Adding internal fuses to Ferranti 255 - A Good idea?

[Old Sparky]

I am currently repairing a Ferranti 255 AM/FM Mains radio, which, after replacing various waxies and a faulty on/off switch & tone control is basically working. On noticing that there is space inside the chassis to add a couple of 20mm fuseholders, I thought of so doing, fitting a fuse in each lead to the mains transformer primary, possibly using a couple of 80 or 100mA(T) fuses for the purpose. This would give added protection, although I have put a 1A fuse in the mains plug, after changing the original two-wire mains lead for 3M of 3-core. I realise that this detracts from originality, but, as noted above, adds extra protection. The fuseholders would be inside the chassis, fitted with covers, and not accessible without at least removing the cover from the underside of the cabinet. Should I go ahead with this mod?
As an aside, I've often wondered why most continental radios of that era are internally fused, and British sets seldom, if ever were.

[Yorkie]

As an aside, I've often wondered why most continental radios of that era are internally fused, and British sets seldom, if ever were.

Because unlike UK 13A plugs, continental mains plugs don't have fuses in them. That said, as we know, UK plugs come with a 13A fuse as standard, which is often not changed for the correct lower value of fuse called for by the appliance. (1A fuses for 13A plug-tops aren't so easy to find). If the radio has the facility to add a couple of fuseholders, why not? The added protection can only be for the better.

[John M0GLN]

Should you fuse both leads to the mains transformer primary? If the one in the neutral lead blows it still leaves the transformer live.

John

[Nick]

Should you fuse both leads to the mains transformer primary? If the one in the neutral lead blows it still leaves the transformer live.

Doesn't guarantee anything - typically, only 1 will blow, and its 50/50 if it'll be the live or neutral, however I'd always assume 100% that it was the neutral and the set would still be live

A two pole breaker would do the tick - if either or both trip, the circuit is isolated...

[Skywave]

I've had to consider this question of fusing & mains wiring many times over the years, when renovating comms. receivers. My approach is as follows (with the rare, occasional exception *) - assuming an a.c. mains only set with a centre-tapped secondary winding on the mains transformer.

1. Route the incoming mains to a suitably-rated DPDT switch first (* the RCA AR88D / LF is a classic exception here) - and then fit a fuse (typically 2-amp. anti-surge) in the "L" feed from that switch to the transformer primary. (Yes, I am aware that there are two schools of thought on this: which comes first: the switch or the fuse? There are pros and cons in either approach.)
2. Fit a fuse between the centre-tap of the transformer secondary to chassis (which is usually the 0v. point). Something like 100 mA to 250 mA anti-surge is usually adequate.
3. All soldered joints, connections and tags that are carrying mains voltages are well-insulated with heat-shrink sleeving.
4. Fit a conventional 3-pin mains plug with a fuse of appropriate rating: usually 3-amp.
5. Always use 3-core mains cable, suitably rated, with the earth wire securely bonded to the receiver's metalwork. Ensure that that cable is mechanically secure to the chassis.

Such changes do indeed sometimes change the originality of a set, but that is a change that I consider worthwhile on the grounds of safety: safety for the set, the set operator and set maintainer. (I speak from personal experience in the last category: ouch!)

Al. / Skywave / March 20, 2014.

[Old Sparky]

Points taken about fusing both connections to the Mains TX primary, folks. All I can say is that Hacker did it that way in some of their record players. I have recently fitted an IEC mains connector to a Grenadier in which the L & N leads to the TX Primary were both fuse protected by Hacker. Returning to the Ferranti 255, it's a conventional valved receiver using an EZ80 full wave rectifier, so obviously has a centre-tapped secondary winding into which a fuse could be inserted.
As to fused and unfused mains plugs, I certainly know that most, if not all, Continental plugs are unfused, but back in the 50s & 60s there must have been many UK homes still using the unfused 5 & 15 Amp mains plugs, meaning that many popular radios (and other appliances) were relying on the fuse links at the ring main for protection.

[Joe]

5A and 15A socket weren't on rings.

That aside, I used to fit a single fuse inside radios in the live before the switch. I used 500mA normal blow for AC sets and 350mA(T) in AC/DC ones. Since getting a supply of 1A plug fuses I haven't bothered.

- Joe