Electronics > Beginners
Connecting system ground to mains earth
johnmx:
Normally I use switching AC/DC power supplies (from XP Power, EOS, Traco Power, etc.) in my prototypes. Some power supplies have the outputs completely isolated from mains earth. Although they usually have the footprint of a capacitor in the PCB to connect the output DC ground to earth, they don’t have this component populated. The problem is that in most of the times appears an AC voltage between the DC ground and power supply’s case which is connected to earth. In some cases can be as high as 90Vac which is enough to give an electric shock (I tried :-X)
How do you recommend connecting the DC ground to the mains earth?
A direct connection eliminates the potential difference but may increase noise (EMI) in system ground.
A 1MOhm resistor in parallel with a 100nF/250V film capacitor may reduce the voltage but doesn’t eliminate it. Still 6Vac measured with a handheld multimeter.
If there is a ‘RF expert’ around here please tell me what you do in this cases, because the system ground is always connected to the equipment case.
Kiriakos-GR:
You had just reply by your self , in your own question ...
--- Quote ---In some cases can be as high as 90Vac which is enough to give an electric shock
--- End quote ---
And so anything lower , that it can not cause shock ( like the 6 Volts ) it does stand as solution.
Zero999:
It shouldn't be a safety issue as the current is too low.
The only problem is it could damage some static sensitive components but as long as the case is connected to the same supply voltage as the circuit you're measuring (either +V, -V or 0V) via a low enough impedance, it shouldn't be a problem.
One thing you could try is powering the 'scope from a traditional mains transformer based power supply or using an isolation transformer with the SMPS.
Feanor:
This 90VAC could be the difference between mains earth and mains neutral. This can occur when the mains voltage "floats". Can be caused by many thins such as poor load balancing on the 3 phase line supplying your area.
I have this problem at home with a poor earth connection. The voltage between neutral of my cellphone charger and my body for example gets up to around 80VAC when the weather has been dry for a few weeks. It can give a stinging tingle if you make contact with it.
The mains neutral and mains earth should be at the same potential and solidly connected to the ground by way of a ground spike. They should be connected together on the supply side of your home distribution board. Connecting earth to neutral on the load side of your home distribution board can lead to your RCD (residual current detection) circuit breaker tripping all the time.
My home is very old and there are no RCD circuit breakers at all, so I just buried a 2 meter piece of copper pipe in the ground and I connect it to the neutral of what ever I am using as the initial power supply (wall wart, cellphone charger, ATX power supply). Problem solved until I upgrade the wiring of the house, when this happens a proper ground spike will need to be put in and the neutral and earth will be connected to this on the supply side of the distribution board.
alm:
--- Quote from: Feanor on December 19, 2010, 04:17:24 am ---This 90VAC could be the difference between mains earth and mains neutral. This can occur when the mains voltage "floats". Can be caused by many thins such as poor load balancing on the 3 phase line supplying your area.
--- End quote ---
This is easy to verify with a DMM. But for ungrounded switchers, I think it's more likely that it's due to a capacitive voltage divider formed by the input filter caps.
--- Quote from: Feanor on December 19, 2010, 04:17:24 am ---I have this problem at home with a poor earth connection. The voltage between neutral of my cellphone charger and my body for example gets up to around 80VAC when the weather has been dry for a few weeks. It can give a stinging tingle if you make contact with it.
--- End quote ---
Cell phone chargers (which are usually double isolated, so nothing to do with ground potential) should not allow any significant current to flow between people touching it and either the live or neutral. Both are considered identical from safety perspective in consumer equipment.
--- Quote from: Feanor on December 19, 2010, 04:17:24 am ---The mains neutral and mains earth should be at the same potential and solidly connected to the ground by way of a ground spike. They should be connected together on the supply side of your home distribution board. Connecting earth to neutral on the load side of your home distribution board can lead to your RCD (residual current detection) circuit breaker tripping all the time.
--- End quote ---
This depends on the local earthing system (eg. TN-C vs. TT). Some systems may even have ground in the middle between the two other conductors (eg. on ships). Connecting any other conductor to ground could cause some serious fireworks in that case. Even in TN-C systems, neutral and ground are usually not at the same potential, since the neutral wire carries current, and the ground wire doesn't, so the neutral wire has an IR drop (this is actually a neat way to measure wiring resistance). Allowing ground to carry current defeats the point, you'll just have to parallel neutral wires, and no additional safety compared to WO2-era appliances (chassis connected to neutral).
In my opinion, it's a bad idea to tell people to mess with safety features of electrical wiring, unless they know what they're doing, but in that case they wouldn't need anyone to tell them what to do. Especially if you don't know someones location, electrical system or regulations.
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