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RaymondR · 17-01-2019, 11:48

Quote:

Originally Posted by evm1024

Sorry to say that he is more correct than not.

Nigel Calder's Boatowner's Mechanical and Electrical Manual 3rd edition page 186 "Conductivity of Metals and Bolted Connections" has a full description of the problem.

The stacking order of a bolted connection is quite important.

Simply stated you want the tabs of the fuse to be in direct contact with the bus bar that it is bolted to. If you put a washer on the stud first then the fuse you have introduced the resistance of the washer into the major path of the current as well as an additional contact.

If the washer is made of silver, copper (high conductivity) you will see minimal resistance.

Brass conducts at a rate that is 26% that of copper, bronze has even less conductivity and stainless steel conducts only 3% that of copper.

If you put a stainless washer under the fuse (between the fuse tab and the buss) they you have introduced a significant resistance and at high currents it will get hot. Perhaps hot enough to start a fire.

This is what he was saying.

Regards.

Yes, in an ideal world the face of a terminal should be in contact with a bus bar however:

Many electrical connections do not involve a bus bar, some involve the terminal being between two nuts and I find a better connection results from having a washer either side of the terminal.

Copper washers are hard to find and a little to soft to be practical. Most auto parts stores and a lot of hardware shops carry brass washers.

The brass washers are so thin as to not cause significant resistance compared to copper however, as my experience with the two incidents illustrates, this is not the case with stainless steel where high currents are involved.

The down side of the cheap Asian goods we now benefit from is that a lot of the stuff is not designed or engineered it is just copied and a chrome plated electrical washer looks a lot like a SS washer and this is what ends up on electrical equipment.

In the case of my anchor winch it was only one of the three terminals which had SS washers the other two being OK with the proper chrome plated electrical washers. I assumed that a previous owner of the winch had made the substitution. The point of the narrative was to avoid SS washers in electrical work and where there is excessive heating of a terminal check for SS washers, particularly if it appears the culprit is a reasonably clean and un-corroded connection.

Montanan · 17-01-2019, 12:14

Quote:

Originally Posted by evm1024

Sorry to say that he is more correct than not.

Nigel Calder's Boatowner's Mechanical and Electrical Manual 3rd edition page 186 "Conductivity of Metals and Bolted Connections" has a full description of the problem.

The stacking order of a bolted connection is quite important.

Simply stated you want the tabs of the fuse to be in direct contact with the bus bar that it is bolted to. If you put a washer on the stud first then the fuse you have introduced the resistance of the washer into the major path of the current as well as an additional contact.

If the washer is made of silver, copper (high conductivity) you will see minimal resistance.

Brass conducts at a rate that is 26% that of copper, bronze has even less conductivity and stainless steel conducts only 3% that of copper.

If you put a stainless washer under the fuse (between the fuse tab and the buss) they you have introduced a significant resistance and at high currents it will get hot. Perhaps hot enough to start a fire.

This is what he was saying.

Regards.

Thank you for that very well stated clarification.

The installation of comparatively poor conductors into primary circuit pathways happens all to often, not just aboard vessels mind you. It takes a lot of conscious consideration and skilled decision making to assemble a circuit as to positioning, connectivity surface contact and conductor sizing, materials, etc.

The picture the OP posted indicate a concern to me of potentially having non-current limiting protective circuitry of opposing polarity cabling in contact. There appears to be negative and positive cables crossing or even tied together which cabling may be in front of [on the battery power source side] of the protective fuse. If so the cables pathways should be redesigned so as to not have any contact on the non-fuse protected portion. The picture does not show the entire circuitry on a portion of the cabling so one can not be certain as to the installed circuit.

evm1024 · 17-01-2019, 13:23

Something to remember is that current flows through all paths on proportion to their (inverse) resistances.

More current flows through low resistance paths than high resistance paths. And the total resistance is equal to: 1 / (1/r1 +1/r2 +1/r3 ...)

When we stack lugs, washers, nuts etc we are creating a number of paths. For this discussion I will only consider 3 paths (and their resistances).

Looking at the drawing we can call path 1 the path that goes out of the lug bottom and directly into the buss (base). Path 2 is out the edges of the hole in the lug where they contact the stud and into the stud. Path 3 is out the top of the lug, through the washer, lockwasher and nut to the stud. I've clumped these all together because of the many paths - to the washer and thus the stud, through the washer to the lock washer to the stud etc).

Clearly the ( path 1) lug to buss contact is low resistance. It has a large contact area and is likely made of (tin coated) copper.

