Grounding Symbol Between Transformer Windings

[TheElectrician]

Hi, in the image below, the grounding is shown between the windings of the transformer. What does this mean, where is it exactly gorunded?

Thanks

 

[retirede]

I believe that indicates a shielded isolation transformer, but I’ve never encountered one (that I know of).

 

[synchro]

I believe that indicates a shielded isolation transformer, but I’ve never encountered one (that I know of).

Yes, that's the electrostatic shield between the primary and secondary windings of an isolation transformer.

 

[infinity]

I agree it is indicating that the transformer is shielded. Isn't an isolation transformer usually a 1 to 1 ratio?

 

[MTW]

From Eaton Spec Sheet.


Application Description
Electrostatically shielded transformers are used to protect sensitive electrical equipment from undesirable high fre- quency signals commonly generated by lighting, switching surges, motors, and SCRs feeding noise back into the line. An electrostatic shield, consisting of a single turn of aluminum foil placed between the primary and secondary winding, is used to provide from
30 to 70 dB attenuation of wide band line-to-ground (common mode) noise.
Typical uses of shielded isolation
transformers include:
■ Suppression of transients and noise which travels from its source to the sensitive load equipment.
■ Suppression of noise and transients at the point where the noise or transients are originating, thus preventing them from backfeeding from the source to the feeders.
■ Transform one voltage level to another.
■ Isolate one circuit from another.

 

[jim dungar]

I have never seen a medium voltage 3.6MVA oil filled electrostatically shielded transformer.

 

[petersonra]

I can't say that I've ever seen a 630 volt three-phase transformer before.

My guess is a symbol is just drawn that way for no real reason I don't think they are asking for electrostatically shielded anything unless it says so in the spec somewhere.

Unless you have to supply the transformer it doesn't make any real difference anyway to how you wire it

 

[Carultch]

I agree it is indicating that the transformer is shielded. Isn't an isolation transformer usually a 1 to 1 ratio?

Not necessarily. It is common to teach that a 1:1 ratio transformer is called an isolation transformer, because that is the main purpose of a 1:1 transformer. Isolation transformer are built in other ratios as well. Most transformers you see, are isolation transformers, and likely just not called that, because the voltage conversion is their main purpose.

Isolation transformer is the opposite of an autotransformer, which means that the magnetic coupling is the only way to pass power through it. Autotransformers also pass through a reference to ground, while isolation transformers create their secondary as a separately derived system.

 

[GoldDigger]

But, as a practical matter, isolation transformers are built in way that minimizes all couplings between primary and secondary except their mutual inductance.

 

[jim dungar]

But, as a practical matter, isolation transformers are built in way that minimizes all couplings between primary and secondary except their mutual inductance.

Only when using industry slang.
Most manufacturers use a two word term, like 'drive isolation', when describing their specialty isolation designs.