Transformer question

[Popoon]

I just installed a mains transformer,it came equiped with bolts that fixed either side of the bell covers using a deformable fibre washer and a steel washer.

I tested this when it arrived and yes the bolts even with the insulating washer do touch the laminations on the way through,i assumed at the time that this was normal.

It's a drop through design and the transformer when fixed has perhaps a tiny bit of clearance and so may not be electrically connected that way.

My question relates to this,should the bellcovers and therefore the laminations be grounded at all and if so is it normal to rely on the vagaries of the bolts touching the laminations as they pass through?

I cannot see how i could prevent the bolts from earthing even if i tried and i do not feel producers of electronic or even electrical equipment would spend hours and hours tightening these bolts "just so".

I used the multimeter and it is connected to the chassis electrically.

I should say the chassis came with an adaptor plate and because the hole didnt seem big enough i enlarged it so the transformers nuts would sit flush with the chassis.

[phsyconoodler]

Not to worry.The washers are just for keeping the bolts from turning and maybe to protect the finish on the transformer.Nothing more.In fact many people use a transformer bolt as a ground source.

[Popoon]

Thats a weight of my mind,as it happens i had enlarged the drop through cutout so the transformer would fit with an additional washer between the nuts and the chassis,removing these addtional washers means the transformer cover is now in full electrical contact as its cover is pressed tightly to the chassis.
What purpose does the earth wire have within the transformer,i ask this as it reads as isolated from everything else.

[VacuumVoodoo]

The earth wire coming out of a power transformer is called protective screen. It is usually connected internally to a single open winding layer or a single open layer of copper foil placed between primary and secondary windings. Its purpose is to divert any high voltage flashover, such as may happen between secondary and primary, seconadry and core , to earth. The protective screen must be connected directly to the earthing lug of incoming mains voltage. It is also a requirement of IEC61558 norm for Class B Isolating Transformers.

The bolts holding end covers should be isolated from the covers. If not they will form a closed single turn secondary winding together with end covers. This will cause drop of transformer efficiency and increased heat losses.
You can easily check if that is the case: if the bolts are hotter than the rest of transformer body you got a shorted parasitic secondary winding formed by bolts and end covers. Bolts touching laminations inside the hole are no problem - laminations are isolated from each other - but to be sure one can put on heat shrink tubing on the section of the bolt that goes inside the hole.

[Popoon]

In anticipation of fixing the problem i rushed out and bought several widths of heat shrink and a heat gun,i went to work and got me some new steel and fibre washers to fit over the newly insulated bolt.
Igot home and was eager to put the solution into effect,i unbolted the transformer from the chassis and offerd the heatshrunk bolt to the bell cover only to find it too big although it would fit the holes through the laminations.
My options were to drill the bell covers or fit the orinal thinner screws,these were weirdly sized falling between M3 and M4 and i have no nylon insert nuts for them and unlke the new ones they have a slot instead of a hex head and are not brass and so fell out of favour.
Examing the laminations i found them laquered though not for insulation purposes as the meter reads continuity across all of them and coninuity with the bell covers even without nuts and bolts.
I gave up at this point and simply reasembled as before.
No lesson learned at all.