Earth wire going into bell transformer, is this ok?

[Jar99]

I want to install Nest Hello doorbell and looking at my current setup there's the earth wire going from the power supply below the transformer into the top between the bell cables. Is that ok?

When i get a new transformer would I do the same or should the earth wire go somewhere else?

Pic attached.

 

[martygturner]

I would be more concerned that the present 240v is terminated to exposed connecters. The earth is not needed and seems to be just tucked down the back of the transformer.

 

[RandomGrinch]

The earth is not needed and seems to be just tucked down the back of the transformer.

Zooming in on the pic, the earth is connected to the centre tap of the secondary side of the transformer; stopping the output from floating.
It won't be needed for the Nest installation.

 

[JohnW2]

Zooming in on the pic, the earth is connected to the centre tap of the secondary side of the transformer; stopping the output from floating.

Indeed, although it doesn't seem to actually be be a 'centre' tap.

However, am I misunderstanding the markings, since I didn't think that 5 + 6 = 8 ?

Kind Regards, John

 

[RF Lighting]

I don’t know exactly how it works internally, but connecting as per the diagram allows a 5v, 6v, or 8v supply so it can’t be a centre tap and shouldn’t be earthed.

As others have said though the main priority here is making safe those easily touchable exposed live parts before someone gets injured.

 

[bernardgreen]

Whether the transformer's output should have a connection to Earth depends on how the transformer is constructed, ( that is my opinion ).

There are three main method of construction offering three levels of isolation between the primary winding ( mains 230 V ) and the secondary winding ( the supply to door bell )

Dual bobbin has two layers of physical separation
Split bobbin has a single layer of physical separation
Layer wound may depend entirely on the enamel coating of the winding wire to provide electrical isolation between the primary and secondary windings. Many layer wound transformers will have a layer of insulating tape between the primary and secondary windings to provide some physical separation but not all.

If the isolation fails then the door bell wiring could become Live and door bell push buttons are seldom rated as being safe at 230 V.

In this image the screen would be the tape.

 

[Jar99]

Hi everyone, thank you for all your feedback.

Regarding the exposed 240v wire, there is a screwed on cover which I removed to take the photo (I did turn off power beforehand). Please see the attached photo with it in place - I assume that is safe.

The bell wires are going into 1 and 3 for 8v and the earth is going into the middle tap / connector - From your comments i believe i should remove it and cap it (I'll use a wago).

Let me know if there's anything else to consider.

Thank you

 

[JohnW2]

I don’t know exactly how it works internally, but connecting as per the diagram allows a 5v, 6v, or 8v supply ....

That's what the markings imply but, as I said, it makes no sense to me. If there is 6V between the left-hand two (of three) terminals and 5V between the right-hand two then, dependent on whether or not those voltages are in-phase, the voltage between the outer two terminals should be either 11V or 1V, shouldn't it? I see no way of getting 8V.

.... so it can’t be a centre tap and shouldn’t be earthed.

As I said, it's clearly not a 'centre tap'. It clearly does not need to be earthed, but I don't see that any harm would result from earthing any part of the ELV output, whether one end of it or some 'intermediate point' (rather than leaving it floating. Furthermore, as bernard has pointed out, earthing some part of the output does afford protection again primary-secondary faults - so I personally would certainly not 'knock' the practice..

As others have said though the main priority here is making safe those easily touchable exposed live parts before someone gets injured.

I would hope that we are all agreed about that.

Kind Regards, John

 

[plugwash]

That's what the markings imply but, as I said, it makes no sense to me. If there is 6V between the left-hand two (of three) terminals and 5V between the right-hand two then, dependent on whether or not those voltages are in-phase, the voltage between the outer two terminals should be either 11V or 1V, shouldn't it? I see no way of getting 8V.

The only explanation that would make sense to me would be that it is a center tap but the quoted voltages are "under full load" and that there are significant resistive losses in the primary winding and/or significant leakage inductance. So the voltages produced by the secondaries when both are loaded to 1A are significantly lower than when only one of the two secondaries are loaded to 1A.

 

[JohnW2]

The only explanation that would make sense to me would be that it is a center tap but the quoted voltages are "under full load" and that there are significant resistive losses in the primary winding and/or significant leakage inductance. So the voltages produced by the secondaries when both are loaded to 1A are significantly lower than when only one of the two secondaries are loaded to 1A.

Perhaps, but if the voltages are all quoted in relation to a 1A load (with, as you imply quite appreciable losses) then goodness knows how high the off-load voltage would be (a state in which a 'bell transformer would be for nearly all the time) !

Kind Regards, John

 

[RandomGrinch]

then goodness knows how high the off-load voltage would be

About 13V, according to this source:

Friedland 752 transformer giving higher than expected reading

Hello, We moved into an old house (Wimpy 1957) and the doorbell doesn't work. I read the AC voltage on the old Friedland 752 transformer, and it's at 13V, not 3 or 5 or 8 as per the options. The solenoid on the bell might be getting too much power. Why might it read 13V? This person also...

www.electriciansforums.net

 

[plugwash]

I remember in the days of transformer based unregulated wall warts it was very common for the off-load voltages to be significantly higher than the nominal voltages. The efficiency on those things was awful and the no-load current was also substantial.

Efficiency regulations basically killed transformer based wall-warts, but there doesn't seem to be anything similar for bell transformers.

 

[JohnW2]

About 13V, according to this source:

Fair enough, although I would have suspected higher.

However, I still can't make much sense of this. If there is 8V across the entire secondary when 1A is flowing through it, then if (per plugwash's suggestion) the tap is a 'centre' one, then one would expect 4V (not 6V or 5V) across the two 'halves', each of which would have 1A flowing through them, wouldn't one?

Even if the tap is at 6/11 of the way from the 'left-hand end', one would expect about 4.36V across the left 'half' and about 3.63V across the right 'half' - again, not 6V or 5V.

What are we missing?

Kind Regards, John

 

[SUNRAY]

That's what the markings imply but, as I said, it makes no sense to me. If there is 6V between the left-hand two (of three) terminals and 5V between the right-hand two then, dependent on whether or not those voltages are in-phase, the voltage between the outer two terminals should be either 11V or 1V, shouldn't it? I see no way of getting 8V.

As I said, it's clearly not a 'centre tap'. It clearly does not need to be earthed, but I don't see that any harm would result from earthing any part of the ELV output, whether one end of it or some 'intermediate point' (rather than leaving it floating. Furthermore, as bernard has pointed out, earthing some part of the output does afford protection again primary-secondary faults - so I personally would certainly not 'knock' the practice..

I would hope that we are all agreed about that.

Kind Regards, John

I've sussed it...

It must be an unbalanced 3 phase transformer

My experience of these transformers are usually 3, 5, 8V or 4, 8, 12V
Be interesting to put a voltmeter on it.