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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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19th Apr 2021, 4:16 pm | #1 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,274
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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!
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Kevin |
19th Apr 2021, 5:09 pm | #2 |
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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.
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19th Apr 2021, 5:56 pm | #3 |
Octode
Join Date: Jan 2007
Location: Dorridge, West Midlands, UK.
Posts: 1,486
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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.
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Chris Wood BVWS Member |
19th Apr 2021, 9:30 pm | #4 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,274
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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.
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Kevin |
19th Apr 2021, 9:47 pm | #5 |
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Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
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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 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
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Re: Step down transformer earthing
Going through the bobbin will release smoke- it'll be a shorted turn!
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20th Apr 2021, 8:32 am | #7 | |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,274
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Re: Step down transformer earthing
Quote:
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Kevin |
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20th Apr 2021, 10:07 am | #8 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Step down transformer earthing
Unless I've missed something I would put it in a box and do it properly.
Lawrence. |