Neutral and Earth Connections

[GrahamN]

I have just (almost) finished my first ever valve radio restoration (a DAC90A), and after a little bit more cleaning will be putting the chassis back into the case. Now I seem to remember (very vaguely) back in the late 60s when I worked temporarily in a radio workshop, that a common practice was to join the neutral and earth wires together in some radios - but I really can't remember why, or if this is appropriate for a live chassis set.

I've pretty much decided to hard-wire the mains (with neutral to chassis of course), rather than use the plug - partly because DAC90A are so plentiful that modifying the set from original isn't so heinous a crime as it would be with a much rarer set, and partly because I can't find a suitable plug anywhere (even eBay don't have any at the moment, and the last ones i saw on their went for about £10 with postage - more than I really want to spend).

I did see someone selling a restored set, and they made a point of saying they earthed the speaker grille - this doesn't seem to make much sense to me as I can't see how that could possibly become live regardless of any plausible fault - but wondered if I should be connecting Earth and Neutral together so that the main chassis is earthed.

Or should I just use 2-core mains cable and forget earth completely?

I should add that I have no intention of selling or otherwise disposing of the set, so it is only me and my family that are likely to use it for the forseeable future.

Thanks in advance for any advice.

[Station X]

There are differing opinions on this. My advice is to hardwire it making sure that the neutral lead is connected to the chassis. Earth the speaker grille if you like. The advantage of this is that it will enable the set to pass a Portable Appliance Test (PAT), as exposed metal parts will be connected to earth.

DON'T join neutral to earth.

[evingar]

You shouldn't join earth and neutral in most installations - A negative to earth short will cause your leakage trip to blow.

[GrahamN]

Thanks.

A negative to earth short will cause your leakage trip to blow

So am I mistaken in my memory about joining earth and neutral? Or is this something that can be done under certain circumstances? Surely the mains supply hasn't changed since the late 60s (or has it?).

I really can't remember why it was done - I just seem to remember it being fairly common in this particular workshop (and we serviced primarily schools equipment so I would have thought that safety would have been fairly important...)

[TrevorM6TSE]

Hi All,

As afar as I know, Earth and neutral are linked, but either at your meter or in the substation. I think there's lots of posts about this. I can confirm that shorting earth to neutral (in my house anyway) does trip your master RCCD. I did it my accident changing a light switch.

I'd agree not to connect neutral and earth in the radio.

All the best.
Trev

[russell_w_b]

Originally Posted by GrahamN

Thanks.



So am I mistaken in my memory about joining earth and neutral? Or is this something that can be done under certain circumstances? Surely the mains supply hasn't changed since the late 60s (or has it?).

Neutral and earth are solidly connected (and are effectively as one) at the incoming side of your premises, but are separate on the consumer side of the RCD. A N - E fault will trip the RCD in the same manner as a L - E fault.

[dseymo1]

I'm in favour of earthing such things as metal grilles etc.
Although the likelihood of them becoming live is remote, the most unlikely faults do sometimes occur, as we all know.
Imagine an output transformer with faulty inter-winding insulation, together with a rubbing speaker voicecoil. Between them these could result in the speaker chassis becoming live, which, in some sets, would be in electrical contact with the grille.
Interestingly, I faced just this question recently when constructing a guitar practice amp. Although the mains transformer was double-insulated, I decided that it was worth earthing the grille and other exposed metal parts, just in case. In practice, it significantly reduced hum pickup, which might be a disadvantage in the case of a radio.

[julie_m]

Neutral and earth are joined upstream of the ELCB, then kept separate. The ELCB measures the difference between the current flowing out of the live and the current flowing back into the neutral, and cuts off the supply if this exceeds 30mA. Joining them together downstream of the breaker will cause unwanted tripping, as some current will invariably bypass the breaker: the resistance from the proper neutral-earth junction via the earth wire to your new junction is in parallel with the resistance from the proper junction, via the ELCB coil and neutral wiring to the new junction, acting like an ammeter shunt.

But it is best for the radio's chassis to be at neutral potential rather than live. Fortunately, our power sockets are always non-reversible, so you always know which leg is the neutral.

You can earth the speaker grille if you think it will help; but as long as there is at least 8mm. between it and any live metalwork at all times, it should be considered adequately insulated. Don't connect the speaker grille to mains neutral, though; because if the neutral wire in the plug ever becomes disconnected, then the speaker grille could become live via the dropper resistor and heater chain (which still have a combined resistance low enough to do you a mischief).

[kalee20]

AJS Derby is right here, 8mm is plenty of clearance to have between exposed metalwork and mains-related metal. This will meet current standards. So would 8mm creepage distance.

The trouble is, the DAC90a can't be guaranteed to be like this. At the loudspeaker, there's about 2mm between the terminal tags and the speaker chassis; and an ill-defined (but probably less than 1mm) between the loudspeaker chassis and the grille. So the rest of the 8mm would have to be made up within the output transformer, between primary and secondary, which is extremely doubtful. Further, the leads to the loudspeaker would have to be double-sleeved (or else use so-called reinforced insulation), since they drape over the mains-related chassis.

If you want to improve safety (and beyond replacing the slightly perished rubber leads to the speaker with modern silicone-rubber leads, I'm not doing anything with my own DAC90a), the advice to take the loudspeaker grille to the earth lead is good. But DON'T connect neutral and earth (as per reasons given in above posts).

[Andy40uk]

If you don't want to hardwire it I have a spare mains connector for one of these you can have FOC. PM me if interested,

Andy

[GrahamN]

Thank you everyone - perhaps my memory is not as good as it used to be, or perhaps it was done for a totally different reason. In any event I shall remember not to connect N & E together!

Andy - I must admit I would much prefer to use a plug, so I shall PM you shortly. Thank you for your generous offer.

[Dave Moll]

Originally Posted by GrahamN

I would much prefer to use a plug.

In which case it would be a good idea to mark it such that you can make sure it is always connected with the chassis to neutral. The connector is reversible, and it can be difficult to tell which way round it is connected.