Does transformer metal core need to be grounded/earthed

[giosif]

Hi,

I am repairing a bench linear power supply which also had small corrosion here and there, mainly on the inside of the metal base of the case and a bit on the metal core of the transformer.
I took the supply apart, cleaned the components the best I could and I am now putting everything back together.
When doing these things, I usually check for the resistance between the earth pin on the mains input socket of the unit and the various parts of the supply that are supposed to be earthed.

And, while the transformer is mounted directly to the metal base of the case using metal brackets and metal screws, when measuring the resistance betwen the earth pin and the metal core of the transformer, I get a value of about 500 Ohms.
Measuring the resistance between the earth pin and the metal base of the case itself, I get close to 0 Ohms.

The questions I have related to the transformer core somewhat floating (i.e. 500 Ohms to earth) are:

• Does this pose any kind of risk?
  I wouldn't think so, since, even if there was a failure with one of the windings and the metal core would become live, a person would not normally come into direct contact with it, when the supply is fully assembled.
• Could it affect the performance of the power supply?
  I am thinking of things I personally can't fully grasp (yet), but wonder whether they might be relevant here: e.g. parasitic capacitance and coupling.
  

Thanks!

[fourtytwo42]

It depends on the insulation rating and style of transformer, if it is a modern split bobbin type then earthing the core is immaterial from a safety point of view, if it is an older varnished paper insulated type it is probably advisable to earth the core for additional safety. It is probably only varnish around the lugs preventing the screws from earthing it.

[giosif]

It depends on the insulation rating and style of transformer, if it is a modern split bobbin type then earthing the core is immaterial from a safety point of view, if it is an older varnished paper insulated type it is probably advisable to earth the core for additional safety. It is probably only varnish around the lugs preventing the screws from earthing it.

Thanks for the answer!
It is a modern transformer, indeed.

[David Hess]

Transformers which include an electrostatic shield usually have it clamped into the transformer core so to make good use of that feature, the transformer core must be solidly grounded.  Otherwise, it is just a safety issue depending on how the transformer is constructed.  There is generally no reason *not* to solidly ground the transformer core.

[giosif]

Transformers which include an electrostatic shield usually have it clamped into the transformer core so to make good use of that feature, the transformer core must be solidly grounded.  Otherwise, it is just a safety issue depending on how the transformer is constructed.  There is generally no reason *not* to solidly ground the transformer core.

Thanks and I agree, particularly with your last statement!
However, I don't think this transformer would have the electrostatic shield you mention as, in that case, I would have expected the original manufacturer of the power supply (respectable company) to have ensured the core is grounded via a dedicated connection, rather than relying on the brackets and screws used to mount the transformer to the case.

[David Hess]

Transformers which include an electrostatic shield usually have it clamped into the transformer core so to make good use of that feature, the transformer core must be solidly grounded.  Otherwise, it is just a safety issue depending on how the transformer is constructed.  There is generally no reason *not* to solidly ground the transformer core.

Thanks and I agree, particularly with your last statement!

However, I don't think this transformer would have the electrostatic shield you mention as, in that case, I would have expected the original manufacturer of the power supply (respectable company) to have ensured the core is grounded via a dedicated connection, rather than relying on the brackets and screws used to mount the transformer to the case.

The ones I have and many of the ones I have seen are coated with a heavy varnish like any other transformer and require a star lock washer to make good contact to ground through one of the mounting bolts.  If the electrostatic shield is present, the wire connecting it can be seen between the winding window and where it is crimped between the laminations and frame.