Electronics > Beginners
Measuring Mains with Rigol DS1052E
ciccio:
--- Quote from: IanB on August 02, 2011, 05:00:12 pm ---Please humour me here as an electronics noob...but if the transformerless device already had it's 0 V rail connected to the chassis and the chassis was connected to mains earth, what might go wrong with connecting a high voltage scope probe under those circumstances?
--- End quote ---
The transformerless device cannot have an external chassis connected to mains earth: it's ground point is connected to mains live or mains neutral (if the plug is reversible),so it will be classified as a "dangerous to touch terminal".
A transformerless device must be built inside an insulating case (old TV sets were made this way).
The headphone socket of my old TV set is driven by an insulation transformer.
The antenna connector is insulated through low value, high voltage capacitors (that limit the current that can be sourced from the antenna socket).
IanB:
--- Quote from: ciccio on August 02, 2011, 05:39:24 pm ---The transformerless device cannot have an external chassis connected to mains earth: it's ground point is connected to mains live or mains neutral (if the plug is reversible),so it will be classified as a "dangerous to touch terminal".
A transformerless device must be built inside an insulating case (old TV sets were made this way).
The headphone socket of my old TV set is driven by an insulation transformer.
The antenna connector is insulated through low value, high voltage capacitors (that limit the current that can be sourced from the antenna socket).
--- End quote ---
That makes sense. But in the UK (where I'm from) reversible two pin plugs are found only with electric shavers. Everything else has a three pin fused plug with an earth connection. Devices can either be "double insulated", which means there must be no potentially live parts that can be touched, or the case must be securely connected to the earth wire in the mains plug. In theory (I think) you can connect the mains neutral to the mains earth inside the device since the two wires should be at the same potential. If someone reversed live and neutral in the mains lead then a short circuit current would flow and blow the fuse in the plug or trip the circuit breaker if someone bypassed the fuse. This idea of connecting earth to neutral inside a device is the bit where I may be off base I think...
ciccio:
Are you sure that neutral on your socket is at ground potential?
Measure it, please.
Here in Italy neutral is grounded only at the electric company's medium voltage to low voltage transformer,
The one serving my home is about 500 meter from me, and in this moment neutral is about 30 V respect to ground...
An old electrician told me: live is hot, and neutral is hot..
Bloch:
--- Quote from: ciccio on August 02, 2011, 06:13:32 pm ---Here in Italy neutral is grounded only at the electric company's medium voltage to low voltage transformer,
The one serving my home is about 500 meter from me, and in this moment neutral is about 30 V respect to ground...
--- End quote ---
One thing to note is that every contry have there own mains standard >:(
Here in Denmark it is "normal" at every house to have you own ground point (a 2 to 6 meter long plug in the ground)
If i was you I would talk to the lokal electrician to check if you own ground point is ok. (My guess is not)
I dont have a englich page but try to see here http://translate.google.com/translate?js=n&prev=_t&hl=en&ie=UTF-8&layout=2&eotf=1&sl=da&tl=en&u=http%3A%2F%2Fcubus-adsl.dk%2Felteknik%2Finstallationer%2Fnulling.php
IanB:
--- Quote from: ciccio on August 02, 2011, 06:13:32 pm ---Are you sure that neutral on your socket is at ground potential?
Measure it, please.
Here in Italy neutral is grounded only at the electric company's medium voltage to low voltage transformer,
The one serving my home is about 500 meter from me, and in this moment neutral is about 30 V respect to ground...
An old electrician told me: live is hot, and neutral is hot..
--- End quote ---
I'm not in the UK right now so I cannot measure, but I know there could be a potential difference between earth and neutral wires. In the UK the earth and neutral are either connected at the nearest substation as you describe, or are connected just before entering the home (in more modern systems). Either way, if there is 30 V between neutral and earth, it suggests there is some significant voltage drop where it ought not to be; for instance there might be an unexpected current flowing in the earth conductor?
I suppose though that I am wrong to think you could connect earth and neutral inside a device; most likely you should not do so and I imagine I would find that out if I could locate the right regulations to read...
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