Transformer neutral reading 28v

[Jako13!]

I’ve seen a 110v control transformer reading 28v on the neatral to earth and 67 on the other leg. Why would this be?

 

[marconi]

Jako13!: Now study this diagram for the situation where the secondary side is imperfectly isolated from ground/earth by say insulation leakage indicated by the three pure resistors. Assume your voltmeter has a very high resistance (say 50M Ohms) which means it draws only a negligible but not zero current to take a voltage reading.

What would the voltmeter measure when connected between earth and L1, L2, L3 and L4? First though draw a circuit diagram for the upper and then the lower 110V circuits.

 

[Jako13!]

I had another look today and the panel has been designed with neutral links fitted for the output of the transformer.

 

[marconi]

JAko13! - Could you say a little more about these links? Pictures are helpful. This way the 'EF Brains Trust' can comment further. In the meantime I am confident you will not alter anything. What is the purpose of the panel?

 

[DPG]

What is the problem you are trying to fix? What made you take your measurements?

 

[Jako13!]

What is the problem you are trying to fix? What made you take your measurements?

Someone’s bypassed the e stop circuit so was having a look to see what they had done.

 

[Pete999]

But it's not. 28 volts was measured, which is almost certainly not a 'real' voltage.
The wiring diagram of the Tx is visible in the OP pic., and the earth connection doesn't connect to either winding, so it's a screen.

What do you mean " a Screen"?

 

[brianmoooore]

What do you mean " a Screen"?

Copper foil sheet between the primary and secondary windings. The sheet is arranged so that it doesn't form a shorted turn. Used when one winding is wound on top of the other.
The other method of making an isolation transformer is to position the windings side by side with a space between them

 

[Pete999]

Copper foil sheet between the primary and secondary windings. The sheet is arranged so that it doesn't form a shorted turn. Used when one winding is wound on top of the other.
The other method of making an isolation transformer is to position the windings side by side with a space between them

So why do we use 110v transformers for electrical safety?

 

[brianmoooore]

So why do we use 110v transformers for electrical safety?

Site transformers produce 55-0-55 AFAIK, so with only 55 volts above earth, it's fairly difficult, but not impossible, for the average site monkey to electrocute themselves.

 

[pc1966]

It might well be connected to earth at the DC side of some power supply, so really don't go changing it unless you have access to the full schematic and design details!

 

[pc1966]

So why do we use 110v transformers for electrical safety?

There are 2 different but related aspects:

One is the danger presented by the secondary voltage itself. So you can easily kill yourself on a 240V isolated supply just but getting both ends of it, and do so without too much more difficulty on 120/110V (as US electrical accidents sadly testify to). Going down to 55-0-55 puts it out of most danger situations, unless very wet (insert joke here...)

The second danger is that of a flashover between the primary and the secondary. One traditional solution is a split-bobbin style of construction so the windings have a mm or so of solid plastic insulation and a cm or so of creapage distance between them. Another solution is to put an earthed screen between the two. This is also an advantage if electrical noise is an issue, as anyone familiar with old valve LW/MW radios will know...

That is, before all broadband modems throwing noise all over the spectrum so folk could wast time on arsebook....