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?

 

[Lucien Nunes]

The output is probably not earth-referenced, i.e. it is electrically separated / floating. The voltages to earth are not defined and the actual readings on the meter are not very meaningful, giving only a hint at the relative amounts of leakage / capacitance from the two lines. They will probably vary according to different instruments and a low-impedance meter might not show any reading. On a floating circuit what you do not want to see is zero on one leg and the full voltage on the other, as that might indicate a short to earth.

 

[Pete999]

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

Center tapped to Earth transformer maybe???

 

[Jako13!]

The output is probably not earth-referenced, i.e. it is electrically separated / floating. The voltages to earth are not defined and the actual readings on the meter are not very meaningful, giving only a hint at the relative amounts of leakage / capacitance from the two lines. They will probably vary according to different instruments and a low-impedance meter might not show any reading. On a floating circuit what you do not want to see is zero on one leg and the full voltage on the other, as that might indicate a short to earth.

So it’s an isolating transformer? If it is can these be modified to make the neutral true 0v?

 

[Lucien Nunes]

Simply connect one end of the winding to earth and call it the neutral. Why do you want to change it? Have you considered why it is arranged this way now? Is anything depending on the electrical separation for safe operation?

 

[Jako13!]

So it’s an isolating transformer? If it is can these be modified to make the neutral true 0v?

Simply connect one end of the winding to earth and call it the neutral. Why do you want to change it? Have you considered why it is arranged this way now? Is anything depending on the electrical separation for safe operation?

It’s simply just a transformer for a control circuit that runs 2 three phase motors for an extraction system it’s industrial so can’t see any reason why it has to be isolating. I’m don’t know weather I’m being silly hear but I thought with isolating transformer I shouldn’t pick up a voltage to earth? Just between each end of the transformer winding?

 

[Lucien Nunes]

Well that would be true if there were exactly zero leakage and zero capacitance to earth. But even before you start to run cables from it, the transformer secondary has capacitance to earth, through which a small amount of AC can flow to give a reading on the meter. The more cabling and equipment, the more leakage can flow. It's a ghost voltage if you like that term, that will probably go away if you use a low-impedance tester.

Floating circuits are single-fault tolerant, and can reduce the voltage and leakage in critical places where insulation is challenged. Don't change the configuration simply because you 'like' one wire to be neutral!

 

[Jako13!]

Well that would be true if there were exactly zero leakage and zero capacitance to earth. But even before you start to run cables from it, the transformer secondary has capacitance to earth, through which a small amount of AC can flow to give a reading on the meter. The more cabling and equipment, the more leakage can flow. It's a ghost voltage if you like that term, that will probably go away if you use a low-impedance tester.

Floating circuits are single-fault tolerant, and can reduce the voltage and leakage in critical places where insulation is challenged. Don't change the configuration simply because you 'like' one wire to be neutral!

Interesting but if it was earthed I can’t see the problem from a safety point? With isolating transfers what’s to protect us on the second fault?

 

[snowhead]

It looks to be in a control panel so as above, it's been designed by someone who wanted it the way it is, don't change it unless you have a good reason and know exactly what the consequences are.

Have you notice the earth wire in the terminal block, presumably not connected, but what's happening before the block?

 

[Lucien Nunes]

May be connected to the interwinding screen in the transformer

 

[Jako13!]

I’m just confused how to tell if it’s an isolating transformer by measurement? And the idea of a second fault on these type of transformer what’s in place to stop us getting a shock on the second earth fault?

 

[brianmoooore]

As Lucien says, it's an ISOLATING transformer, with an earthed screen between the windings to ensure it stays isolating in the case of transformer failure.
If you connect anything from the supply side to the secondary, including earth it is no longer isolating.
Any fault to earth anywhere on the secondary side, or what's connected to it, can only produce a maximum of 110 volts to earth at any point, which should be non lethal. If you get a second fault to earth at any point, other than one that is at the same potential as the first fault, then the fuse blows.

 

[Pete999]

Center tapped to Earth transformer maybe???

Each leg 55v to earth

 

[brianmoooore]

Each leg 55v to earth

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.

 

[marconi]

JAko13!: Good morning. A good observation which shows you are thinking about your electrical measurements and what they mean or tell you.

I have drawn the circuitry for a perfectly isolated secondary side; why does the meter read 0 V or better - why is the meter current I zero Amps - when the meter lead touches either L1, L2 , L3 or L4?