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Old 4th Jun 2021, 3:18 pm   #1
unitelex
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Default Laminated iron transformer leakage current.

I have noticed on some vintage AC sets, and also relatively more modern (eg B+O Beocenter 2600) that there can be notable AC leakage to the chassis.

It can be felt as a buzzing feeling if the chassis is touched lightly, particularly if the hand is moving slowly over the chassis. Once I have noticed, it bugs me.

The Beocenter 2600 I have in the workshop measures 112V AC on the extruded front panel but at only 0.2uA to ground according to my Fluke 87

I am aware that this is not uncommon especially on modern Switched-mode power supplies where there are class Y capacitors from primary to secondary.
The safety standards allow up to 3.5mA IIRC for plug-in equipment. So this unit is well within the limits. Nevertheless it irritates me.

When I received this set the 2 core cable into the mains plug was wired in reverse. Blue to Live and Brown to Neutral. Assuming this was a mistake by the previous owner I had corrected it then later noticed the leakage issue on the chassis. I tried going back to the reversed configuration and the leakage current all but disappeared. So I now assume the L-N reversal in the plug was either intentional or serendipity. If intentional the engineer ought to have a note left within the plug and within the set to warn future repairers that internal blue wiring is live and brown is neutral (IMHO).

Cause?
There is no Y cap between live and chassis on this set, the AC goes via the on-off switch and voltage selector direct to the transformer.

If I disconnect live directly at the transformer the leakage disappears.
The transformer is mounted via insulating plastic pillars, isolated from the chassis. So it seems that the leakage path is between one end of the primary winding and the secondary winding. Possibly stray capacitance? Dampness within the transformer is possible although it has been running for several days and therefore not dried out?

I will put the plug wiring back to standard and reverse the connections at the voltage selector. I will put a note inside to that effect.

I decided against changing to a 3 core cable and earthing the chassis as this could lead to earth loop problems with external inputs.
I daresay that any external equipment connected via the DIN socket eg record deck which is separately earthed would provide a return for the leakage current so it would then not be noticeable.

Does anyone have any similar experiences/solutions or do you ignore it if the leakage current is within safety limits ?
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Old 4th Jun 2021, 4:21 pm   #2
snowman_al
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Default Re: laminated iron transformer leakage current

As there is no connection via a capacitor or resistor from the mains to the chassis, consider the voltage is 'induced'.
All transformers work by induction, good old Faraday, so the mains induces a voltage in the secondary and also into any conductive parts in the magnetic field round the transformer.

Try a little experiment with a normal EI transformer. Wire up the mains side and then measure the AC voltage from the metal core or frame to mains ground. Bet you measure 20 to 50 volts AC depending on the way round the mains is connected. Now put a load on the secondary and the induced voltage from the core to earth will increase as the magnetic field is greater due to the load.

I'll leave the ''do I fix it or not'' to others to comment on...
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Old 4th Jun 2021, 5:06 pm   #3
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Default Re: Laminated iron transformer leakage current.

I very much suspect that what you're observing is due to capacitive coupling between the primary 240V 'hot end' and the core of the transformer. When you reverse the mains connection, the start of the winding closest to the core is changed from live = 240v ac, to neutral = close to earth potential, so in this condition there is very little coupling.

0.2uA would correspond to only about 3pF.

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Old 4th Jun 2021, 6:19 pm   #4
unitelex
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Default Re: Laminated iron transformer leakage current.

Al, John,
DC resistance between primary and secondary reads infinite on my Fluke

So yes would agree, mostly capacity coupling.

It is surprising that the manufacturer connected the start of the winding (adjacent to the end of the secondary winding) to mains live rather than neutral.

There is clearly less leakage with it the other way around.
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Old 4th Jun 2021, 9:21 pm   #5
marty_ell
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Default Re: Laminated iron transformer leakage current.

I would be quite concerned about the Beocenter 2600. This is class II, double insulated from a top manufacturer and even though it is from the '70-80s you really should not be feeling any leakage with the item stand-alone on the bench. Assuming the power transformer itself has no physical damage I would be looking for frayed/cracked wire insulation, previous water ingress to the trf., mains switch etc (even if now thoroughly dry). Even years of dust and grime getting into the on/off switch is a possibility.

You really shouldn't rely on plugged-in external equipment to provide an earth return for an unknown leakage current! The day might come when you forget, switch on the 2600 stand-alone and receive a nasty whack.

Worse, if the 2600's problem cause does fail it is likely to take out the external equipment as well - the ersatz earth connection may then be lost leaving the 2600 live.

To prevent hum pickup in audio set-ups where all separates are safety-earthed, often the simplest solution is to plug them all into one socket strip, or at least to plug to adjacent wall sockets. And keep your mains cables neat!


In the general case, mains-chassis leakage values depend much on on just how 'vintage' is the equipment in question. The impregnation of power transformers in the 'good old days' was seriously lacking by modern standards, particularly just post-WWII when quality materials were scarce; especially for those cheap radios/amps built down to the last penny.