Path 2 is much more variable. The lug is copper and the stud "may" be copper. But the key takeaway is that the contact area is small and thus the resistance is "high".

Lastly path 3 has a "high" resistance in most cases. Unless the washer, lock washer and nut are made of copper or other low resistance materials there will be significant resistance.

Using the formula above we get: total resistance = 1/ (1/low resistance + 1/high resistance + 1/high resistance) which equals a low resistance with the vast majority of the current flowing through path 1 directly from the lug into the bus.

No problem. Very low voltage drop.

But, Lets put a stainless washer under the lug. Stainless with less than 3% of the conductivity of copper now causes path 1 to be of much higher resistance than a copper path.

This is not to say that stainless is a poor conductor. All metals conduct electricity quite well compared to an insulator. See this webpage for conductivity of metals:

Conductivity Of Metals Sorted By Resistivity | Eddy Current Technology

But, when path 1 includes a stainless washer the path resistance is now much closer to the path resistance of path 2 and path 3. And thus a much greater portion of the total current flows through path 2 and path 3. Where as with no stainless washer (copper to copper contact in path 1) almost no current flows in proportion through path 2 and path 3.

As a result we now have the full circuit current flowing through some "high" resistance paths with their associated voltage drops and thus power dissipation as heat.

If the resistance of each path is equal the current will divide equally. And as such we will end up with 1/3 of the total current flowing through path 2. Which is to say through the tiny contact "patch" of the lug to the stud.

Instant heat.

That's my story and I'm sticking to it.

mitiempo · 17-01-2019, 15:47

Quote:

Originally Posted by Zil

DC. Use the proper sized wire for least voltage drop. Use a fuse sized to the capacity of the wire. That is the only correct choice.

The fuse should be larger than the load and smaller than the wire's ampacity. If the wire is sized correctly this gives a wide range of amperages to choose a fuse from. If the wire is too small and therefore has an ampacity too close to the size of the load the range of fuse sizes is too small.

**Wotname** · 17-01-2019, 16:17

Quote:

Originally Posted by Zil

If there are washers on a terminal they must be in the position that NO current will pass through the washer. Not any kind of metal.

Regardless of what other's think you are saying, there is simply NO way to ensure current will not flow though any washers unless they are insulated from the post, nut and fuse.

Just repositioning them does not prevent them passing some of the current; of course, having them "at the top" is a good start in reducing the heat of the junction. In fact, it is better for them to pass current than not; it lowers the total resistance of the junction.

**Wotname** · 17-01-2019, 16:20

Quote:

Originally Posted by evm1024

Something to remember is that current flows through all paths on proportion to their (inverse) resistances.

More current flows through low resistance paths than high resistance paths. And the total resistance is equal to: 1 / (1/r1 +1/r2 +1/r3 ...)

When we stack lugs, washers, nuts etc we are creating a number of paths. For this discussion I will only consider 3 paths (and their resistances).

Looking at the drawing we can call path 1 the path that goes out of the lug bottom and directly into the buss (base). Path 2 is out the edges of the hole in the lug where they contact the stud and into the stud. Path 3 is out the top of the lug, through the washer, lockwasher and nut to the stud. I've clumped these all together because of the many paths - to the washer and thus the stud, through the washer to the lock washer to the stud etc).

Clearly the ( path 1) lug to buss contact is low resistance. It has a large contact area and is likely made of (tin coated) copper.

Path 2 is much more variable. The lug is copper and the stud "may" be copper. But the key takeaway is that the contact area is small and thus the resistance is "high".

Lastly path 3 has a "high" resistance in most cases. Unless the washer, lock washer and nut are made of copper or other low resistance materials there will be significant resistance.

Using the formula above we get: total resistance = 1/ (1/low resistance + 1/high resistance + 1/high resistance) which equals a low resistance with the vast majority of the current flowing through path 1 directly from the lug into the bus.

No problem. Very low voltage drop.

But, Lets put a stainless washer under the lug. Stainless with less than 3% of the conductivity of copper now causes path 1 to be of much higher resistance than a copper path.

This is not to say that stainless is a poor conductor. All metals conduct electricity quite well compared to an insulator. See this webpage for conductivity of metals:

Conductivity Of Metals Sorted By Resistivity | Eddy Current Technology

But, when path 1 includes a stainless washer the path resistance is now much closer to the path resistance of path 2 and path 3. And thus a much greater portion of the total current flows through path 2 and path 3. Where as with no stainless washer (copper to copper contact in path 1) almost no current flows in proportion through path 2 and path 3.

