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Old 5th Nov 2011, 6:42 pm   #15
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
Default Re: adding earth lead to vintage radio

Quote:
Is it indeed safe to use a model that you cannot add earth wires to
There's a whole spectrum covering all aspects of safety, not just electric shock risk. After all, what's the use of earthing a radio chassis if it later catches fire and burns the house down. In general terms, a chassis that can be earthed will normally be safer once it is earthed, although with well-constructed sets the difference is not a great one unless something else is amiss.

If I were leaving a radio in the house for daily use by anyone in the family including children, who might not spot trouble as readily as experienced users, I would reckon it worthwhile to add an earth where possible unless the integrity of the insulation afforded by the case is really excellent. I would certainly power it via an RCD, and possibly strap a thermal fuse to the mains transformer. For my own purposes only, the RCD is enough.

It is important to distinguish between transformerless (AC/DC or universal) sets and ones with a mains transformer. Transformerless designs are much more likely to present a shock risk, although this can sometimes be mitigated by ensuring the mains lead is connected with the correct polarity. In general they cannot be earthed, so you are stuck with the residual risk, albeit a low one if you have RCD protection on your electrical supply. Any set with a mains transformer is likely to be safer than a universal, regardless of whether it has a 2-core or 3-core mains lead. These are often described as AC-only sets in contrast to AC/DC.

For comparison it's worth noting that well into the 1980s many TVs had 'live' chassis just like 1940s universal radios. Worse, in some respects, because the chassis couldn't be connected to the neutral side of the supply let alone earthed, due to being connected after the rectifier. They would have been built using more modern insulation techniques but the principles are the same, yet we wouldn't consider these especially dangerous today.

You will rarely get a useful answer on these points from a non-technical seller, as they won't understand the electrical theory that governs the safety aspects of receiver design. You can always ask here if you have the model number or look up the set's specification online, and please don't be afraid to ask for clarification of unfamiliar terms.

Lucien

E2A Steve's post beat me to it covering some of these points already!

Last edited by Lucien Nunes; 5th Nov 2011 at 6:50 pm. Reason: Additions
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