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Grounding - safety and best practices

Madjalapeno

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While there is much talk on here about ground loops and lifting of grounds, I didn't find much on the safety issues when working with mains voltages, and what the best practices are, so I thought it would be good to get something together so we don't add to the Electric Shock thread.

A lot of issues can be avoided with the use of external power supplies, but whenever using mains voltages we need extra safety, and consider worst case scenarios and have multiple safeties.

Some of this information comes from IEC 60601-1 Medical Design Standards for Power Supplies (I'm not suggesting using medical filters), as it is just a good basis for safety, and some comes from other posts here, based on tear-downs, etc. (Thanks to @Doodski, @restorer-john, @Buckeye Amps and many more).

The terms Earth and Ground seem to be used interchangeably depending on nationality.

I am not an EE, so please take precautions. This should be a bare minimum. Please add to this, and correct me if I'm wrong.

IEC 60601-1​


The following notes are from 60601 - but I've only copied small, relevant sections. It’s a huge, 420 page document, but is a good guide if you’re interested in this subject. Section 8.6 is the most important for us.

Subsection 8.6.2

Screws for internal PROTECTIVE EARTH CONNECTIONS shall be completely covered or protected against accidental loosening from the outside of ME EQUIPMENT.

Where an APPLIANCE INLET forms the supply connection to ME EQUIPMENT, the earth pin of the APPLIANCE INLET shall be regarded as the PROTECTIVE EARTH TERMINAL.

The PROTECTIVE EARTH TERMINAL shall not be used for the mechanical connection between different parts of the ME EQUIPMENT or the fixing of any component not related to protective earthing or functional earthing.


Subsection 8.6.4

PROTECTIVE EARTH CONNECTIONS shall be able to carry fault currents reliably and without excessive voltage drop.

Additionally, the impedance between the protective earth pin in the MAINS PLUG of any DETACHABLE POWER SUPPLY CORD supplied or specified by the MANUFACTURER, when attached to the ME EQUIPMENT, and any part of the ME EQUIPMENT that is PROTECTIVELY EARTHED shall not exceed 200 mOhm


8.6.5 Surface coatings

Conductive elements of ME EQUIPMENT that have surface coatings of poorly conducting material such as paint, and between which electrical contact is essential to a PROTECTIVE EARTH CONNECTION, shall have the coatings removed at the point of contact

Subsection 8.9

CREEPAGE DISTANCES and AIR CLEARANCES

I’ll summarize this one as it’s long:

Electricity can travel along dry insulating surfaces (CREEPAGE) and jump across gaps (CLEARANCE). For 250V, the minimum CREEPAGE distances are 4mm, and CLEARANCE distances are 2.5mm. There should be nothing closer to live terminals than those distances.

So in layman's language, what does this mean?

  • General safety – while we know ventilation is important, we need to be aware of the size of those vents. Section 5.9.2.1 defines the use of a test finger and making sure that nothing deforms, or that a live connection can not be touched. Ideally vents should be smaller than 3mm (or less if you have kids). All panels and fuses should require the use of a tool to remove.
  • Ground connections should be made to bare chassis metal. We should try and make sure every panel is grounded, either by removing anodization where panels touch, or the use of ground leads.
  • Rather than scraping or filing by hand there are special wire brushes that have a pilot bushing to remove anodizing. (https://www.audiosciencereview.com/...-multichannel-amp-teardown.27648/#post-955548, https://www.yardstore.com/drilling-and-cutting/sanding-discs-and-acc/wire-bonding-brushes, https://www.mcmaster.com/4912A22/)
  • Ground connections must be secure. Ideally a PEM stud would be used as there is a direct metal contact between the stud and the chassis panel, and it can't be accidentally loosened from the outside.
  • The wire from the power input module is terminated with a closed loop connector which is fixed to the earthing stud or screw with a star or lock washer and a nut. Other parts of the product that need to be earthed are connected by closed loop connectors to the same stud and locked with an additional nut. It is important that the earth wire from the power supply cord is located at the bottom of the stud and locked with its own nut. (source: https://www.601help.com/Requirements/earthing.html)
  • Cables need to be protected from abrasion. They should be anchored at regular intervals with clamps or cable ties that are securely fastened to the enclosure. Adhesive backed clamps should be avoided unless you want to annoy @amirm. Additionally, I like to make sure that wires carrying mains voltages are inside a protective sleeve or wrap. Apart from anything else it makes things neater.
  • Fuses should be included, even though many SMPS’s have inbuilt fuses. It’s much easier to replace a fuse on the back panel than it is to remove the fuse from the power supply. The fuse-holder should have the type and full rating marked adjacent to the fuse-holder (Subsection 7.2.12)
  • Power switches used to control power to equipment should either be marked with symbols IEC 60417-5007; or indicated by an adjacent indicator light; or indicated by other unambiguous means.
  • Any conductive material less than the creepage and clearance distance from the mains voltage should be protected with the use of an insulating material. Live terminals should be protected anyway with heatshrink or another method of insulation.
 
