Black finish screws used for earthing to metal case.( possibly non conductive)

Thread Starter

aspiespot

Joined Dec 14, 2018
20
We have noticed on some earthed equipment that the pan head machine screws that connect the earthing wires to the metal case are black in colour.( shake proof washers are also used)

Looks like some of these black finishes are conductive ( like black nickel / Zinc-Iron / black chromate ) and some are not conductive (like a oxide finish )

I would welcome any help or advice on whether black finish machine screws should be used ? ( especially the non conductive oxide ones)Although I understand the black finish on these screws is so so thin and maybe wears off when screwed together ,
 

djsfantasi

Joined Apr 11, 2010
7,764
We have noticed on some earthed equipment that the pan head machine screws that connect the earthing wires to the metal case are black in colour.( shake proof washers are also used)

Looks like some of these black finishes are conductive ( like black nickel / Zinc-Iron / black chromate ) and some are not conductive (like a oxide finish )

I would welcome any help or advice on whether black finish machine screws should be used ? ( especially the non conductive oxide ones)Although I understand the black finish on these screws is so so thin and maybe wears off when screwed together ,
Just sand off the color before assembly.
 

jpanhalt

Joined Jan 18, 2008
11,088
We have noticed on some earthed equipment that the pan head machine screws that connect the earthing wires to the metal case are black in colour.( shake proof washers are also used)

Looks like some of these black finishes are conductive ( like black nickel / Zinc-Iron / black chromate ) and some are not conductive (like a oxide finish )

I would welcome any help or advice on whether black finish machine screws should be used ? ( especially the non conductive oxide ones)Although I understand the black finish on these screws is so so thin and maybe wears off when screwed together ,
Test them. Some "black" SS are not much more than a very thin layer of dye, it seems. In any event, removing any insulating oxide under the head and on the thread is trivial, if you like the looks..
 

Hymie

Joined Mar 30, 2018
985
Where a conductive enclosure is required to be earthed for electrical safety purposes, it will have passed the relevant earth bond test – demonstrating a low impedance connection to the protective earthing terminal of the product.

During type test, it is sometimes necessary to scratch the paint finish of the product to make an electrical connection in order to conduct the earth bond test.

Bear in mind that where the conductive enclosure is separated from mains circuits by double/reinforced insulation then there is no requirement for the enclosure to be connected to safety earth.
 

shortbus

Joined Sep 30, 2009
8,723
What difference would it make if the screws were nylon? None. The screw is only used to keep the ground terminal AGAINST the equipment's base not to conduct through it. Unless you have some kind of equipment that is smaller than the head of the screw?
 

djsfantasi

Joined Apr 11, 2010
7,764
What difference would it make if the screws were nylon? None. The screw is only used to keep the ground terminal AGAINST the equipment's base not to conduct through it. Unless you have some kind of equipment that is smaller than the head of the screw?
I agree. If the case is not painted or anodized at the attachment point. However, by removing the color from the back of the screw head, you have a better connection. A tiny piece of sandpaper and a minute of sanding is cheep insurance.
 

MaxHeadRoom

Joined Jul 18, 2013
23,381
We have noticed on some earthed equipment that the pan head machine screws that connect the earthing wires to the metal case are black in colour.( shake proof washers are also used)
Looks like some of these black finishes are conductive ( like black nickel / Zinc-Iron / black chromate ) and some are not conductive (like a oxide finish )
What is the nature of the equipment?
I would suspect that maybe these earthing conductor points may have been supplied by the installer etc.
Any decent or recognized industrial enclosure manuf. that I have come across has always supplied the correct metallic hardware.
Max.
 

Tonyr1084

Joined Sep 24, 2015
6,079
What difference would it make if the screws were nylon? None. The screw is only used to keep the ground terminal AGAINST the equipment's base not to conduct through it. Unless you have some kind of equipment that is smaller than the head of the screw? {bold underline my edit}
The material the screw is made of DOES have an affect. Use of a non-conductive screw to ground is far weaker than that of a conductive screw. In standard practice it has always been to put an internal tooth star lock washer between the head of the screw and the chassis. This serves two purposes, to bite into the metal and to prevent the screw from backing out. PC boards with ground pads typically get a screw and a star washer, usually internal tooth. Again, both for fastening strength and electrical bonding. The teeth of the washer bite through minor oxidation and improve the grounding.

