Step down transformer earthing - UK Vintage Radio Repair and Restoration Discussion Forum

McMurdo · 19th Apr 2021, 4:16 pm

Here's a '220 to 110' portable chinese autotransformer arrived with me to repair. Someone's smashed the socket outlet.

What strikes me is that there's no earthing of the unit other than one end shroud to mains earth and the opposite end shroud to the outlet earth. No continuity between mains and output and nothing to chassis.

The only route for wires from the in-side to the out-side is through a gap in the bobbin.

The laminations have what looks like a mumetal welded sleeve around them, pinned in place by the end shrouds.

Suggestions please!

19th Apr 2021, 5:09 pm

Probably saving a bit of wire (the fools) to increase profits. If I was asked to fix this I would either do a proper job with earth continuity or scrap it. The former would be my preferred route, knowing that they bought a similar item instead that caused injury would bother me a lot.

chriswood1900 · 19th Apr 2021, 5:56 pm

That seems to be the norm with these, you just prompted me to look into the unit I have and it is wired as you describe. There is continuity but its far from perfect I doubt these conform to any known standard and look to be made to the standard of typical US electrics where there seems to be a more casual approach to the provision of earthing on 110V volt equipment if it's provided at all.

McMurdo · 19th Apr 2021, 9:30 pm

I'm more concerned with the correct way to earth it, it's a 3kw transformer and is used for a heating application. (ie well-loaded). Connecting the end caps by pushing an earth wire through the middle of the bobbin would surely leave it open to eddy current damage, what do people think? Is it likely to be an issue? Also, how can I earth the outer sleeve?
The unit is used in an area outfitted with full stainless steel work benches, it's supplied via an RCBO circuit but it's no excuse for defective earthing.

joebog1 · 19th Apr 2021, 9:47 pm

I would be using a bit of copper shim gently pushed under the "mu-metal" shield. fold it under end bells until it's down past the endbell "ends" . Solder a normal green/yellow earth wire on both sides, one to input, and one to output. Wire ends go to the input and output sockets/connections. Going thru the bobbin is not a good idea, although there is room to do so. We have the same transformers available here.

Joe

Herald1360 · 20th Apr 2021, 12:25 am

Going through the bobbin will release smoke- it'll be a shorted turn!

McMurdo · 20th Apr 2021, 8:32 am

Quote:

a bit of copper shim

That's what I was thinking yesterday, I ordered some shim as the straps I have here are too thick. The owner of this thing is a prodigious importer of dodgy equipment, one item being a 40ft long bottle liquid-filling production line all in stainless steel and not an earth in sight!

ms660 · 20th Apr 2021, 10:07 am

Unless I've missed something I would put it in a box and do it properly.

Lawrence.

19th Apr 2021, 4:16 pm	#1
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,271	Step down transformer earthing Here's a '220 to 110' portable chinese autotransformer arrived with me to repair. Someone's smashed the socket outlet. What strikes me is that there's no earthing of the unit other than one end shroud to mains earth and the opposite end shroud to the outlet earth. No continuity between mains and output and nothing to chassis. The only route for wires from the in-side to the out-side is through a gap in the bobbin. The laminations have what looks like a mumetal welded sleeve around them, pinned in place by the end shrouds. Suggestions please! Attached Thumbnails __________________ Kevin

19th Apr 2021, 5:09 pm	#2
Guest Guest Posts: n/a	Re: Step down transformer earthing Probably saving a bit of wire (the fools) to increase profits. If I was asked to fix this I would either do a proper job with earth continuity or scrap it. The former would be my preferred route, knowing that they bought a similar item instead that caused injury would bother me a lot.

19th Apr 2021, 5:56 pm	#3
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,483	Re: Step down transformer earthing That seems to be the norm with these, you just prompted me to look into the unit I have and it is wired as you describe. There is continuity but its far from perfect I doubt these conform to any known standard and look to be made to the standard of typical US electrics where there seems to be a more casual approach to the provision of earthing on 110V volt equipment if it's provided at all. __________________ Chris Wood BVWS Member

19th Apr 2021, 9:30 pm	#4
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,271	Re: Step down transformer earthing I'm more concerned with the correct way to earth it, it's a 3kw transformer and is used for a heating application. (ie well-loaded). Connecting the end caps by pushing an earth wire through the middle of the bobbin would surely leave it open to eddy current damage, what do people think? Is it likely to be an issue? Also, how can I earth the outer sleeve? The unit is used in an area outfitted with full stainless steel work benches, it's supplied via an RCBO circuit but it's no excuse for defective earthing. __________________ Kevin

19th Apr 2021, 9:47 pm	#5
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Step down transformer earthing I would be using a bit of copper shim gently pushed under the "mu-metal" shield. fold it under end bells until it's down past the endbell "ends" . Solder a normal green/yellow earth wire on both sides, one to input, and one to output. Wire ends go to the input and output sockets/connections. Going thru the bobbin is not a good idea, although there is room to do so. We have the same transformers available here. Joe

20th Apr 2021, 12:25 am	#6
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Step down transformer earthing Going through the bobbin will release smoke- it'll be a shorted turn! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

20th Apr 2021, 10:07 am	#8
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Step down transformer earthing Unless I've missed something I would put it in a box and do it properly. Lawrence.