Transformer Centre Tap Tied To Earth

[toddster900]

Hi All

Firstly I apologise for any incorrect explanations or terminology. I am systems engineer not a spark and my interest is purely "night time reading".

I am struggling to understand how it can be that the center tap of a transformer can be tied to earth. I understand that if the center tap is tied to earth, meaning that it is at the same potential as earth, then one end of the secondary winding is +55V and the other is -55V relative to earth. However, what confuses me is that I would expect the secondary winding voltage to be induced in such a way that, the center tap would be at +55V relative to earth. If this was the case then clearly tying the center tap to earth would create a short and blow the transformer.

What am I missing and/or not fully understanding?

Thanks

 

[Knobhead]

You can tie any point of a secodary winding to earth. The winding is isolated from earth through out its lengh. So long as only one point is tied to earth there is nowhere for the current to go. If you then get an earth fault it then forms a fault path, thus blowing the fuse. (Hopefully).

 

[toddster900]

Ah i see. So in this case although there is a potential differance between the center tap and earth no current can flow as it has no return route.

ThanksTony!

 

[stef]

Tony didn't say that there is a potential difference between centre tap and earth because there isn't one. He said the secondary winding is isolated from earth.
If you remove the earth from centre tap and put your voltmeter across you probably
would measure a voltage difference but it has no relation to earth as it is galvanic
isolated. I call this a floating voltage, probably the wrong term. It can't cause a current flow to earth. By connecting the centre tap to earth you just short out the voltage difference but no current flow is involved. The idea behind this is that whatever part of the secondary winding under a fault condition can give you only a non lethal 55 v.