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Madjalapeno

Madjalapeno

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An additional section.

When using multi-strand wire, it's a pain to make sure every strand ends up in the receptacle. One of those strands can be enough to cause a problem. The use of Wire Ferrules is required in Europe to receive a CE mark, but it's a good idea to use anyway.


Bootlace%20ferrule%20colour%20code.jpg


 
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Madjalapeno

Madjalapeno

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This one is part electrical safety, and part general safety.

Buy a deburring tool. Drilled holes have sharp edges that will knick the insulation on a wire or cut you.

0603A195-E5CC-4270-B8E1-88B9C252A943_1_105_c.jpeg


Only takes a few swipes to remove the burr on an edge and make things safer.
 

restorer-john

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This one is part electrical safety, and part general safety.

Buy a deburring tool. Drilled holes have sharp edges that will knick the insulation on a wire or cut you.

View attachment 174278

Only takes a few swipes to remove the burr on an edge and make things safer.

Notice how that nice hole is not centred with the crosshair/markers? ;)
 

audio2design

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An alternative to (badly) soldering splices and joins in wires is using WAGO connectors, which are UL approved.

wago-connector-image-4.png



I liked the old gray ones better. They seemed to hold better. Avoid the Tyco ones. Very poor alternative.

If you like those above, I find these really good too. They are very gentle on the wires, but hold really well.

https://quickbit.co.uk/wago-24a-grey-2-5mm-2-way-service-connector-224-201/


1640296350974.png
 
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audio2design

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Never solder a wire that will be held by non-spring loaded compression. That means:

- Never solder a wire prior to using it in a crimped terminal
- Never use a soldered wire in a screwed down connection
- Do not tin your wired before using them in speaker or amplifier terminals unless those terminals are spring loaded

The solder flows. Slowly, but it flows. The connection will gradually come loose. In an AC connection, this could lead to a thermal event.
 
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Madjalapeno

Madjalapeno

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Edges of sheet metal can also be razor sharp, but is harder to deburr. Think of getting a paper cut, but from a sheet of metal.

It's a good practice to either use edge protectors or grommets on all holes or edges near to where cables will be. Just make sure it is flame retardant.

1640300534234.png
 

bigjacko

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Interesting stuff, thanks for sharing. I have a dumb question, why ground the chassis instead of other place like the module itself? I am thinking even if we ground the chassis, the chassis has to connect to module's ground, right? Otherwise if we place the amp on an insulated surface, then grounding to chassis will lead to nowhere.
 
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Madjalapeno

Madjalapeno

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Interesting stuff, thanks for sharing. I have a dumb question, why ground the chassis instead of other place like the module itself? I am thinking even if we ground the chassis, the chassis has to connect to module's ground, right? Otherwise if we place the amp on an insulated surface, then grounding to chassis will lead to nowhere.
We're trying to make it all as safe as possible to avoid electrocution, or a fire, by giving the simplest, lowest resistance path to ground.

Grounding the chassis, which is a large metal box, means that even if a wire comes loose or frays that the current has a simple path to ground and will blow a fuse, rather than damage the module, or electrocute anyone touching the box.

With an aluminum amplifier case the anodized surface acts as a poor quality insulator, which is why it's important to make sure that each panel has a connection to ground either with a wire, or by removing the anodization so there is a good metal to metal contact. It's important to make sure the path to ground has enough current handling ability to blow the fuse.

Using a mains cable with a ground wire we have a clear path will act as the ground even if the unit is on an insulated surface, rather than shocking the user. The alternative is a called Double Grounding, where a product is designed that even if an internal wire comes loose it can never make contact with any conductive part of the outer surface. It can be done with plastic cases, but it's more complicated.

 
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- Never use a soldered wire in a screwed down connection
this is actually a valid solution to the "endcaps" or what they are called in english (aderendhülse in german) as far as i know, even allowed by VDE
its just not allowed to use flexiable cables without anyway because strains can get "screwed off" by the screw otherwise

there are full copper crimps for cables, they cost a little much because of "audiophile" but they are worth it imo, you dont know what materal the 0815 ones are

- Do not tin your wired before using them in speaker or amplifier terminals unless those terminals are spring loaded
specially the spring loaded ones are made for blank flexiable cables as far i know, i would avoid solder their just to avoid the additonal layer of "not so good" conducting material tho

Never solder a wire prior to using it in a crimped terminal
true! tho im pretty sure they stop fitting too (atleast if you use the right size crimps for the same size cables)
 
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