in and current carrying capacity. The grade 8 will not stretch with heat/cool and will stress whatever material they are fastened to leading to a failure. IIRC our switchgear uses grade 3 and some larger utility gear we just removed used copper bolts, washers and nuts on the buss work.


Tim.


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## oliquir

only copper bolt would be acceptable the way you describe it. standard bolt are acceptable for mechanical holding of two lugs together, no current passes through the bolt.


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## Lone Crapshooter

We use stainless steel bolts locks flats and nuts never had any fail. For bolting the bus together in the gear always use what the manufacturer supplies.

I have seen silicon-bronze used on the SWGR cable terminations but it is one use only. That is hard to police unless you are on top of the job.

Always use a torque wrench on the terminations.

LC


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## farlsincharge

As far as I know these are factory connections, except for the lugs which I changed. As you can see the only thing carrying the current through the fibreboard are zinc plated 8.8 bolts.


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## frenchelectrican

I am hopeing that ya useing the bellvule washer on that connection.

I would not use grade 5 or higher bolts due the thermal cycle that will loosen up over the time.

Copper or brass bolt and nut will work just fine for current carry conductors.


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## farlsincharge

frenchelectrican said:


> I am hopeing that ya useing the bellvule washer on that connection.
> 
> I would not use grade 5 or higher bolts due the thermal cycle that will loosen up over the time.
> 
> Copper or brass bolt and nut will work just fine for current carry conductors.


I just put it back the way I found it. No washers of any sort present. I actually added some internal star washers before the last nut.


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## telsa

That is -- flat-out -- rotten engineering.


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## glen1971

farlsincharge said:


> In those cases is the bolt actually carrying current or just providing clamping force?


Clamping force, like a lug to a transformer bushing..

Seeing the picture later on in the thread I think I know what you are asking now.. I think a common one I've worked on like that would be on cathodic rectifiers.. Some of them have a bar that connects to studs on the barrier for each tap. I think they are stainless..


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## farlsincharge

So should I special order silicon bronze hardware or just leave it as is?


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## paulengr

That’s not how lug connections work. The bolt provides pressure to squeeze the lugs and bus bars together. It is not intentionally part of the circuit. I wire up motors and generators all the time and that’s all we use most of the time. Silicon bronze is better from a corrosion point of view but practically I’ve seen no issues with grade 5 steel if the connections are waterproofed...88 backwrapped, 130C, 88. It’s kind of old school but I never know what connectors the customers will have.


Sent from my iPhone using Tapatalk


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## Tonedeaf

Unless the generator manufacture specifies something else...I would use Grade 5 hardware with lock washers....we use it for everything else....if you use the wrong alloy or stainless you could end up with heating issues at the connection point. The bolts are not suppose to carry the current they provide pressure for the conducting material.


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## paulengr

Thermal expansion in busbar applications is taken care of by Bellville washers. Given it's ductility I don't even know how you could use copper bolts and nuts for a compression application. Bronze yes copper no...hence why bus bars are usually either steel or silicon bronze bolts. Here is a copy of the busbar standard for how to properly joint bus bars. Jump to figure 70 for a nice illustration showing both the Bellville washer approach and the double nut approach.
http://admin.copperalliance.eu/docs...s/section-6-0-jointing-of-copper.pdf?sfvrsn=2

Be careful here though with the double nut picture right next to it because it's wrong. The thin nut should go on first and then the big nut, not the other way around! The following article shows the engineering behind this and why it doesn't work as shown.
http://www.boltscience.com/pages/twonuts.htm

Split washers are not "lock" washers. The exact term is "helical spring washer". That doesn't even sound like "locking" to me. The intended purpose is to take up some of the slack similar to a Bellville washer when loosening occurs but only in "general purpose" applications as per ASME. I'm not sure which applications they are talking about because once you compress it down to flat (the normal situation) at best, they do nothing. Maybe when I was a kid and tried to put something together with pliers where the whole thing would fall apart by the time I rode my bike to grandma's house but not in any kind of residential, commercial, or industrial plant where I'm going to flatten that washer completely. NASA agrees and states in NASA Fastener Design Manual RP-1228, "In summary a lockwasher of this type is useless for locking."
https://snebulos.mit.edu/projects/reference/NASA-Generic/NASA-RP-1228.pdf

