Regarding rewiring EM games, could someone please be so kind as to explain to me the purpose and logic of attaching the earth/ground wire to the frame of the transformer?

Following an incident a few years ago (see below) I no longer include the transformer frame when introducing grounding to a game. Instead I create a central hub on the bottom board to which I attach the ground wire, and from which I radiate earthing braid to all exposed metal parts.

But I know that when grounding started to be introduced on EM games the ground wire was always attached to the transformer frame (only) so it leaves me wondering if I am missing something

Any comments will be much appreciated, as I have been pondering this for some time and only just come across this thread

Thanks, David

The incident I referred to relates to a 1964 Williams Heat Wave which has been in my possession since 1980 and was my second ever game.
It had been played constantly over the years at home with no problems, but when I was due to take it to a public show in 2015 it needed to be able to pass a UK "Portable Appliance Test" and so needed grounding.
Following all the advice, I replaced the mains cable with a three core cable, and ran the ground to the foot of the transformer, and from there to all exposed metal parts.
When the game arrived at the show, my son went to play test it, and reported getting powerful electric shocks from the flipper buttons (which being metal, had been grounded)
It turned out that my crimped connection on the mains ground wire to the transformer foot had not been done well (moral - always tug on new crimping!), and had come apart in transport, hence severing the main ground.
As a result, mains voltage leaking from the old transformer (240v in the UK) was being distributed via my grounding connections to every exposed metal part on the game - the exact opposite of what was intended.
Whilst the problem arose from my faulty original connection, it made me aware of the potential danger of grounding games in this way, and since I could see no obvious danger to the player from the transformer itself I decided in future to leave it out of the grounding loop since as seen it is a potential source of shock in itself.

Thanks for the comment vid1900 but that doesn't really answer why to include the transformer in the grounding, and why manufacturers started running a ground wire just to the transformer

I was intrigued by your suggestion that there was an undescribed fault in my game so I did a little experiment. I took 4 "factory state" ungrounded games and measured for each the voltage from one of the mounting lugs of the transformer to a grounded point on a grounded game. As I am not an electrician please be gentle with me if that was not an appropriate method of testing.

The results I got were as follows:
Gtb Hearts & Spades 83v
Wms Trade Winds 117v
Gtb Sing Along 140v
Gtb Cow Poke 165v

I find it hard to believe all these games are also faulty, although I could understand if it is the case that transformers get more "leaky" over time

Clearly any of these voltages if transmitted to the flipper buttons would be enough to give a nasty shock

I began to wonder if the purpose of manufacturers running a ground wire to the transformer only was to carry away safely this leaking voltage - if so it would seem totally inappropriate to then link the transformer frame into ground wire radiated to all the metal touch points

I may be talking total BS here - I'm just trying to gain an understanding, that's all - so all comments are welcome

Pin-Pilot No, I haven't tried a separate transformer in Heat Wave - but my other results would suggest Heat Wave isn't an isolated example

YeOldPinPlayer It was set on AC. I'll find another meter and try that, but the one I was using is my regular use one

What would be helpful would be if someone with an early or mid 60's game could replicate my situation
If with an ungrounded game, measure from a transformer mounting lug to a ground point somewhere.
If with an already grounded game, then remove the main incoming ground link to the transformer and then see what voltage is measured between a transformer mounting lug (or any grounded point) and the loose main ground

Obviously our base voltage here is much higher (240v compared to your 110v) but it is really the principle that is of interest - as I understand it from the previous comments in this thread you should find 0v AC when measuring as above

I don't know if this is also a potential cause:
https://americas.hammondpowersolutions.com/en/resources/faq/general/can-you-operate-a-60hz-transformer-at-50-hz
Some earlier games I think used the same transformer for both 50hz and 60hz countries, whereas I think later export games were fitted with a different transformer specifically designed to operate at 50hz (our frequency in UK) (and with a 230v tap)