IPC-A-610E section 4.1 Hardware, figure 4-14 shows an external tooth lock washer between the head and the lug. It also shows a flat washer, which can also be representative of the chassis. The nut is either a stand alone piece of hardware or it can be a threaded hole in the chassis. This is the preferred method of grounding as the lug is held firmly against the chassis via the lock washer. This lock washer also electrically bonds the lug to the head, which is a part of the mechanical assembly, providing contact to both sides of the lug to ground Through the screw. A nylon screw does not conduct.
1608826031851.pngIPC-A-610E-2010, pg. 4-7
 

Tonyr1084

Joined Sep 24, 2015
6,079
You wouldn't want to use a painted screw on a painted surface. There is no electrical bonding. Bare metal and a conductive screw are required to form an electrical bond, whether it's ground or some other power plane.

Some lugs have a built in Internal Tooth Lock washer. This is for electrical bonding AND for mechanical fastening that prevents the screw from coming loose. Here's an example of some internal tooth solder lugs.
 

MaxHeadRoom

Joined Jul 18, 2013
23,381
Here's an example of some internal tooth solder lugs.
You may get away with it on those small lugs, but the regulation states "Earth Wire Connections that depend solely upon a solder connection cannot be used for grounding connections"
The reason is that when equipment grounding carries a current, it can heat.
It is permissible however to mechanically secure the connection first and then apply solder to the joint.
Max.
 

Tonyr1084

Joined Sep 24, 2015
6,079
@MaxHeadRoom; the conditions I have in mind are along the lines of low voltage electronics. I've spent over 30 years in the Quality Control sector of manufacturing PCB's from all phases, raw boards to Printed circuit boards to stuffed, tested and pressed into service. I've also worked with heavy gauge, high current equipment such as in the aerospace industry, and yes, soldering those is not allowed for the very reason you highlight. However, whether using a #8 screw or a 4mm screw for grounding - hardware stickup is generally as shown. I say "Generally" because there are a variety of differing circumstances where stack-up is critical. Such as on a terminal strip with studs and multiple ring terminals. The way the belly of such crimps (or soldered terminals) face as well as whether it has a flat washer against the terminal backed up with a split ring lock washer and a nut. Or multiple nuts where there are a large number of terminals bolted on.

Anyway, for the way I took the TS question, I assumed we were talking about something like the grounding lug on a small frame of some sort like a low voltage power supply. Assuming is a dangerous venture for sure. But off the cuff, that's the way I took it.
 

MaxHeadRoom

Joined Jul 18, 2013
23,381
Anyway, for the way I took the TS question, I assumed we were talking about something like the grounding lug on a small frame of some sort like a low voltage power supply. Assuming is a dangerous venture for sure. But off the cuff, that's the way I took it.
Hence my question in #8 ! ;)
 

shortbus

Joined Sep 30, 2009
8,723
Here https://en.wikipedia.org/wiki/Anodizing is an article on anodizing. Most of the time the coating is non-conductive.
I have my doubts that any of the metals that can be anodized are used in grounding screws.

If the screws are black they are treated like they do socket head screws, black oxide, like gun "blueing".
" When fasteners are dipped into a black oxide bath material the harsh chemical components convert the top layer into magnetite." From - https://www.fastenersolutions.com/socket-head-cap-screws.html
 
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On electrical plugs I see colored screws all the time. Brass, silver and green. Off hand I don't recall which is the hot and which is the neutral, but the ground screw (hex head) is always green and always on the neutral side. Every one of those screws is conductive.

Agreed. Brass is a plating. The silver is probably plated. I have no idea where green comes from.
the silver screw is the white wire. The brass screw is the black or hot wire and green is ground.

Aluminum, I have seen red, blue and black. mag-lite flashlights are anodized. Aluminum heat sinks are usually anodized black.
 

MisterBill2

Joined Jan 23, 2018
9,034
It should be a quite simple test to verify as to the adequacy of the ground connection, which should answer the question quite well, far better than a bunch of guesses and reading to us from the code book. For a system with 120 volt mains simply use a 100 watt incandescent light bulb connected between the mains line wire and the ground connection in question. If the light illuminates to full brightness, as can be verified by measuring the voltage across the light terminals, then the ground is OK.
For additional verification, use a ground bond checker, which is a portion of most ground safety check instruments.
 

shortbus

Joined Sep 30, 2009
8,723
For a system with 120 volt mains simply use a 100 watt incandescent light bulb connected between the mains line wire and the ground connection in question.
Why would anyone do that? It would both be safer and easier to just measure the resistance the an ohmmeter between the screw head and the metal frame of the equipment. Doing it your way would both expose the person to open mains voltage and prove nothing if the ground and neutral weren't bonded at the breaker box.
 
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