AC/DC sets with 3-core mains leads (!)

[Nickthedentist]

Have a look at this chap's (Richard Thorp) web page on restoring a DAC90A: http://steamradio.tripod.com/dac90a/

He advocates using a 3-core mains lead, with the earth conductor being used exclusively to earth the speaker grille as a precaution against its becoming live. I'd never seen this done before, but it seemed like a good idea, and I planned to do the same to my DAC90A when I get round to restoring it.

But when I mentioned it on the forums yesterday, Paul Stenning (no less) said that it was a very bad idea!

And indeed, I'm sure it is. But I can't work out why! Could you possibly explain, Paul (or anyone, for that matter)?

Thanks,
Nick.

[ppppenguin]

You are not meant to have any earth connection to an appliance that is double insulated. The idea of double insulation is that any touchable metal parts are sufficiently well insulated from human contact (by means of 2 separate insulating layers or by suitably reinforced insulation) that there is a negligible chance of them becoming live.

In a DAC90A I would make sure that the grille is well insulated from any other metalwork, ensure that it was plugged in with the chassis to neutral and, ideally, have an RCD somewhere in the mains supply.

[Nickthedentist]

Thanks Jeffrey, good advice there, and I agree wholeheartedly about using an RCD (especially as most trip when there's an E-N short too).

But I still can't see what harm it would do to have it earthed. After all, most modern hifi equipment is double insulated and has a 2-core cable, but its metalwork becomes earthed (via the screen of the audio cables) as soon as it's connected to a component which has a 3-core mains cable (not unheard of, even with brand new AV equipment).

And I would have thought that 50-year old double insulation is probably not terribly reliable.

But obviously I'm missing something!

[ged_drinkwater]

I would agree that, normally, there is only a minuscule chance of the speaker grill becoming live. Nevertheless, the possibility remains that the insulation of the speaker leads might become damaged, leading to the possibility of contact with chassis; should the latter become live, as a result of (say) an incorrectly orientated rear connector, or perhaps a shorted live pole on the mains switch, a potential hazard will be present. On balance, then, in the case of my own DAC90a, I thought it safer remove the 2-pin plug and hard wire a 3-core mains cable, connecting earth to the grill. Like 'Nickthedentist' I don't see any harm in this. Sure, an insulation failure could still occur, but at least I can be assured that the speaker grill can never become live.

Regards,
Ged

[Nickthedentist]

Thanks Ged. I'd hard wire as well; the reversible connector seems to be an accident waiting to happen!

And sorry, I got the people wrong, it was Radio Dave and not Paul Stenning who didn't like the earthed grille idea. Apologies to both of you.

[ppppenguin]

I suppose I was trotting out the standard "party line" on double insulation. I'm not sure I can find any more convincing reason for not earthing the grille. But I still wouldn't fit a 3 core mains lead.

The correct way to use a fixed mains lead is to wire it directly to the tags on the back of the 2 pole mains inlet. NOT!

[Nickthedentist]

Don't worry Jeffrey, I wasn't going to do that. But I'm sure someone, somewhere has.

[paulsherwin]

I agree with Nick, I can't see a problem with this mod, other than perhaps lulling a future restorer into a false sense of security. It can't be a good thing to have a large expanse of exposed floating metal. This mod would force a blown plug fuse in the event of a fault, which has got to be better than a dead person

It goes without saying that the chassis of an AC/DC set shouldn't be earthed (that comment for the benefit of any radio newbie who stumbles on this thread )

Best regards, Paul

[Ed_Dinning]

Hi Gents, I have seen some ac/dc sets that used an earth "rail" that was seperated from the chassis. A capacitor then connected this rail to chassis. I believe this was used on some of the lower cost communications sets that had metal cases. Definately a bad practice with the dodgy capacitors available in the 50's and probably not a lot better even with a modern class Y cap.
It is possible to use class Y caps in this position as they are intended to be connected from a live mains pole to a safety earth and are specially constructed to fail open circuit. They are only available in lowish values and should not be used on pluggable equipment with values above 4700pf (total value of all caps used) as above this value the current passing through the capacitive reactance is deemed to be capable of giving a shock and will probably also upset the RCCD's.

Ed

[ppppenguin]

Some random musings on a sunny morning.

I sometimes wonder how we all survived the era of live chassis sets, many of which had insulation that was less than excellent. BS415 (date?) eventually specified rules for accessibility to live parts. Perhaps in the old days everyone knew that electrical stuff was potentially dangerous and natural selection weeded out those who didn't.

If you get a tingle from the speaker grille, don't touch it again!

