Building safe equipment: grounding practices

[C12VR]

Hello,
I am, like many human beings, interested in staying alive while using my equipment and avoiding lawsuits from the other people who use it as well. I understand that the primary means of ensuring equipment safety is to have a solid mains earth point connected to the chassis and any metal components throughout the system, and minimize the potential for shorts to chassis. Is it enough to wire the earth pin of the IEC mains to a large bolt? How can I confirm that equipment is 100% safe?

 

[Tubetec]

Thats how its done ,
You will remove the paint on the chassis where the bolt passes through ,and use washers and toothed washers to get the best possible connection .
The moment any fault current flows to ground the mains should cut out,
easiest way to test your mains is working properly is a test plug that tells you the wiring is correct , then you also need to test the circuit breaker opperates correctly .

 

[Newmarket]

Is this for new equipment that you are manufacturing or maintenance / safety checks on existing kit ?

 

[ruffrecords]

The first safety rule is the IEC earth tag must be connected to a dedicated bolt close to the connector using shakeproof washers. The second rule is there must be continuity between this point and ALL exposed metal parts of the device. The third rule is the earth leakage from the mains input to the safety earth must be below a certain value.

In the UK, health and safety laws insist that all portable appliances (like kettles, drills, mixers, amps etc) must be tested annually for leakage and continuity and have a sticker attached to prove they have been tested. You can buy an approved portable appliance tester (PAT tester) for a couple of hundred pounds. I safety test all my mains powered builds this way.

Cheers

Ian

 

[C12VR]

Is this for new equipment that you are manufacturing or maintenance / safety checks on existing kit ?

Just diy equipment, preamps and mic power supplies. Not a manufacturer.

The first safety rule is the IEC earth tag must be connected to a dedicated bolt close to the connector using shakeproof washers. The second rule is there must be continuity between this point and ALL exposed metal parts of the device. The third rule is the earth leakage from the mains input to the safety earth must be below a certain value.

In the UK, health and safety laws insist that all portable appliances (like kettles, drills, mixers, amps etc) must be tested annually for leakage and continuity and have a sticker attached to prove they have been tested. You can buy an approved portable appliance tester (PAT tester) for a couple of hundred pounds. I safety test all my mains powered builds this way.

Cheers

Ian

What is a safe earth leakage value? With all the tube mics I have there is a small (25mv) voltage and a resistance of 15ohms between mic body and chassis earth, but only when they are on; in some of these mics the voltage and resistance are negative. When the mics are off resistance is equal to meter lead resistance (less than .5ohm)

 

[ccaudle]

resistance... negative

Since that is a physical impossibility, that is a clue that you are not actually measuring what you think you are.

What is a safe earth leakage value?

The health safety limit is on the order of a small number of mA. I work with servers a lot in my day job, and a typical rating for server power supplies is under 1mA leakage current, but you may have multiple power supplies installed in a single chassis.

Note that the leakage currents which are below the limits of safety concern can cause audio performance problems. If you have not made some mistake like getting metal dust into your chassis wiring any leakage current would typically be either very small currents through transformer parasitic capacitance, or through intentional capacitance in power entry EMC filters. For most DIY equipment I would advise against putting in a high current industrial EMC power entry filter, stick with an unfiltered entry power, or use a medical style filter which has only line-to-line filtering and not line-to-earth filtering and you will not have any problem with leakage current.

 

[JohnRoberts]

Hello,
I am, like many human beings, interested in staying alive while using my equipment and avoiding lawsuits from the other people who use it as well. I understand that the primary means of ensuring equipment safety is to have a solid mains earth point connected to the chassis and any metal components throughout the system, and minimize the potential for shorts to chassis. Is it enough to wire the earth pin of the IEC mains to a large bolt? How can I confirm that equipment is 100% safe?

The safety agencies (like UL) have accumulated decades of experience to establish safety guidelines. Much too complicated to detail here. That said there are a few short cuts we can use to insure personal safety for us and our friends.

#1 GFCI (RCD) outlet protection devices. These devices will trip when they detect more than a few mA (nominally 6 mA) of current leaving the outlet and not returning as expected. This will break the electrical circuit and protect against lethal stray currents harming humans.

#2 double insulated power supplies... Popular wall warts use double insulation to forgo the need for safety ground connections. External wall warts have probably faded in popularity, but small OEM switching PS modules could be mounted inside chassis and as long as the modules are UL approved they should be safe.

Safety ground bonded chassis are the old school way to protect against stray paths energizing exposed metal. This works as long as the ground bond is robust enough to trip the fuse/current breaker.

JR

PS; I am not a fan of old fashioned 3 lamp outlet testers, they can be tricked by RPBG reverse polarity Bootleg ground mis-wiring into thinking an outlet with energized safety ground is still OK.

For way TMI check my... improved outlet tester