That's a nice sentiment but the reality is even worse. Helical spring washers actually help to enhance loosening under actual self-loosening type conditions as explained in the link below. These in particular need to be thrown away and not used.
http://www.boltscience.com/pages/Why_nuts_and_bolts_can_self-loosen.pdf

"Star" type washers are intended as true "lock" washers. As per ASME: “3.1 The tooth‐lock washers covered in this Standard are intended for general applications. The tooth‐lock washers serve to increase the friction between the
screw and the assembly. Internal tooth‐lock washers are preferred where it is
desirable to provide a smooth periphery.”

However these also have a problem, too. As per NASA RP-1228 above that gives the most clear explanation: "Although this washer does provide some locking action, it damages the mating surfaces. These scratches can cause crack formation in highly stressed fasteners, in mating parts, or both, as well as increased corrosion suscepitbility". In other words right at the point where the stock (non-"locking") components are already stressed to their limits, the star lock washer comes "to the rescue" and helps make the joint fail faster.

In summary there are NO "lock" washers out there that are recommended for use in ANY bolted joint unless you have someone that is ordering you to use them.

What does work is that there are all kinds of "messed up thread", Nylock, and similar lock NUTS and bolts, safety wiring, nut caps, you name it. When I say "messed up thread" that's the best way I can describe all kinds of flattened/oversized/misshapen designs where the threads are purposely made to have more friction than a normal thread on the nut. There are probably as many designs out there as there are fastener engineers and they all work on the same principle. In electrical applications the messed up thread approach works best because Nylocks don't handle heat very well but everywhere else Nylocks are almost a go-to solution for mechanical holding. In extreme conditions such as aircraft, safety wiring and nut caps are the rule of the day.

Another approach is...Loctite. In other words glue it together. There are temperature limits to Loctite though so be careful of that.

Finally there is the problem of getting the correct tension. IAEI did a test on hundreds of professional and non-professional electrical folks and found that when we tighten bolts "correctly" as in with your choice of tools, we basically suck at it.
https://iaeimagazine.org/magazine/2...-a-torque-wrench-improves-system-reliability/

Using thread lubricant (Molykote or similar molybdenum disulfide pastes or sprays) or Loctite if you are going to be using a locking compound as a thread lubricant plus a proper torque wrench or torque screwdriver lets you hit the mark within +/-10%.

But that's only about 10-15% of the torque translates into actual clamping force and everything else is overcoming fastener friction, worrying about torque in some ways is a waste of time. DTI's (direct torque indicating washers) are even better and good for +/-5% but unfortunately I've looked everywhere and I can only find them for structural, not for electrical. These are special washers that have indents that compress down and a slot that you measure with feeler gauge to tell when the bolt is tight enough, sometimes filled with orange form-a-gasket type material that squirts out at the right tension. The best I've found so far for electrical work is that there is an ultrasonic machine that very accurately measures the amount of stretch on the bolt that directly translates into bolt tension if you know what the bolt material is. This is probably the best way to do it but not really practical to haul around a laptop and the sensor is still pretty big so it's not going to work very well on common electrical sizes like 1/4-20's and smaller. It would be good though on the larger 3/8" and 1/2" and larger sizes.

So for right now unless you're in a nuclear plant with money to burn, buy yourself a couple bottles of Loctite or Molykote, and use a torque wrench/screwdriver when you are doing critical joints, and get the thing calibrated once in a while because they do drift over time. I think most high end automotive shops calibrate their torque wrenches something like every 1000-2000 fasteners.


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## splatz

I don't get creative with electrical connections but with general mechanical applications, these are excellent and I have used them with good results: 

http://www.nord-lock.com/nord-lock/wedge-locking/washers/introduction/


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## gnuuser

steel bolts would be ok but use brass or copper washers and nuts.
read someplace in one of the old books that steel washers and nuts would act as a closed loop transformer and get hot enough to melt the conductors.
(on ac circuits)


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