Thermal fuse specifics for clocks. - UK Vintage Radio Repair and Restoration Discussion Forum

tawnyman · 24th Apr 2013, 11:46 am

Hi there, I recently acquired a handsome Ferranti synchronous clock and wanting to play it safe I'd like to fit a thermal fuse to the coil. How would I go about doing that and how do I know what temperature rating the fuse should have?
Having read the threads on here I shall be changing the mains cable. The case is obviously insulated but is it worth attaching an earth to the metal parts inside? I was going to use a repro fabric covered 0.5mm sq cable, I understand the current is very small on these clocks. I'm guessing the cable that is currently fitted is original, a transparent greenish plastic coating?
Many thanks
Julian

Nickthedentist · 24th Apr 2013, 12:25 pm

Hi John and welcome.

Yes, the current is miniscule, about 10mA or so (a couple of watts), so the current rating of the mains flex is immaterial. However, it has to be quite narrow to go through the opening in the case. The cable you see fitted may indeed be original: two core twisted, insulated in clear(-ish) PVC. Commonly also used as the dropwire for pendant lights.

I would do what you propose, and earth the metalwork with a 3-core flex.

As for the thermal fuse, I use pretty much the lowest temp. rating I can get my hands on, soldered into place, using extreme caution not to overheat it while soldering - tin the things it's going to be soldered to first, be damn quick with your iron, and spray some aerosol freezer on the fuse before picking up the iron. Insulate the mid-air joint with heat-shrink sleeving (same caveat as for soldering!). Note that not all thermal fuses are rated for mains use, so choose carefully. Picture here: https://www.vintage-radio.net/forum/...8&d=1333904252

Most people don't bother though, as they feel that the coil wire is so thin that it will be its own fuse in the case of shorted turns or whatever. A 1A fuse in the plug (a proper ceramic one, not a 20mm glass one botched to fit) is a good idea - you can get them on eBay.

At the other end of the spectrum, there are people who think all ancient mains powered clocks are too dangerous to use, and who re-wire their coils with thicker wire and operate them from a low-voltage AC supply from an external transformer. It's your choice!

Nick.

tawnyman · 24th Apr 2013, 12:48 pm

Thanks Nick. Yes, good point about the size of flex. I see from the photo that the thermal fuse just sits against the outside of the coil. If I decided to go down the route of rewiring the coil what kind of thickness wire and no of turns would I be looking at?
Julian

24th Apr 2013, 11:46 am	#1
tawnyman Retired Dormant Member Join Date: Apr 2013 Location: Reading Posts: 4	Thermal fuse specifics for clocks. Hi there, I recently acquired a handsome Ferranti synchronous clock and wanting to play it safe I'd like to fit a thermal fuse to the coil. How would I go about doing that and how do I know what temperature rating the fuse should have? Having read the threads on here I shall be changing the mains cable. The case is obviously insulated but is it worth attaching an earth to the metal parts inside? I was going to use a repro fabric covered 0.5mm sq cable, I understand the current is very small on these clocks. I'm guessing the cable that is currently fitted is original, a transparent greenish plastic coating? Many thanks Julian Attached Thumbnails

24th Apr 2013, 12:25 pm	#2
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,846	Re: Thermal fuse specifics for clocks. Hi John and welcome. Yes, the current is miniscule, about 10mA or so (a couple of watts), so the current rating of the mains flex is immaterial. However, it has to be quite narrow to go through the opening in the case. The cable you see fitted may indeed be original: two core twisted, insulated in clear(-ish) PVC. Commonly also used as the dropwire for pendant lights. I would do what you propose, and earth the metalwork with a 3-core flex. As for the thermal fuse, I use pretty much the lowest temp. rating I can get my hands on, soldered into place, using extreme caution not to overheat it while soldering - tin the things it's going to be soldered to first, be damn quick with your iron, and spray some aerosol freezer on the fuse before picking up the iron. Insulate the mid-air joint with heat-shrink sleeving (same caveat as for soldering!). Note that not all thermal fuses are rated for mains use, so choose carefully. Picture here: https://www.vintage-radio.net/forum/...8&d=1333904252 Most people don't bother though, as they feel that the coil wire is so thin that it will be its own fuse in the case of shorted turns or whatever. A 1A fuse in the plug (a proper ceramic one, not a 20mm glass one botched to fit) is a good idea - you can get them on eBay. At the other end of the spectrum, there are people who think all ancient mains powered clocks are too dangerous to use, and who re-wire their coils with thicker wire and operate them from a low-voltage AC supply from an external transformer. It's your choice! Nick.

24th Apr 2013, 12:48 pm	#3
tawnyman Retired Dormant Member Join Date: Apr 2013 Location: Reading Posts: 4	Re: Thermal fuse specifics for clocks. Thanks Nick. Yes, good point about the size of flex. I see from the photo that the thermal fuse just sits against the outside of the coil. If I decided to go down the route of rewiring the coil what kind of thickness wire and no of turns would I be looking at? Julian