Fuse Confusion
The AT&E 10/2 PAX (10 line electromechanical phone exchange) I’m restoring has some interesting fuses in it.
They’re designed so that when the piece of fuse wire blows, the flag pops up to let you know which fuse has blown, and a strip of metal pops down to make contact with a bar running the length of the fuse board. This bar is connected to an alarm circuit. If a fuse blows, the alarm circuit activates and a light comes on to tell you about it.
The idea is similar to the fuses I’m used to seeing on the UAX/PABX kit we’ve got at the railway, although they’ve just got two sprung contacts instead of cool little flags.
I’ve done a reasonable amount of googling for every search term I can think of, but can’t find any information about this particular type of fuse (let alone a source for replacements!)
I assume that the colour of the flag (red, blue or black) denotes the value of the fuse, and whilst there are part numbers stamped on the ends, there are no values. The markings are as follows:
| Fuse Colour | Horizontal Slot | Vertical Slot | Makers Mark? |
| Blue Fuse | L27317 | 31/1 | U.C.A |
| Red Fuse | L27318 | 31/2 | U.I.C |
| Black Fuse | L27319 | 31/3 |
I assume the marks in the Horizontal Slot column are model numbers, but I’m not sure what the “31/n” bit is about, especially as it doesn’t match up with the last three digits of the model number.
So – has anyone got any ideas where I could get some more of these fuses? Has anyone got any ideas what value fuse wire I should attempt to repair (or replace) them with?
Interesting, I’ve not seen these before. If any are intact, there are two ways you could work out what to replace them with: crudely by measuring the wire diameter with a micrometer (and replacing with similar), which might work if there’s only a stub of broken fuse wire remaining, and a ‘one-shot’ method of gradually applying more current until a working fuse blows. The latter is useful only if you have some unbroken fuses of course and the bravery to break them in the hope of future replacement.
I’m going to ask for some help from some local geeks here, I know a few telecoms people.
I thought you might like these! I’ve done some asking around through the Telecommunications Heritage Group mailing list, and apparently they are “type 31″ fuses, and the number after the slash is the current rating. So the red ones are “Type 31, 1A” and so on.
I’m told that “type 44″ fuses are direct replacements, and that these are the same type of fuses we use on the strowger exchanges at the railway.
Now all I need to do is find a source of 1A/2A/3A fuse wire!
great – looks like you have at least a couple of options then. If you can’t find 1A fuse wire a slightly more expensive option would be an inline fuse such as http://uk.farnell.com/jsp/displayProduct.jsp?sku=1162750&CMP=e-2072-00001000
I had thought of using inline fuses, unfortunately if I do that I don’t get the alarm circuitry functionality (so may never know a fuse has gone, only that something isn’t working as expected)
I have however been sent a table which shows which gauges of which types of metal wire blow at which currents – so although I’ve not found any actual fuse wire at those ratings (it all seems to start at 5A which is the lowest you’d go in a house) I can find some spools of equivalent wire.
Failing that, I’ll just use some Type 44 fuses as they seem to be comparatively easy to come by.