What might appear a good insulation resistance chassis-to-mains when a valve radio is sitting on the bench, cold, might quickly drop to a too-low value after only a short warm-up period. For the power trf. itself this is due to a build-up of contamination of the 'wax' due to years of in-out gassing, plus general degradation and other causes. Sadly there is no remedy short of replacement, or a full rewind if you wished to maintain the appearance.

As a matter of course I always replace two-core mains cables with three-core when refurbushing old kit (not for AC-DC sets, obviously!) thereby guaranteeing the equipment is safety-earthed. For me the future risk in not providing a solid earth would be too high even if an insulation test appeared acceptable after soak test.

Hope this helps, Marty
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Old 5th Jun 2021, 7:38 pm   #6
unitelex
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Default Re: Laminated iron transformer leakage current.

Hi Thanks for your thoughts.
The measured leakage is 0.2 micro Amps, the safety directives limit is 3.5 milli Amps so no need to be concerned as far as the safety standards are concerned unless it gets more than 1000 times worse.

The root cause of the leakage was determined to be within the transformer itself and winding to winding rather than perished wiring insulation or switch etc. This was deduced through a logical process of elimination.
It was determined to be non-galvanic by measurement therefore induced rather than conductive leak.

I was not proposing external equipment as a source of safety earth- far from it, my meaning was only to suggest the 0.5uA leakage current would not be felt on the chassis if it had a path to ground.
There is no nasty whack scenario because the leakage current is orders of magnitude below the internationally accepted safe limit.
This is a transistor set, relatively modern. The transformer was made in 1979 and uses what looks like modern Mylar insulation. It is as you say classed as double insulated with the double concentric squares symbol showing.
I concur with Al and John on the conclusion this is primary-secondary capacitive coupling. Its current amplitude is fixed by the mains frequency, physical dimensions of the transformer, the thickness and dielectric constant of the winding insulations. I doubt these are likely to change over time.

I largely agree with your comments on earlier vintage equipment, these are valid and sensible precautions.

Having reversed L and N at the voltage selector, the leakage current is now undetectable. I think this lends further credibility to the capacitive coupling from the end of the primary winding closest to the secondary winding into the secondary winding

I will consider converting the 2 core cable to 3 core, although this should not be necessary in equipment meeting the double insulated category.

Thanks again for all your comments
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Old 5th Jun 2021, 8:00 pm   #7
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Default Re: Laminated iron transformer leakage current.

Quote:
Originally Posted by unitelex View Post
It is surprising that the manufacturer connected the start of the winding (adjacent to the end of the secondary winding) to mains live rather than neutral..

I wouldn't fret about which 'end' of a transformer's primary is would closer-to-the-core.


Remember that in most-of-the-world (including most-of-Europe) mains-plugs were [and still are] unpolarised - so there's no real concept of preserving 'live' and 'neutral' polarity in an appliance connection.

Hence it's perfectly-normal to find a single-pole on/off switch wired in one supply-line and the appliance-protection-fuse in the other line.

Depending on which-way-round you inserted the appliance's plug into the wall-outlet, this could mean the fuse, or the switch, was in the 'live' side.
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Old 6th Jun 2021, 2:35 pm   #8
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Default Re: Laminated iron transformer leakage current.

Which has the interesting side effect that if the fuse has blown and the unit is switched off, it's isolated from the line either way round
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Old 6th Jun 2021, 11:49 pm   #9
unitelex
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Default Re: Laminated iron transformer leakage current.

Quote:
Originally Posted by G6Tanuki View Post
Quote:
Originally Posted by unitelex View Post
It is surprising that the manufacturer connected the start of the winding (adjacent to the end of the secondary winding) to mains live rather than neutral..

I wouldn't fret about which 'end' of a transformer's primary is would closer-to-the-core.


Remember that in most-of-the-world (including most-of-Europe) mains-plugs were [and still are] unpolarised - so there's no real concept of preserving 'live' and 'neutral' polarity in an appliance connection.

Hence it's perfectly-normal to find a single-pole on/off switch wired in one supply-line and the appliance-protection-fuse in the other line.



Depending on which-way-round you inserted the appliance's plug into the wall-outlet, this could mean the fuse, or the switch, was in the 'live' side.
Ah yes of course. I had not considered the original design would be for the Danish 2 pin plug. So the original design/market could not control which side of the primary gets Live vs Neutral. Probably explains why this Danish design is as it is.
Nevertheless we have the advantage in the UK of the 3 pin plug giving consistent L+N allocation (assuming domestic sockets are wired correctly)
So I am happy to continue with the minor improvement.
Yes, and I think the various country plug types and their advantages/disadvantages have already been discussed elsewhere so no need to re-iterate here in case anyone was tempted to...

Thanks for all the feedback.
Mods, I think this thread can be closed - thankyou.

Last edited by unitelex; 6th Jun 2021 at 11:51 pm. Reason: some of the response was embedded in the quote, moved it out
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