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Old 17th Mar 2008, 12:14 pm   #1
RetiredHal
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Default Restoration. Add Fuse and Earth?

Hi, moderators, please move this thread if it is in the wrong category.

I have trawled his forum for Earthing and Fusing. I have also scanned other forums and schematics.

My question is why is the primary transformer side fused , rather than the secondary side. My restoration follows this rule. I replace the twin flex with a three core flex, ground the chassis, insert 2A fuse in the active lead. This seems to be the consensus on this forum.

It seems to me that a DC problem can damage the transformer well before the fuse in the primary will blow.

Note, I have not found a schematic that protects the secondary side by way of a fuse. (plenty that protect the primary side).

Is this just a "financial shortcut" or are there legitimate reasons of which I am not aware?

Cheers John
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Old 17th Mar 2008, 12:36 pm   #2
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Default Re: Restore Add fuse and Earth

Primary fuse can prevent fires if the transformer develops shorted turns or leakage to earth. Secondary fuse can protect transformer from rectifier and DC faults. Both have their uses.

In all cases the first duty of the designer is to prevent danger to the user, from fire or shock. The second duty is to prevent equipment damaging itself. This is much less important than the first.

No fusing regime is perfect and there will always be some faults, notably small overloads, that will cook wiring or components rather than blowing fuses.
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Old 17th Mar 2008, 12:50 pm   #3
Ian - G4JQT
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Default Re: Restore Add fuse and Earth

Perhaps the non-fusing was a cost-cutting exercise, but there must have been other reasons. I don't think I can remember encountering a fuse in a vintage radio chassis, but I've seen them in old TVs – presumably the increased component count made them potentially more fault prone.

Many older radios would have been originally used in houses that didn't even have a fuse in the mains plug, so the lack of any fusing on the radio was surely potentially quite dangerous in the event of a short circuit somewhere.

On professional equipment, both sides of the transformer are usually fused. On anything I build - using valves or solid state - I always add appropriate fusing: between primary and mains lead, between secondary and rectifiers (to protect the transformer if a diode rectifier or voltage regulator component goes short circuit) and in all the DC rails to protect the regulator circuits, rectifiers and transformer.

Incidentally, earthing vintage gear that uses a mains transformers that was not originally earthed can cause problems. The insulation between the mains winding and the metal core may break down; when the chassis was floating there was no potential between the mains winding and metal laminations. As soon as the chassis is earthed, depending upon the construction/quality of the mains transformer, there is suddenly mains voltage between the windings and the laminations.

I've got away with it more often than not, but had one set that kept tripping the mains ELCB due to leakage, before it broke down completely with a "BANG"!

Regards,

Ian

Last edited by Ian - G4JQT; 17th Mar 2008 at 12:55 pm.
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Old 17th Mar 2008, 2:32 pm   #4
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Default Re: Restore Add fuse and Earth

Ian , I am probably missing something important here
Quote:
Incidentally, earthing vintage gear that uses a mains transformers that was not originally earthed can cause problems. The insulation between the mains winding and the metal core may break down; when the chassis was floating there was no potential between the mains winding and metal laminations. As soon as the chassis is earthed, depending upon the construction/quality of the mains transformer, there is suddenly mains voltage between the windings and the laminations.
I thought that if I grounded the chassis (by way of a three pin plug) I would only create a small earth loop (and I am not sure if this is true with a transformer as only the secondary windigs are earthed to chassis). Typically the difference between Earth and Neutral will be around 0.3V in a home where Earth and Neutral are tied at the fuse box and 6-10 V in high rise at the eleventh floor.

My point being that the insulation on the transformer should be able to cope with a potential difference of say 3 volts between Earth (chassis) and Neutral (one of the transformer legs).

What am I missing here? NB I am really trying to learn something here.

Cheers John

Last edited by Station X; 17th Mar 2008 at 3:30 pm. Reason: Quote fixed.
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Old 17th Mar 2008, 2:51 pm   #5
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Default Re: Restore Add fuse and Earth

If the insulation between the primary winding and the core is poor, then it's possible that the voltage between the live end of the primary and the core could cause a breakdown. If the core is earthed this will blow fuses and/or trip RCDs.

IMHO, if a mains transformer cannot survive a reasonable insulation test then it's dangerous to use the set. Unless you can treat the set like a live chassis design and keep all parts that could become live safely apart from humans.
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Old 17th Mar 2008, 3:19 pm   #6
Ian - G4JQT
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Default Re: Restore Add fuse and Earth

Hi John,

Yes, there is normally no more than about 5 volts between earth and neutral - it can vary depending the current in the neutral, but is not relevant here regarding insulation. What is relevant is the insulation between any wiring at 'live' potential (240VAC) and chassis.