In the UK we have a tradition of earthing. Elsewhere in Europe I think there was greater use of double insulation. Earthing is a great way to protect people but fails completely when the earth wire falls off somewhere. Double insulation, properly done, tends not to fail. In the USA they don't need any of this fancy protection because you can't get a decent belt from 110V

A story for those who think that RCDs are the cure for all evils.
http://www.bbc.co.uk/london/yourlond...stigator.shtml

And I still have a worry, possibly irrational, about connecting an earth wire to something that is supposedly double insulated. I suppose class Y capacitors are OK but the ones in mains filters can hold enough charge to give you a belt when you touch the mains plug. Or is that the class X caps across the mains.

[Aerodyne]

This mod would force a blown plug fuse in the event of a fault, which has got to be better than a dead person

Couldn't agree with Paul more, though there is certainly something slightly creepy about fitting an earth lead to an AC/DC receiver with 'live chassis' technology. I guess, if we are to follow this to its logical conclusion, any set of live chassis form with metal grille or cabinet parts should be treated in the same way.
-Tony

[Nickthedentist]

Thanks everyone.

I agree, there's something just "plain wrong" about fitting an earth wire to an AC/DC set, but it does make sense in a way. Still no word from "Radio Dave" though...

I also agree that if something was safe enough then, then it's probably safe enough now. But an earth conductor is such a simple and cheap precaution that it would seem daft not to provide one when a set's being restored. I certainly always fit a 3-core cable to AC only sets if one wasn't fitted originally - stops that un-nerving tingling if nothing else!

RCDs certainly aren't without their problems. Some of the plug-in variety only have single-pole switching, so would fail to isolate the live conductor if the socket into which they're plugged is wired "backwards". I can recall a news story 20 years ago in which someone was killed in their garden by a faulty appliance in spite of an RCD, for this very reason.

The chap with the home made water heater in Jeffrey's article was a bit silly (to put it politely) - but he'd have been luckier if it had happened more recently as new immersion heaters have to have overheat protection now (for new installations anyway).

By the way: Ed, with the comms receivers you mention, were their cabinets connected to true earth via a 3-core lead, or were they just tied to the neutral-potential chassis with an isolation cap? I presume it was the latter, otherwise I can't see why the cap would be necessary at all (but I'm happy to be corrected!).

Nick.

[Sideband]

Some American AC/DC sets had an isolated chassis. They had an insulated earth 'bussbar' and all the earth returns were connected to this. A capacitor was connected between the bussbar and chassis probably rated at about 500vAC (OK with 110v mains) and this allowed a three-core lead to be fitted.


Still not keen on this idea!!


Rich.

[Radio_Dave]

Originally Posted by Nickthedentist

Still no word from "Radio Dave" though...
Nick.

Hi Nick,

I’ve been away from this forum for a couple of days and seem to be the cause of this discussion .

You all seem concerned with about the speaker grill becoming live. I really cannot see how this could happen. I have looked at one of my DAC90a’s and this can only happen if, firstly, the speaker frame becomes live and secondly, the speaker frame actually makes a contact with the grill. I think Jeffrey’s idea of insulating the grill is a good one. A simple ring made out of plastic and fitted between the speaker and the grill should do the trick

I am more worried about having an earthed grill and a potentially live chassis (depending on how the 5 amp plug is connected). Picking the radio up incorrectly (I can touch the tuning cap though the slots in the back cover) or if a knob were to break or fall off it would be very easy to get an electric shock?


I am the first to admit that I am no expert at this kind of thing. If I have got something fundermentally wrong here I stand to be corrected



Best Regards

David

[paulsherwin]

Originally Posted by Radio_Dave

I am more worried about having an earthed grill and a potentially live chassis (depending on how the 5 amp plug is connected). Picking the radio up incorrectly (I can touch the tuning cap though the slots in the back cover) or if a knob were to break or fall off it would be very easy to get an electric shock?

That's a very good point David, I hadn't thought of that. Holding a live control shaft with one hand while touching the earthed grille with the other would produce a dangerous shock across the chest. Unlikely, but certainly a possibility.

Best regards, Paul

[Nickthedentist]

Yes, David's idea is not something I'd given much thought to either. I'm still not 100% convinced, one way or the other though

[GJR 11L]

My most recent Bakelite acquisition, a DAC90A, was offered to me by the vendor with a choice between a 'proper, safe modern 3-core lead' (the seller's words, I hasten to add) and an original Bush lead to use in the original manner (albeit with a modern{ish, it's a brown MK item} 3 pin mains plug and clear markings to indicate correct orientation). As luck would have it I bought well, the set's a little corker, but I was reminded of the cautionary tale of the older person who, on buying a new, earthed appliance of the kitchen variety, could not fathom out how to utilise the device's 'extra' lead in the 2 pin plug necessary to connect the device to the archaic wiring in their cottage.
Whether this is an urban myth I know not, but it is claimed that our older person, unable to decide what to do with the earth core, divided it evenly between the live and the neutral...... ;-)