As a result we now have the full circuit current flowing through some "high" resistance paths with their associated voltage drops and thus power dissipation as heat.

If the resistance of each path is equal the current will divide equally. And as such we will end up with 1/3 of the total current flowing through path 2. Which is to say through the tiny contact "patch" of the lug to the stud.

Instant heat.

That's my story and I'm sticking to it.

I missed this post (

) and it makes my previous post somewhat redundant.
Very well explained

**Wotname** · 17-01-2019, 16:25

Quote:

Originally Posted by RaymondR

Yes, in an ideal world the face of a terminal should be in contact with a bus bar however:

Many electrical connections do not involve a bus bar, some involve the terminal being between two nuts and I find a better connection results from having a washer either side of the terminal.

Copper washers are hard to find and a little to soft to be practical. Most auto parts stores and a lot of hardware shops carry brass washers.

The brass washers are so thin as to not cause significant resistance compared to copper however, as my experience with the two incidents illustrates, this is not the case with stainless steel where high currents are involved.

The down side of the cheap Asian goods we now benefit from is that a lot of the stuff is not designed or engineered it is just copied and a chrome plated electrical washer looks a lot like a SS washer and this is what ends up on electrical equipment.

In the case of my anchor winch it was only one of the three terminals which had SS washers the other two being OK with the proper chrome plated electrical washers. I assumed that a previous owner of the winch had made the substitution. The point of the narrative was to avoid SS washers in electrical work and where there is excessive heating of a terminal check for SS washers, particularly if it appears the culprit is a reasonably clean and un-corroded connection.

To say nothing of the material of the post on "some of the Asian stuff".

masonc · 17-01-2019, 18:10

Quote:

Originally Posted by rom

Hello everyone, I have a fuse holder that allegedly handles 750A but I can hardly reach 400A for a few minutes before it gets cray hot. Temperature increases around as well of course. 95mm2 cable on the left, copper bar to the right. Right now the fuse is a 425A, maybe not a true ANL ? Any ideas ?Attachment 183815
Also, is there a way to tell what should be the max. 230VAC amperage (fuse) if I decide that 400A is the max 12VDC ? (let's pretend the inverters work at 90% efficiency)

Put a voltmeter across the fuse holder, see what the voltage drop is.
Get hold of a thermal camera and take a picture of the fuse holder at full current flow, that'll identify the problem.

masonc · 17-01-2019, 18:12

Even if your cable is rated at 105C, the terminals aren't.

RaymondR · 17-01-2019, 19:45

Just had another look at post #1 and expanded it as much as I could. Are those nice shiny bolts SS or chrome plated copper?

I agree that brass has about four times the resistance of copper but in the real world is it significant when the current path is around 1/16"?

On the Asian thing, we all know there is Asian and Asian, the second one is what I was referring to.

rom · 18-01-2019, 02:06

Quote:

Originally Posted by witzgall

I don't think you can software limit the output of a multi, only the AC input.

Chris

And you are right ! I got mixed up with so many interesting features... Back to square one, only solution is to limit 230VAC with a breaker.

rom · 18-01-2019, 02:11

Quote:

Originally Posted by mitiempo

No it is not dangerous. In any proper installation the wiring is sized for the max load plus a margin beyond that. The fuse is not there for anything but a short circuit. If a fuse blows under load then it is the incorrect fuse.

google:

"a safety device consisting of a strip of wire that melts and breaks an electric circuit if the current exceeds a safe level"

wikipedia "fuse":
"fuse is an electrical safety device that operates to provide overcurrent protection"

wikipedia "overcurrent"
"Possible causes for overcurrent include short circuits, excessive load, incorrect design, or a ground fault"

I am going to use a breaker that among other things will limit excessive loads on the AC side.

rom · 18-01-2019, 02:43

Quote:

Originally Posted by masonc

Put a voltmeter across the fuse holder, see what the voltage drop is.
Get hold of a thermal camera and take a picture of the fuse holder at full current flow, that'll identify the problem.

Best advice so far. @400A the fuse/holder dissipates about 36W, the BEP Marine switch right next to it connected to the copper bar and rated 600A continuous dissipates about 40W, I have to admit I underestimated that one.

Zil · 18-01-2019, 04:48

I hope none have washers between the contact surfaces of their electrical shtuf. Lug to lug or lug to post. or lug to anything else you may dream up for sake of argument. Where can I buy those silver washers? How does a lug temperature be rated less than 105c?