Do Away With Ring Final Circuits?

[JohnW2]

Yes I agree too. And on that basis I'm sure you'll agree this is a single phase load: ... Despite the fact the socket it's plugged into (represented by the red line)is this:

Yes, your right in being sure that I would agree with you.

Asa I've said, if a load has only two 'terminals', hence only sees the pd between two supply conductors, then, from the POV of the load, there is no meaning to the concept of 'phase' - so there can't be more than one of them!

Kind Regards, John

 

[Harry Bloomfield]

Yes, your right in being sure that I would agree with you.

Asa I've said, if a load has only two 'terminals', hence only sees the pd between two supply conductors, then, from the POV of the load, there is no meaning to the concept of 'phase' - so there can't be more than one of them!

Effectively, so far as a a load is concerned it will not care whether it is two phases, or one phase and neutral - ignoring the voltage. However, try sticking a 'scope across it.

 

[SUNRAY]

Assuming the 3 following examples are of similar device, something like a resistive heating element or incandescent light bulb for example the only difference to be seen on a scope will be the voltage or current value, they will all be a sine wave.
1/

2/

3/

 

[Harry Bloomfield]

they will all be a sine wave.

Out of phase with each other by 120 degrees across two phases.

 

[SUNRAY]

Out of phase with each other by 120 degrees across two phases.

Which one?

 

[JohnW2]

Effectively, so far as a a load is concerned it will not care whether it is two phases, or one phase and neutral - ignoring the voltage.

As I said, if it is a '2-terminal load', then the concept of phases/phase differences is meaningless.

However, try sticking a 'scope across it.

If you connected it just across the two terminals, you would see just a sine wave. Nothing to compare it with, so no question of anything to do with 'phases'.

However, if you used your (2 channel) scope to look at the pd between each of those two terminals and some external reference (e.g. 'earth' or some sort of neutral) then, with some of the sort of situations SUNRAY has been considering, you would see two voltages which were 'out-of-phase' (with respect to the reference) - but that woulkd still be meaningless in terms of what was across the load.

Kind Regards, Joihn

 

[SUNRAY]

As I said, if it is a '2-terminal load', then the concept of phases/phase differences is meaningless.
If you connected it just across the two terminals, you would see just a sine wave. Nothing to compare it with, so no question of anything to do with 'phases'.

However, if you used your (2 channel) scope to look at the pd between each of those two terminals and some external reference (e.g. 'earth' or some sort of neutral) then, with some of the sort of situations SUNRAY has been considering, you would see two voltages which were 'out-of-phase' (with respect to the reference) - but that woulkd still be meaningless in terms of what was across the load.

Kind Regards, Joihn

Agreed.

 

[BS3036]

If you connected it just across the two terminals, you would see just a sine wave.

However, if you took that to mean connecting the scope earth to one of the two terminals, you would see just smoke. You would need at least a differential scope. Just saying in case someone tries it.

 

[SUNRAY]

However, if you took that to mean connecting the scope earth to one of the two terminals, you would see just smoke. You would need at least a differential scope. Just saying in case someone tries it.

This was actually going through my mind too but I thought: 'Surely anyone who owns, or knows how to use, an oscilloscope would be aware of this' but of course you are right to raise it.

 

[Harry Bloomfield]

Agreed.

I had to think that through, but yes I agree too.

 

[Harry Bloomfield]

However, if you took that to mean connecting the scope earth to one of the two terminals, you would see just smoke. You would need at least a differential scope. Just saying in case someone tries it.

I have such an instrument, but lack the two mains phases (3-Ph), only a single phase supply here.

 

[JohnW2]

However, if you took that to mean connecting the scope earth to one of the two terminals, you would see just smoke. You would need at least a differential scope. Just saying in case someone tries it.

Sure, anyone trying it would have to make sure that they were using a scope which had both sides of its input (or two inputs) isolated from earth.

Kind Regards, John

 

[plugwash]

Most scopes also have "CAT 1" inputs so they shouldn't really be connected directly to the mains.

The proper soloution is to either use something like an "industrial scopemeter" or use an isolated differential probe with appropriate safety ratings, but both are quite expensive.

 

[bernardgreen]

You could use a couple of step down transformers and connect the scope leads to the ELV secondaries.

 

[SUNRAY]

You could use a couple of step down transformers and connect the scope leads to the ELV secondaries.

If you used this approach and had 4 traces & transformers this is roughly what would be seen

Excuse the American wiring colours.
The green trace is the PD across black and red phases.