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.
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?
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.