Think of it this way: if you had a poorly insulated live lead running against an UNEARTHED chassis, probably nothing would happen as there is no voltage potential between the chassis and the live lead. As soon as you connect the chassis to earth (or neutral for that matter - but don't do that!) you then have 240 VAC between the chassis and poorly insulated lead, leading to possible insulation break down.

If the mains transformer has the live side of the winding going on the bobbin first, separated by perhaps only a layer of waxed paper, and thus being very close to the laminations bolted to the earthed chassis, this may have the same affect as the above example.

In practice, it's probably the secondary winding that goes on first, with the earthed side closest to the laminations. This would make sense, although I don't actually know if this is always done in practice.

With modern materials and construction methods it probably doesn't matter (although the winding connections are clearly marked), but with something made 50-odd years ago and kept in dubious storage conditions, you can see there may be an issue.

Adding a good earth via earth rods or water pipes will have the same effect as adding a mains earth.

If you have a Mega (spelling?) you can test the insulation between the primary and the laminations, but only up to about 400 - 500 volts, this will give a clue to the state of insulation, but I would not want to rely on it! The Portable Appliance Test (PAT) tests at 2kV I think.

That's a rather rambling explanation, sorry...

Regards,

Ian

Last edited by Ian - G4JQT; 17th Mar 2008 at 3:26 pm.
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Old 17th Mar 2008, 4:49 pm   #7
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Default Re: Restore Add fuse and Earth

I would just like to add that although one should always respect the difference between Live & neutral. It often occurs that the two are accidentally switched, sometimes at the electrical intake. Therefore it's best to always treat the neutral as if it were live & insulate it accordingly. Even today sometimes equipment is plugged in to non-polarised outlets, and of course many of our Europeans friends still have non-polarised outlets. I have taken several valve sets to friends in Italy.

I have also noticed that several 110volt auto transformer units connect the US neutral pin to the middle tap of the transformer making it 120volts live to earth. The 110v live contact either at true mains neutral or phase. So the supply voltage is correct across the US plug pins, but could be 120 0r 240 volts above earth. Not good for a very old set that probably had poor insulation designed for only 110 to earth.
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Old 17th Mar 2008, 5:26 pm   #8
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Default Re: Restore Add fuse and Earth

Yes I fully agree on fitting a fuse to the live side of the mains, I use 1 amp plug top fuses if its feeding a radio and a 2 amp if its a vintage TV (black & White). Luckily I have loads of these small value fuses that are plug top sizes.
Regarding transformer leakage its really quite rare but when it does occur its normally been caused by a duff rectifier or an HT short damaging the tranny.
Most sets with a mains tranny will have an earth connection and I always use the earth but not the mains earth as it can be too noisy. I have a few earth rods around my house and use them, I do check though the voltage difference between chassis and earth and its never above 10v although I did come across a set I did for a customer with about 100v leakage, I sold him a plug in RCD and fitted an earth via the plug but the trip has never gone in 3 years.
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Old 17th Mar 2008, 9:34 pm   #9
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Arrow Re: Restore Add fuse and Earth

Quote:
Originally Posted by Ian - G4JQT View Post
Hi John,
In practice, it's probably the secondary winding that goes on first, with the earthed side closest to the laminations. This would make sense, although I don't actually know if this is always done in practice.
In my experience - and this checks out with various standard reference books on the subject - it is usual for the primary (in a.c mains supply transformers) to be wound first - that is, primary closest to the core.

Al / Skywave.
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Old 17th Mar 2008, 9:44 pm   #10
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Exclamation Re: Restore Add fuse and Earth

Just a quick one from me on a potential hazard of fusing on the sec'y side of a mains transformer.

It is not uncommon to find a full-wave valve rect. cct. with a fuse in the centre-tap of the secy. to 0v / Gnd. This is OK. What is not OK is when someone has decided to replace the full-wave rect. valve with a pair of silicon diodes & not change this aforesaid fusing arrangement, since if both diodes go S/C, you then have a dead short across the sec'y. of the transformer; the centre-tap fuse will afford no protection against this failure.

Result = one burnt out mains transformer.

Prevention: add a fuse in each leg of the HV winding of the sec'y.

Al / Skywave.
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Old 18th Mar 2008, 11:57 am   #11
RetiredHal
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Default Re: Restoration. Add Fuse and Earth?

Thanks, yes I can now see the problem of insulation when you earth the chassis with a replacement three core flex. I never thought about it as a hazard. It appears I will have to buy myself a "megger" to test the insulation before I do a restoration.

Cheers John
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